View Full Version : colt 6933 vs cut down colt 6720
chapperjoe
08-21-11, 13:16
other than the facts that:
-I can chop the barrel on the 6720 to 12.5" (I prefer that to 11.5")
and
-the profile on the 6933 gets a little heavier forward of the FSB..
....what are the differences between these two choices?
charmcitycop
08-21-11, 14:02
........
SpaceWrangler
08-21-11, 15:26
I'm not sure if the 6720 has an F marked FSB.
It has been discussed here recently that they do not. A Colt 6720 has a regular Front Sight Base, so you need to install a taller Front Sight Post.
chapperjoe
08-21-11, 15:27
I was more concerned with port size but that's a good difference to know...
They come with a matech so I assumed it was f hieght.... Maybe someone will chime in.
chapperjoe
08-21-11, 15:29
It has been discussed here recently that they do not. A Colt 6720 has a regular Front Sight Base, so you need to install a taller Front Sight Post.
The plan would be to cut it down, so I guess that could be rectified.
CaptainDooley
08-21-11, 15:38
From what I have read it's not F-marked but has a taller post to compensate.
Double check here: http://m4carbine.net/showthread.php?t=85670
I was more concerned with port size but that's a good difference to know...
They come with a matech so I assumed it was f hieght.... Maybe someone will chime in.
SpaceWrangler
08-21-11, 16:32
The plan would be to cut it down, so I guess that could be rectified.
Gotcha.
I believe the posts are the same, the one on the 6720 is just turned up a little higher to compensate for the lower fsb.
QuietShootr
08-21-11, 21:56
I don't think it's going to matter much in New York, since SBRs are illegal.
chapperjoe
08-21-11, 22:47
Within 2-4 weeks I should be making my second escape from NY.
This one permanent.
other than the facts that:
-I can chop the barrel on the 6720 to 12.5" (I prefer that to 11.5")
and
-the profile on the 6933 gets a little heavier forward of the FSB..
....what are the differences between these two choices?
I'd buy one of these instead of modifying the Colt barrel.
https://danieldefense.com/cold-hammer-forged-barrels/12-5-inch-chf-barrels/12-5-5-56mm-lightweight-carbine-chf-barrel-w-fsb.html
I really don't know why you want this barrel style in a 12.5" anyway. I'd rather have an 11.5" barrel.
LowSpeed_HighDrag
08-21-11, 23:38
I really don't know why you want this barrel style in a 12.5" anyway. I'd rather have an 11.5" barrel.
I'd like to hear this as well. Ive always thought the 6933 was regarded as an optimal SBR?
Magic_Salad0892
08-22-11, 03:23
IMHO, cut the 6720, leave the gas port at .063'', get VLTOR A5, get LMT Enhanced carrier, and you have an 11.5'' carbine that will function 99% optimally suppressed, or unsuppressed with 5.56mm NATO ammo.
12.5" barrels were developed to use a reflex style silencer with a low pro gas block and have the shortest barrel length possible with a reflex silencer.
chapperjoe
08-22-11, 09:09
that is the eventual plan.
not to get too theoretical, but how about hanging a reflex can on a lightweight barrel - too much POI shift or not an for most?
other than the facts that:
-I can chop the barrel on the 6720 to 12.5" (I prefer that to 11.5")
and
-the profile on the 6933 gets a little heavier forward of the FSB..
....what are the differences between these two choices?
I wouldn't recommend cutting a 6720 barrel to any length. The 6520/6720 barrel is flared at the end to support the crush washer. The shoulder created during threading will not be sufficient to support the crush washer properly.
chapperjoe
08-22-11, 09:15
and that's why M4c is what it is, thank you!!!!
turdbocharged
08-29-11, 09:24
Please correct me if I am mistaken, however,
I thought it was a best practice to run a thicker barrel profile for a supressor, since you have so much dead weight out front that can cause more POI shift on the thinner profiles.
I wouldn't recommend cutting a 6720 barrel to any length. The 6520/6720 barrel is flared at the end to support the crush washer. The shoulder created during threading will not be sufficient to support the crush washer properly.
Correct.
Please correct me if I am mistaken, however,
I thought it was a best practice to run a thicker barrel profile for a supressor, since you have so much dead weight out front that can cause more POI shift on the thinner profiles.
This is also true.
chapperjoe
08-29-11, 09:27
that's what I was referring too with the POI shift.
I think if I go the 'cut down' route, a 6920 or 6940 is a better choice.
that's what I was referring too with the POI shift.
I think if I go the 'cut down' route, a 6920 or 6940 is a better choice.
There is no reason to cut a Colt barrel.
There is no reason to cut a Colt barrel.
Agreed. Buy the Colt upper half/barrel in the length you want, and sell off the other upper.
Magic_Salad0892
08-30-11, 02:10
Agreed. Buy the Colt upper half/barrel in the length you want, and sell off the other upper.
Not agreed. Milspec gas ports are huge if you're a suppressor user.
If not, then they're fine.
Well, then what would be optimal port size for running with and without can?
Magic_Salad0892
08-30-11, 05:10
Well, then what would be optimal port size for running with and without can?
