View Full Version : SBRs for civilians
In my searching of this site, I came across this thread:
https://www.m4carbine.net/showthread.php?t=12856&highlight=self-defense
Given the conclusion drawn by many that there are good reasons not to use an SBR for home defense, is there any practical reason for someone like myself to pay the feds more money to have something shorter than 16"? I am neither in Law Enforcement or the military...
I draw a clear distinction between what I consider a "practical reason" and "because I want one" - "because I want one" is a 110% valid reason in my book. Its just that I see an awful lot of people who talk about their SBRs, and as cool as I think it would be, I can't at this point justify the expenditure, unless it was for what I consider to be the purely practical purpose of defending my own home.
So, help convince me that I should build one specifically for home defense. Or convince me to stop thinking about it so that I can start thinking about other options for my next gun... :D
Thanks!
In my searching of this site, I came across this thread:
https://www.m4carbine.net/showthread.php?t=12856&highlight=self-defense
Given the conclusion drawn by many that there are good reasons not to use an SBR for home defense, is there any practical reason for someone like myself to pay the feds more money to have something shorter than 16"? I am neither in Law Enforcement or the military...
I draw a clear distinction between what I consider a "practical reason" and "because I want one" - "because I want one" is a 110% valid reason in my book. Its just that I see an awful lot of people who talk about their SBRs, and as cool as I think it would be, I can't at this point justify the expenditure, unless it was for what I consider to be the purely practical purpose of defending my own home.
So, help convince me that I should build one specifically for home defense. Or convince me to stop thinking about it so that I can start thinking about other options for my next gun... :D
Thanks!
A 16" weapon will work just fine. An SBR though, will allow you to maneuver around a CQB environment much better. I simply like them because I shoot a with a can.
C4
A 16" weapon will work just fine. An SBR though, will allow you to maneuver around a CQB environment much better. I simply like them because I shoot a with a can.
C4
See, that would be the ideal situation as I see it. Have a can on a very short AR, making it very maneuverable, and not ear-shatteringly loud should it ever need to be fired. It would also allow me to hear the BG when he is yelling at me to tell me he is giving up after I've shot once, which doesn't seem like a bad thing, all things considered.
So, I assume you don't buy the argument(s) presented in the thread I linked to? If not, why not?
I don't really want to fire any unsuppressed gun indoors, especially an AR, so if I can be convinced of the "a good shoot is a good shoot" position over the "juries / judges / prosecutors want to hang anyone who uses any NFA weapon in self-defense" position, then an SBR with a can will probably be AR #2...
Thanks for the input.
See, that would be the ideal situation as I see it. Have a can on a very short AR, making it very maneuverable, and not ear-shatteringly loud should it ever need to be fired. It would also allow me to hear the BG when he is yelling at me to tell me he is giving up after I've shot once, which doesn't seem like a bad thing, all things considered.
So, I assume you don't buy the argument(s) presented in the thread I linked to? If not, why not?
I don't really want to fire any unsuppressed gun indoors, especially an AR, so if I can be convinced of the "a good shoot is a good shoot" position over the "juries / judges / prosecutors want to hang anyone who uses any NFA weapon in self-defense" position, then an SBR with a can will probably be AR #2...
Thanks for the input.
I do not "buy" the arguments. If you are involved in a questionable shoot, you are screwed no matter what the length the barrel is.
I would also say that it also kind of depends in what State you live in and or what your local police are like. Since I live outside of city limits (way out), the county Sheriff has jurisdiction. The majority of them are either personal friends or owe me favors. ;)
C4
As mentioned in the referenced thread, this can all change based on the "where" you are if used for self defense.
I honestly could care less and would want to use the best tools at hand, if I had the abiltiy to prep for it. That said, I know a bunch of folks for good reason would recommend not using your machine gun, but a regular gun. I know much can be domonized by a lawyer, but I dont see an SBR as much over the line as a machine gun. Yea tell that to the jury...
I tend to think an SBR is a decent, easly way to go.
You can always get a class 3, then any barrel size works!
I do not "buy" the arguments. If you are involved in a questionable shoot, you are screwed no matter what the length the barrel is.
I would also say that it also kind of depends in what State you live in and or what your local police are like. Since I live outside of city limits (way out), the county Sheriff has jurisdiction. The majority of them are either personal friends or owe me favors. ;)
C4
That's kinda what I figured... Assuming I go with an SBR, I'll be in St. Paul, MN by the time I get together. I've heard that the city police are quite gun friendly, while the county Sheriff is not. Either way, it isn't likely we'll be in either St. Paul or Ramsey county for more than 2-3 years anyway, as we hope to be moving to at least the outskirts of the metro area. I'm on a couple of forums specific to the Twin Cities - I'll post my question there to see what the locals think.
