View Full Version : Question of Length- Cutting an M4 Barrel
If an M4 barrel were cut & threaded to remove the M4 notch, would the barrel length come out to 10.5 inches?
I think it works at 11.5 and 12.5.
TerdFergison
10-12-13, 10:14
If an M4 barrel were cut & threaded to remove the M4 notch, would the barrel length come out to 10.5 inches?
Why would you want to do this?
I didn't think you could cut a chrome lined barrel and get a good crown on it.
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Also how would you redo the gas system?
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SilverBullet432
10-12-13, 10:32
After labor costs and all, why not just buy a new barrel?
Yes, you can cut a chrome-lined barrel and get a good crown- no problem at all.
If you already own the barrel, cutting means you get the barrel you want for less than $100, instead of dropping another $250-$300 for it.
And to the OP, yes- the notch is eliminated and does not affect you with a cut to 10.5. As indicated above, you can even squeeze an 11.5 out (not sure about 12 without looking- I've not done one).
Edit - Missed the gas port - it's a pretty easy thing to open up the gas port. No other change is needed.
Wow.. a lot of fail in this thread.
Yes, you can cut a chrome-lined barrel and get a good crown- no problem at all.
If you already own the barrel, cutting means you get the barrel you want for less than $100, instead of dropping another $250-$300 for it.
And to the OP, yes- the notch is eliminated and does not affect you with a cut to 10.5. As indicated above, you can even squeeze an 11.5 out (not sure about 12 without looking- I've not done one).
Edit - Missed the gas port fail - it's a pretty easy thing to open up the gas port. No other change is needed.
The issues raised were perfectly reasonable for the most part, so the "a lot of fail" comment was unwarranted. While you would be likely to save some money after all is done you still risk a less than ideal outcome.
The issues raised were perfectly reasonable for the most part, so the "a lot of fail" comment was unwarranted. While you would be likely to save some money after all is done you still risk a less than ideal outcome.
Fixed.
However, what less than ideal outcome is possible?
Fixed.
However, what less than ideal outcome is possible?
Fixed what? You quoted me verbatim.
The less than ideal possible outcomes could include an uneven crown or a misaligned gas port.
Edit: Never mind the first part, I see you corrected the initial statement. Fair enough.
The less than ideal possible outcomes could include an uneven crown or a misaligned gas port.
If the OP were to do the work him in his garage with a hacksaw and Dremel, I suppose these outcomes would be possible. I guess I'm making the assumption he either possess the tools and skill necessary to do it or intends to send it to a shop that does.
Either way, the crown will come out just fine, and the gas port isn't moving, so misalignment isn't possible.
If the OP were to do the work him in his garage with a hacksaw and Dremel, I suppose these outcomes would be possible. I guess I'm making the assumption he either possess the tools and skill necessary to do it or intends to send it to a shop that does.
Assumptions can be problematic and even good gunsmiths make mistakes. Assuming he knows a good gunsmith.
Either way, the crown will come out just fine, and the gas port isn't moving, so misalignment isn't possible.
How do you know the crown will come out just fine? And are we not talking relocating the FSB? Also, what about the potential for a less than perfect thread job?
You're making it sound essentially risk free and that simply isn't the case. Not looking to nitpick but there's no other way to put it.
I'm weighing out options. I'm thinking I could get a complete Colt 6920 to SBR. The 16" bbl with the carbine length gas system starts off with a smaller gas port for when I get a suppressor and would allow me to shoot the rifle until the SBR paperwork clears. Just thinking it might be a practical alternative to ordering a bbl with a custom gas port size. I really like how Grant set up his dual purpose SBR in this thread-
https://www.m4carbine.net/showthread.php?t=33743
Going this route would give me more control over port size and concentricity of the threads to reduce POI shift when shooting with or without the suppressor.
Thanks for the responses. I don't have an M4 bbl to measure
Assumptions can be problematic and even good gunsmiths make mistakes. Assuming he knows a good gunsmith.
How do you know the crown will come out just fine? And are we not talking relocating the FSB? Also, what about the potential for a less than perfect thread job?
You're making it sound essentially risk free and that simply isn't the case. Not looking to nitpick but there's no other way to put it.
FSB does not need to be relocated, the port simply needs to be widened about a hundredth of an inch. The crown and threads can be cut just as easily as from the manufacturer. Can you get a bad barrel from the manufacturer? It happens. Can you get a bad cut from a qualified shop? Less likely, but it also happens.
This isn't an arcane art- it's a very common service. Problems are uncommon.
Assumptions can be problematic and even good gunsmiths make mistakes. Assuming he knows a good gunsmith.
How do you know the crown will come out just fine? And are we not talking relocating the FSB? Also, what about the potential for a less than perfect thread job?
You're making it sound essentially risk free and that simply isn't the case. Not looking to nitpick but there's no other way to put it.
Cutting & crowning the barrel isn't that difficult and if I were simply planning to use an A2 birdcage, I could probably figure out how to re-thread the muzzle
FSB does not need to be relocated, the port simply needs to be widened about a hundredth of an inch. The crown and threads can be cut just as easily as from the manufacturer. Can you get a bad barrel from the manufacturer? It happens. Can you get a bad cut from a qualified shop? Less likely, but it also happens.
This isn't an arcane art- it's a very common service. Problems are uncommon.
My ASSumption (told you they can be problematic) was that if you are going past the M4 notch then there would not be adequate room for threading leaving the FSB in place, but looking at one now I see that isn't necessarily true I suppose. Still there is risk, even if minimal, and it will cost money unless you take further risk and do it yourself. But it's not my decision and so be it.
This discussion reminds me of a similar thread that highlighted some of what I mentioned. Will see if I can find and then post the link.
justin_247
10-12-13, 12:48
Robb Jensen successfully cut down a barrel with an M203 notch on it and turned it into an SBR. You may want to contact him and see what all he did.