With: .056'' (Will cycle full power 5.56mm unsuppressed.)
Without: .071'' (Even if I prefer .062'')
Both sizes are kind of subjective, and it's my opinion.
Others are free to chime in.
Not agreed. Milspec gas ports are huge if you're a suppressor user.
If not, then they're fine.
I disagree. I mean.... I run my can on a Bushmaster for Pete's sake. That is a big port. Just bump up the buffer to H2 or H3.
Not agreed. Milspec gas ports are huge if you're a suppressor user.
If not, then they're fine.
If he cuts the barrel, the gas port is no longer spec for that length of barrel.
I disagree. I mean.... I run my can on a Bushmaster for Pete's sake. That is a big port. Just bump up the buffer to H2 or H3.
If he cuts the barrel, the gas port is no longer spec for that length of barrel.
You two are spinning your wheels. I am convinced this cat uses a pair of dice in his decision making process.
Having the gas port sized smaller for a suppressor is a backwards approach.
The gas port should be sized for mil spec ammo unsuppressed. That way it insures reliable functioning when the gun is dirty.
I wouldn't recommend cutting a 6720 barrel to any length. The 6520/6720 barrel is flared at the end to support the crush washer. The shoulder created during threading will not be sufficient to support the crush washer properly.
If you understand this and select a muzzle device that will index on the muzzle not the shoulder then it's not a problem.
Selecting a barrel that is larger at the muzzle end but the same under the handguards to reduce POI shift seems odd to me.
The number of people who shoot suppressed only rifles is very small, that's why there are no rifles made that way. Rifles with suppressed only gas ports should not be considered reliable without the silencer.
If you understand this and select a muzzle device that will index on the muzzle not the shoulder then it's not a problem.
Is there a suppressor manufacturer that advocates this method of indexing for a muzzle device? What cutting tool would you use in the counterbore to ensure that the bottom was square to the bore of the barrel?
Please correct me if I am mistaken, however,
I thought it was a best practice to run a thicker barrel profile for a supressor, since you have so much dead weight out front that can cause more POI shift on the thinner profiles.
Which only matters if you are going to be taking the can on and off. With a fulltime can there is no shift.
I'm working on a carbine now with a 10.3" Daniel Defense 0.625" dia. barrel. If it works out I'll likely be putting together an identical upper for fulltime suppressor use, and won't give POI shift a second thought.
AAC does, the length of threads needed to index on the muzzle is in their thread specs. The bottom of the mount is as square to the bore as the threads and back if they are cut in the same op.
I guess I'd have to see one cut in half to fully grasp how it's finished. It would seem that a boring bar would have to be used to relief cut the corners of the counterbore and in my mind it wouldn't leave enough purchase between the face of the muzzle and the suppressor mount to rely on square fit. AAC gets the benefit of the doubt though.
It has internal thread relief and the face cut square with a boring bar I assume. It's not like an A2 with the angle of the drill at the bottom.
Magic_Salad0892
08-31-11, 04:24
I disagree. I mean.... I run my can on a Bushmaster for Pete's sake. That is a big port. Just bump up the buffer to H2 or H3.
Have you run it without lubrication, for extended periods of time?
I have, both suppressed, and unsuppressed. This port size works well for me with 5.56mm pressure ammo.
If he cuts the barrel, the gas port is no longer spec for that length of barrel.
There is no gas port specification for suppressor use. For unsuppressed use though, I gotcha. But there are middle-grounds. Like for a suppressed gun where the suppressor will NEVER come off it'd be something like .048'' or .050'' or something super tiny. The gun wouldn't even cycle unsuppressed. .056/.060'' or more, will cycle unsuppressed, and will be reliable.
Rifles with suppressed only gas ports should not be considered reliable without the silencer.
Agreed. That's why mine is slightly bigger. I subscribed to Grant's train of thought, and built my guns based on his theories, and findings. After he did them first. I trusted his decisions, and the overall builds have worked for me.
VLTOR A5 will make the bolt lock back unsuppressed. Just a footnote, btw.
The gas port should be sized for mil spec ammo unsuppressed. That way it insures reliable functioning when the gun is dirty.
That's if the gun will be shot primarily unsuppressed. Mine is ported for (close to) milspec ammo suppressed. Actually it's ported slightly larger than that.
Hope the replies make sense. I was in a hurry.
Was it established that the 6720 and 6933 have different size gas ports, and if so do we know the sizes, or are we just assuming they are different based on the different barrel lengths.
Magic_Salad0892
08-31-11, 04:46
It's my understanding that 6720 has .062'', and 6933 has .071''. (or .090'' that I read from a Robb Jenson post.)
Is there a suppressor manufacturer that advocates this method of indexing for a muzzle device? What cutting tool would you use in the counterbore to ensure that the bottom was square to the bore of the barrel?
The AAC miter mount indexes off the muzzle face.
The AAC miter mount indexes off the muzzle face.
So I hear. I'm just trying to wrap my arms around their decision to do it. I'm am not saying it is a bad idea, I am just want to know what the design benefit is over indexing against the shoulder.
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