Thanks Grant!
And of course, if anyone else wants to chime in, feel free. I don't think there is really a blanket easy answer to this question, and more input will help me to really figure out what I think the right thing to do is for my specific situation...
As mentioned in the referenced thread, this can all change based on the "where" you are if used for self defense.
I honestly could care less and would want to use the best tools at hand, if I had the abiltiy to prep for it. That said, I know a bunch of folks for good reason would recommend not using your machine gun, but a regular gun. I know much can be domonized by a lawyer, but I dont see an SBR as much over the line as a machine gun. Yea tell that to the jury...
I tend to think an SBR is a decent, easly way to go.
You can always get a class 3, then any barrel size works!
I'm still pretty new to all this, so please clarify for me - if I file the paperwork & pay the fee (tax?), I can use a barrel that has an overall length of less than 16", right? If I want to add a can to that, do I fill out more paperwork, or am I already covered? And just so I'm clear, if I wanted a full-auto weapon, that would be yet another set of paperwork? I'm not really interested in getting anything full-auto, I just want to add to my knowledge base with that question. But for me, the whole reason for paying for permission for the shorter barrel is to put a can on it, so that question I'm acutely interested in!
I should just get out there - I am looking into SBR'ing a couple of lowers. I do Class III stuff, so that trumps all. My understanding is paperwork for registering is similar in that you must go local LEO sign-off and then federal(with your $200 check) - no SBR upper allowed on until Form 4 in hand.
As far as a suppressor, same deal, but it is an entity to itself. Does not need an SBR or MG to be bound to or used with. Their is nothing that binds the two together(i.e. I can use my AAC 7.62SD can on my 18" title I AR-10 and move it to a regular 20" AR gun no problem - assuming I have the QD mount on the AR as well)
I should just get out there - I am looking into SBR'ing a couple of lowers. I do Class III stuff, so that trumps all. My understanding is paperwork for registering is similar in that you must go local LEO sign-off and then federal(with your $200 check) - no SBR upper allowed on until Form 4 in hand.
As far as a suppressor, same deal, but it is an entity to itself. Does not need an SBR or MG to be bound to or used with. Their is nothing that binds the two together(i.e. I can use my AAC 7.62SD can on my 18" title I AR-10 and move it to a regular 20" AR gun no problem - assuming I have the QD mount on the AR as well)
Thank you very much! Especially the info about the suppressor not being tied to any particular gun. I only plan on building one SBR, but it would be very nice if the one suppressor I would get to go with it will also be available for any other 5.56N AR that might want to put it on.
Great info - thanks again.
I do not "buy" the arguments. If you are involved in a questionable shoot, you are screwed no matter what the length the barrel is.
I would also say that it also kind of depends in what State you live in and or what your local police are like. Since I live outside of city limits (way out), the county Sheriff has jurisdiction. The majority of them are either personal friends or owe me favors. ;)
C4
I concur.
variablebinary
04-20-08, 01:39
If anything, SBR/PDW style weapons are becoming increasingly popular among civilians, LEO and military.
For a couple of reasons.
Bullet designs have improved so you still get decent ballistics, barrier penetration and soft tissue damage
The designs are dialed in so reliability is good.
Cans are more popular across the board as well
In addition, cutting the pie in the confines of your average home is easier with an SBR.
On a side note, they are very light, so training becomes easier because your support arm doesnt take a beating.
All around, SBR's kick ass and are very practical for civilians.
Jay Cunningham
04-20-08, 02:53
SBR's, by their very nature, have no "sporting purpose" so in my book they are ideally suited for home defense.
Seriously - a short 5.56mm carbine is an ideal defensive weapon... and the case for a suppressed version is even stronger. I agree with the others who expressed the sentiment that a good shoot is a good shoot and a bad shoot is a bad shoot regardless of weapon used.
According to the data that I have seen, the risk of over penetration from an AR type weapon inside a structure is actually LESS than that of common handgun calibers as well as slugs fired from a 12 gauge.
To me that's a GOOD argument as to 'why did you choose this weapon over granddaddy's shotgun' or something more 'politically correct'.
Depending upon your domicile there might be someone on the other side of that wall.