Safetyhit. Please take the drama back to the GD Area. What Dead man stated earlier was essentially true. A competent shop can do what is required. As to whether the cost of it is worth it, probably not.
Safetyhit. Please take the drama back to the GD Area. What Dead man stated earlier was essentially true. A competent shop can do what is required. As to whether the cost of it is worth it, probably not.
Hey thanks IG. I'll suppose this comment has nothing to do with the cologne thread and try to clarify a bit further.
Here is the old thread I was referring to. The question was about the feasibility of cutting a 16" M4 down to a 12.5" and sure enough there you are in the first post stating "My advice is not to cut the Colt barrel." and it went on from there.
Others you would perhaps deem more reputable chimed in, agreed and even mentioned some of my same stated concerns.
https://www.m4carbine.net/archive/index.php/t-75063.html
K.L. Davis
10-12-13, 13:15
The barrel at the notch should be 0.61 dia - if you square off the tapered step at the back of the notch, then cut and thread the "notch" section of the barrel, you should come out with a barrel that is ~11.3 inches, measured from the bolt face.
I have no idea what you mean by the cologne thread or why you can't seem to understand what I said. I was referring to the claim being made about cutting/crowning and the opening of the gas port. I also never said it was something I would do. In fact, aside from your misguided attempt to make it look as if I was being contradictory I have oft stated I would buy a good barrel rather than cut one down.
Hey thanks IG. I'll suppose this comment has nothing to do with the cologne thread and try to clarify a bit further.
Here is the old thread I was referring to. The question was about the feasibility of cutting a 16" M4 down to a 12.5" and sure enough there you are in the first post stating "My advice is not to cut the Colt barrel." and it went on from there.
Others you would perhaps deem more reputable chimed in, agreed and even mentioned some of my same stated concerns.
https://www.m4carbine.net/archive/index.php/t-75063.html
Cutting, crowning, threading, and re-porting a barrel is child's play to any capable gunsmith. In fact, machining 4xxx series steel is a treat. If the barrel at 16" is no longer of value to the owner then it is all gain. You would likely loose the value of the machine work when trying to sell a used barrel. I would proceed.
I have no idea what you mean by the cologne thread or why you can't seem to understand what I said. I was referring to the claim being made about cutting/crowning and the opening of the gas port. I also never said it was something I would do. In fact, aside from your misguided attempt to make it look as if I was being contradictory I have oft stated I would buy a good barrel rather than cut one down.
If I misreprented what you said it was unintentional. Needless to say your initial statement to me was less than complimentary yet I was still only relaying what I thought was the truth regarding a semi-complicated decision, not looking for drama.
All I would ask at this point is if you perceive the stated modifications to be so risk-free (as do most in this thread apparently) then why advise against such modifications if it's less expensive to make them with a capable gunsmith instead of just buying new? Keep in mind that it actually wasn't your post that formed my initial opinion, but when I found the thread I was looking for there you were also advising against.
MistWolf is a knowledgable guy and I wasn't looking to derail his thread. Was just trying to help and apparently it backfired. Hopefully at least some more detailed knowledge of the scenario was passed on to our fellow members.
Here is the M4 profile from Randall's site.
Add .75" to the lengths shown for actual measurements to account for the bolt face to shoulder dimension.
If you cut it at 10.625", the FH shoulder is at 10.000"
The total bbl length is 10.625+.750=11.375"
http://www.ar15barrels.com/tech/m4a1.jpg
http://www.ar15barrels.com/tech/ar15bolt-extension.gif
Here is the other option for a 12.75" bbl, the FH shoulder is at 11.375"
http://img822.imageshack.us/img822/8980/adco1.jpg
The answer to your question is yes. If your goal is to cut it short enough to eliminate the notch, it could be made between 10 & 11.3 inches.
I had a 6920 barrel cut to an even 10 inches several months ago. The gas port was left alone to use as a mostly suppressed SBR. It's soft shooting w/ not much gas to the face while using my Surefire 212.
The combination of 10in barrel w/ 9in handguard makes a seemless transition from handguard to can. I haven't noticed a decrease in accuracy after the chop.
Here is a thread on the same question.
https://www.m4carbine.net/showthread.php?t=18155
justin_247
10-12-13, 20:00
Here is a thread on the same question.
https://www.m4carbine.net/showthread.php?t=18155
Thank you! That's exactly the thread I was looking to send to the OP.
Robb Jensen's post is on pg. 2 of that thread.
If it's just cut and threaded at 11.5", you will still see some of the taper unless more machine work is done. Rguns sold cut-down Colt M4 barrels as Commando barrels in the past. It was pretty obvious.
I got one years ago and had Adco cut it to 10.3" and open the gas port to make a MK18 upper. Don't shoot it that much, but it's worked fine.
I thought the first CQBR/MK18 barrels were made by cutting down M4 barrels?
ETA: I posted a pic of the 11.5" barrel, before it was cut to 10.3", in the thread linked above.
FWIW, ADCO's charge for this job is quite inexpensive. Last week I sent them a DD M4 barrel that I wasn't using anymore to get this done.
Cut, crown, thread, and gas port modification was only $65.
ClearedHot
10-13-13, 06:55
12.5" is a good length to cut it down to. I don't believe that the gas port even needs to be opened up if you cut it down to that length.
BoyScout4Life
10-13-13, 07:44
I think it works at 11.5 and 12.5.
Roger That....
The measurements shown are breach threads to crown.
http://i1363.photobucket.com/albums/r704/BallistaSystems/OFFICE-PC/Drawings/BarrelLengths_zps80b61de9.jpg
That is an excellent print. Thank you
That is an excellent print. Thank you
You got it! ;)
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