Unfortunately, I just discovered that suppressors are a no-no in Minnesota... For a state with such good carry laws, I'm more than a bit surprised, but it is what it is. The suppressor was, to my way of thinking, the biggest reason to go with the SBR in the first place - now what?
Obviously, an SBR will be more maneuverable, but its going to be even louder than my 16" middy, right? Any other reasons to go with an SBR if it can't be suppressed? Other than the "cool" factor, I mean... :p
Unfortunately, I just discovered that suppressors are a no-no in Minnesota... For a state with such good carry laws, I'm more than a bit surprised, but it is what it is. The suppressor was, to my way of thinking, the biggest reason to go with the SBR in the first place - now what?
Obviously, an SBR will be more maneuverable, but its going to be even louder than my 16" middy, right? Any other reasons to go with an SBR if it can't be suppressed? Other than the "cool" factor, I mean... :p
I'm in a "no can zone" as well here in Massachusetts, but we can have SBRs - I have one in process for my Uzi Model B. I suppose "cool factor" comes into play, but my thoughts were simply to bring the SA IMI as close to it's FA forefather as possible. I do not use my Uzi for home defense - though the thought of adding a rail and a light has crossed my mind a time or two. I picked up a case of Speer GDHP +P and was impressed with how it handled in the Uzi. Still, I remain a Benelli fan when it comes to home defense and have my SOCOM-16 as a back-up plan - the 110gr TAP in the SOCOM are on standby. With an AR on the way, I'm really going to sit down do some reading and perhaps make some re-considerations on my HD plans.
As to the appearance of using an EBR as a home defense tool and the aftermath of a shoot within your home, I concur. In many states or municipalities, you're screwed no matter what - better have a good attorney. Though I haven't Googled it, my cobwebby brain seems to remember a shop owner in Cleveland back in the late 80's/early 90's timeframe who was dragged through the legal dirt and kicked like a dog by the prosecutors in Cleveland for defending his home/shop with an Uzi.
Personally, I'll deal with the aftermath, as I'm a proponent of using as much gun as necessary to defend my family and myself from harm....
Gutshot John
04-20-08, 08:50
If you're going to significantly shorten an AR barrel, you're going to want a can. Indoors... it won't be "fun" to shoot otherwise.
For a home defense SBR my preference would be for a pistol cartridge. I'm not worried about overpenetration, I just think it will be more shootable/compact.
If you're going to significantly shorten an AR barrel, you're going to want a can. Indoors... it won't be "fun" to shoot otherwise.
So, if you were living in, say, Minnesota ( :cool: ) and couldn't legally have a can, would you stick with a 16" (or 14.5" with permanently attached flash hider) AR? And by "fun", I assume you are referring to the sound when fired - is it that much louder out of a 10.5" barrel than out of a 14.5"? I assume the difference is measurable, but won't my ears be bleeding (figuratively, I hope) from either one?
So, if you were living in, say, Minnesota ( :cool: ) and couldn't legally have a can, would you stick with a 16" (or 14.5" with permanently attached flash hider) AR? And by "fun", I assume you are referring to the sound when fired - is it that much louder out of a 10.5" barrel than out of a 14.5"? I assume the difference is measurable, but won't my ears be bleeding (figuratively, I hope) from either one?
There's some fairly good research out there on audio and visual exclusion during times of high stress. Though I do not hunt anymore, a more simple example can be found with hunters. When I used to hunt, I used my grandfathers Winchester Model 12 with slugs and buckshot. At 15, these rounds would kick my ass at the range and ring my ears heavily if I wasn't using protection. However, with adrenaline and endorphins surging as I got the bead on a whitetail, I never ever recall feeling recoil or losing my hearing in any way. Though excited - I was clear a a cucumber after the shot.
Not that you would, but if you endeavored to touch off a round from a 12ga in a 15x15 room with no hearing protection just for fun, you would rattle your skull. The same scenario under stress, (i.e. intruder) however would likely have much less effect on your hearing or vision. Besides, singular event hearing damage will generally clear up - whereas the multiple incident damage over extended periods tends to be more permanent in nature....
There's some fairly good research out there on audio and visual exclusion during times of high stress. Though I do not hunt anymore, a more simple example can be found with hunters. When I used to hunt, I used my grandfathers Winchester Model 12 with slugs and buckshot. At 15, these rounds would kick my ass at the range and ring my ears heavily if I wasn't using protection. However, with adrenaline and endorphins surging as I got the bead on a whitetail, I never ever recall feeling recoil or losing my hearing in any way. Though excited - I was clear a a cucumber after the shot.
Not that you would, but if you endeavored to touch off a round from a 12ga in a 15x15 room with no hearing protection just for fun, you would rattle your skull. The same scenario under stress, (i.e. intruder) however would likely have much less effect on your hearing or vision. Besides, singular event hearing damage will generally clear up - whereas the multiple incident damage over extended periods tends to be more permanent in nature....
I've read similar info elsewhere, and have had similar experiences in my own hunting days. I guess that leads to the conclusion that -
While a 10.5" barrel will give a measurably louder report than would a 14.5 " or 16" barrel, if I ever have to fire either under stress, I am not likely to (1) notice a difference or (2) experience much hearing loss anyway due to the effects of adrenaline, etc (whatever it is the body actually does to protect itself in these kinds of situations).
Does that sound about right?
Gutshot John
04-20-08, 13:04
So, if you were living in, say, Minnesota ( :cool: ) and couldn't legally have a can, would you stick with a 16" (or 14.5" with permanently attached flash hider) AR? And by "fun", I assume you are referring to the sound when fired - is it that much louder out of a 10.5" barrel than out of a 14.5"? I assume the difference is measurable, but won't my ears be bleeding (figuratively, I hope) from either one?
The more gas that burns outside the barrel/unburned propellant...the louder it will be. Moreover I don't think you'll notice much difference between a 5.56 16 and 14.5 in terms of loud...both will be defeaning indoors. Not bleeding...but try it sometime and you'll see what I mean. Definitely choose a FH over a brake. I've not shot a 10.5 AR, but there is significant difference between a 20 and 14.5. I would imagine that difference would be even greater as you got to 10".
Using a pistol round with a relatively long barrel 10-12" would be lots more comfortable to shoot than a similar 5.56 especially if you can't have a can. I've always wanted either an 9mm or .45 SBR. An MP5 would be pretty cool. I also fondled the B&T 9mm at the DSA booth at SHOT. Unlike the MP5 it's pretty reasonably priced for an SBR.
All this depends on what you want. There are lots of valid reasons for using an 5.56 and to be fair even a pistol round can be ugly in confined spaces with a short barrel.
While a 10.5" barrel will give a measurably louder report than would a 14.5 " or 16" barrel, if I ever have to fire either under stress, I am not likely to (1) notice a difference or (2) experience much hearing loss anyway due to the effects of adrenaline, etc (whatever it is the body actually does to protect itself in these kinds of situations).
Auditory exclusion under stress: true.
Magical effect that protects your actual hearing when firing an unsuppressed SBR in close quarters: false.
The recoil analogy is perfect. While a hunter may not feel the recoil, certainly there is still exactly the same amount of force being imparted by the gun when it fires. You may not hear the rounds going off, but the exact same amount of energy is battering the stereocilia inside your cochlea.
For every three dB increase in sound, NIOSH estimates that safe exposure time gets cut in half. It's 15min at 100dB. So by 115dB it's less than 30 seconds, and by 130dB it's about 0.88 seconds.
Parabellum9x19mm
04-20-08, 23:42
Sorry that this is a somewhat off topic question, but what about the use of body armor and/or electronic hearing protection in a home defense situation?
I would guess that in the unfortunate event of a home defense situation I'd probably be wearing only underwear :p
But hypothetically, does anyone think that protecting one's body with armor or protecting one's ears (and increasing the sense of hearing electronically) would be used against someone, in investigation and prosecution?
I think all of us would consider it prudent and a tactical advantage, but would police or prosecutors look at it as evidence of malfeasance and malice of forethought?
Like I said, I don't see myself gearing-up when I hear a bump in the night, just wondering if doing so would have negative implications.
MikeCLeonard
04-21-08, 00:30
Auditory exclusion under stress: true.
Magical effect that protects your actual hearing when firing an unsuppressed SBR in close quarters: false.
The recoil analogy is perfect. While a hunter may not feel the recoil, certainly there is still exactly the same amount of force being imparted by the gun when it fires. You may not hear the rounds going off, but the exact same amount of energy is battering the stereocilia inside your cochlea.
For every three dB increase in sound, NIOSH estimates that safe exposure time gets cut in half. It's 15min at 100dB. So by 115dB it's less than 30 seconds, and by 130dB it's about 0.88 seconds.
+1
In a high stress/adrenaline situation, you would also not feel much pain if you were punched in the face but the damage is still done whether it hurt as bad as it normally would or not.
Interesting question. In the case of earpro I can't see where that would be a negative.
An armor package, boy I don't know about that. Hrmmm....
variablebinary
04-21-08, 01:17
Guns are loud. No matter how you cut it, they are loud.
You're ability to resist your eardrums getting blown out is completely genetic and differs from person to person. I dont know if I'd let the prospect of losing my hearing determine what I use when my ass is on the line. Learn Karate if that's the case. Not saying SBR's arent more harsh than carbines, just know a .22 pistol can blow your ears just as quick.
I wouldnt want to use an MG for SD because I dont want to lose it. They are over $15k these days.
If MG's were not banned I'd have one by the bed. The ability to perforate someone from crown to crotch in 2 seconds at 15ft is self defense at its finest.
Sorry that this is a somewhat off topic question, but what about the use of body armor and/or electronic hearing protection in a home defense situation?
I would guess that in the unfortunate event of a home defense situation I'd probably be wearing only underwear :p
But hypothetically, does anyone think that protecting one's body with armor or protecting one's ears (and increasing the sense of hearing electronically) would be used against someone, in investigation and prosecution?
I think all of us would consider it prudent and a tactical advantage, but would police or prosecutors look at it as evidence of malfeasance and malice of forethought?
Like I said, I don't see myself gearing-up when I hear a bump in the night, just wondering if doing so would have negative implications.
Unless the folks you are shooting at are actually LEOs, or your ear protection was shot during the confrontation, no one would ever have to know that you were wearing any kind of ear protection, electronic or not. Of course, this will only work if it is really dark or you actually kill the BG, since otherwise he'd be able to testify that you were wearing them.
I'd guess that wearing body armor could be seen in the same light, if you could get it off and stashed before LEOs arrive.
But in the end, I think that most of us aren't going to have either the forethought or the time to put on ear protection or especially body armor when we hear a "bump in the night"...
All of which leads me back to the next best solution - an SBR w/a can. But a can just isn't a possibility for now. Maybe I'll run for the state congress and get this asinine restriction changed?
I'm in a can friendly state, and own a can. But I still run my SBR unsuppressed for my Home D gun.
To me... if I'm going to add the extra length of a can, I'd rather just have the extra Barrel length and bullet velocity.
I was asking an instructor about this scenario just last weekend. He's on the firearms training division full time, but did many years on the SWAT team. I asked him because I've seen his 10.5" M16 entry gun, and figured he must have an opinion on the topic. Anyway, he said that in some older homes with hard interior walls in the internal rooms that it could be a little harsh firing an SBR. But even then it wasn't that bad.
I'm in a can friendly state, and own a can. But I still run my SBR unsuppressed for my Home D gun.
To me... if I'm going to add the extra length of a can, I'd rather just have the extra Barrel length and bullet velocity.
I was asking an instructor about this scenario just last weekend. He's on the firearms training division full time, but did many years on the SWAT team. I asked him because I've seen his 10.5" M16 entry gun, and figured he must have an opinion on the topic. Anyway, he said that in some older homes with hard interior walls in the internal rooms that it could be a little harsh firing an SBR. But even then it wasn't that bad.
I understand what you are saying, but I would think that the lower muzzle velocity really isn't going to make a difference at home defense distances, and that same lower muzzle velocity might actually help make the bullet slightly less likely to be lethal should it strike an innocent after going through a wall or two.
Of course that would never be an issue for any of us since we'd never miss. :cool:
Am I wrong to make this assumption? (Not the never missing part, of course...)
Anything is possible. But I'll always err on the side of hitting the Bad Guy as hard as possible with the bullet.
I don't know all the ins and outs of SBR rounds vs. 16" rounds thru various mediums.
SBR's have a fragmentation range (depending on which ammo you choose) to about 50-60yds. This is generally longer than most peoples homes.
I also believe that one of our forum member's has a kill with a 10.5 and an M855 round well past 150yds. Shot placement is everything.
C4
Gutshot John
04-21-08, 23:36
SBR's have a fragmentation range (depending on which ammo you choose) to about 50-60yds. This is generally longer than most peoples homes.
I'm not really familiar with the term "fragmentation range." Could you expound?
I presume you mean that inside of 50-60 yards 5.56 from an SBR has sufficient velocity to fragment when it strikes something rather than overpenetrate?
Correct me if I'm wrong.
I'm not really familiar with the term "fragmentation range." Could you expound?
I presume you mean that inside of 50-60 yards 5.56 from an SBR has sufficient velocity to fragment when it strikes something rather than overpenetrate?
Correct me if I'm wrong.
Your description of the term "fragmentation range" sounds about right, though it is highly dependent on bullet selection. I've read ballistics studies that show some bullets out of a 10.5" barrel will reliably fragment for only about 15 meters (sorry, I can't seem to find a link to them right now...) - but again, it depends on the bullet. Proper bullet selection will help extend this, and I have no reason to doubt Grant's data of 50-60 yards with the right ammo.
Of course, I'm not rich (yet!), so I've never lived in a house or apartment where I'd have to take a home defense-type shot at greater than 45-50 feet, so it remains a non-issue for the intended purpose.
Gutshot John
04-22-08, 07:58
Your description of the term "fragmentation range" sounds about right, though it is highly dependent on bullet selection. I've read ballistics studies that show some bullets out of a 10.5" barrel will reliably fragment for only about 15 meters (sorry, I can't seem to find a link to them right now...) - but again, it depends on the bullet. Proper bullet selection will help extend this, and I have no reason to doubt Grant's data of 50-60 yards with the right ammo.
Of course, I'm not rich (yet!), so I've never lived in a house or apartment where I'd have to take a home defense-type shot at greater than 45-50 feet, so it remains a non-issue for the intended purpose.
By way of comparison does anyone know what the fragmentation range would be if the same round was fired from a 20" barrel?
By way of comparison does anyone know what the fragmentation range would be if the same round was fired from a 20" barrel?
I managed to find a couple of the links I couldn't find earlier. I can't vouch for their reliability, but the info they provide jives pretty closely with info I've read elsewhere. Take it with however many grains of salt you deem appropriate.
http://www.ar15.com/content/page.html?id=173
http://ammo.ar15.com/
http://ammo.ar15.com/#sbr
The first one compares 20", 16", & 14.5" barrels. It doesn't cover anything shorter than that, but you can see that the fragmentation threshold drops by 15-30 yards when going from the 16" to the 14.5" barrels. Draw your own conclusions as to what will happen when you take it down to 11.5" or 10.5".
The second one has all sorts of links to a lot of other interesting info. At least I found it interesting...
The third link is to the section within the second link that specifically addresses SBRs. Based on what I've read here, I'd guess that some will dispute this info, and that's OK. Make your own decision based on info from sources you trust.
Again, all of this only addresses fragmentation. While this is a big deal, a well placed shot will still kill ya' well past the "fragmentation threshold". My own feeling is that, for home defense ('cause that is what I'm concerned about with this thread), I'm going to rely on well placed shots over fragmentation to 'stop the threat' from whatever BG has made the very bad decision to enter my house uninvited. Of course, I'll take the fragmentation if it happens, but I'm still only concerned with stopping the threat.
My belief - which I'm sure will be corrected if it is demonstrably wrong - is that by choosing to use a 10.5" SBR for home defense, I'm choosing to give up the likelihood of fragmentation for the benefit of maneuverability and a reasonable length suppressed weapon. I've already given up on the suppressed part as MN won't allow it, but if it is easier to get an SBR on target than it is to get a 16" m4gery, it might still be worth it to me.
That obviously goes a lot farther than a simple answer to your simple question. I'm basically using you guys as a sounding board to help me figure out what to do, so thanks for your patience & indulgence.
lowspeed4u
04-22-08, 14:13
By way of comparison does anyone know what the fragmentation range would be if the same round was fired from a 20" barrel?
Yea, Hi guys this is my first posting on M4, but I have been reading around here for awhile now. To answer your question about the 20'' barrel.
First if your using M855 62 grain green tip (military round) out of a 20'' barrel the muzzle velosity is about 3040 FPS give or take alittle; round set back, temp outside, temp of the round its self, and amount of powder (this shouldn't be to major though). So using the same bullet out of a M4 14.5'' the muzzle velosity is about 2700 FPS, the same factors affect the round.
Ok, next the round reliabaly fragments at a velosity above 2500 FPS. For ease of the information you drop about 200 FPS in the fist 100 yards or so, then about 100 FPS every 100 yards. not excat I know sorry just a knuckle dragger.
So not going to much into the math of the numbers i just said, a 20'' barrel will maintain a muzzle velosity above 2500 FPS till about 250-200 yards, the M4 will maintain that out to about 75-100 yards. not sainging it won't break up further but not reliably so you can't count that really; and as soon as you know that your find out you really know nothing about ballastics because nothing is a guarntee. hope that helped alittle, if not sorry for wasting your time and I wish I could give you back the 3-4 min you took to read this.
P.S haha, round configuring and muzzle velosity are the keys to the fragamentation range for high velosity rifle rounds.
Derrick
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