Barrel cutting/dwell time

General Firearms NFA
1

I have a DDM4v7 (mid, 16" barrel). I’m wanting to cut it to somewhere between just in front of the gas block and an inch inside the rails (closer to the gas block the better). I’m also planning to have it threaded for a .30 suppressor I have ordered. I’m fairly certain the suppressor will maintain the needed dwell time for operation.

However, what about if I run it without? Would be getting a KX3, both for increased back pressure and so the bullet exits past the end of the rails. However, it would be the .30 version to match threads, so not sure if it would be as effective. Considering I might need an A5, too?

I realize this may be a bit theoretical at this point, but am I in the right ballpark with my thinking, or is trimming that close to the block destined to be single fire, even with a suppressor or other compensations?

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2

Sounds wonky. Seems the first question is: why not use a known good configuration? What are you hoping to gain?

3

The path to the choice of SBRing my 5.56 is a bit long. I initially had no plans to do so. My first plan was to shoot as is and ultimately get a short .300blk upper (saving all my 5.56 brass for reloading). However, due to costs, a 2nd upper is farther down the line than I like. Also, figured that since I’m going to SBR the lower anyway, why not also do it to my existing barrel? This would give me the added benefit of my 5.56 fitting in my carry bag without removing the suppressor.

The location of the cut is more aesthetic, as I prefer the mount to be inside the rails for a cleaner, more integrally suppressed look and figure an extra inch or so wouldn’t make too much of a difference.

I could get a new barrel and carbine-length gas system, but that would come threaded for 5.56, needing an adapter, as well as being more expensive.

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4

An 11.5" CAR barrel with a small gas port would be a better choice.

5

Not really what I was hoping for. :frowning: However, can I easily get one threaded for .308 (5/8x24 or whatever it is) or unthreaded?

Also, seems most new barrels are out of stock, which crimps that, but may be different by the time my SBR papers go through.

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6

The issue is the thread-pitch on the mount you’ll be using, right? Does no one make a conversion from 1/2 28 to use with .308 suppressors? I know people use .308 suppressors with 5.56 guns- surely they don’t all have to have a barrel custom threaded

7

There is a thread adapter, I was just hoping to eliminate it. Not a major deal, but if I can remove one extra attachment/seam on the end of the barrel, I’d love to. Kind of a no brainer if I’m cutting/threading anyway, or can get it new that way without much expense.

Although, what would/might I be sacrificing with my original plan? I’d honestly be using the suppressor on this at least 90% of the time. Was thinking non-suppressed just because the SBR paperwork will probably go through a few months before the suppressor does (suppressor wasn’t in stock). With all the back pressure a suppressor adds (even a .308 can with 5.56 bullets), that would surely give the dwell time to reliably cycle? Unless there’s another factor I missed. If the issue comes down to the non-suppressed configuration, I’m comfortable foregoing that capability and just not cutting until the suppressor finally arrives.

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8

Are you using a thread-on suppressor?

9

Aye. Has to be thread-on to fit with my plan to have the attachment point inside the rails. Although, since I don’t plan to move it between barrels much, I like the more solid feel of threaded, anyway.

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10

If it were me and you are set on cutting the barrel, cut it to 11.5" and leave it. You should be fine.

11

A few points to consider.

A silencer inside the handguard gets hot real fast on a rifle. Sure it looks cool but better left for a 22LR or 9mm in my opinion.

Thread on silencers tend to come loose on a semi in my experience. Several QD options fit on mounts with multiple thread options, washing the expense of the mount with not having to get your barrel rethreaded.

12

Your’e doing it wrong… seriously, I don’t want to sound like a jackass, but when form is placed over function - more often than not the later suffers.

That said, I would think that 11.5" is going to be a little short… I am sure you will have problems unless you open the gas port and probably leave the suppressor on.

13

Yes an 11.5" MID gas is NOT going to work, way too little duration.

11.5" CAR gas should work great.

14

I just realized he wants to cut a middy barrel. Ooops. Bad idea.

15

I’m pretty sure I’m tracking with you, the motivation here is to get a bit smaller hole and make the rifle smoother, correct?

Am I also correct in thinking that this will only work suppressed?

16

The OP hasn’t specified where exactly he wants to chop it. When we get into low-dwell setups, even a half an inch can make a pretty big difference. The 11.5 is one inch longer than the 10.5, which has a good 3" of dwell, and is considered to make a pretty big difference in overall odds of reliability. Sounds like the OP is talking about chopping down to less than 3" of the port on a mid, which would be a pretty substantial decrease in dwell ratio than even the 10.5.

It would be experimental… maybe it would work, maybe it would just be a pain in the ass. You could probably tune it to work dedicated suppressed, but unsuppressed it would operate totally different.

If you want to try it just to try it, go for it. If you’re wanting to try this for the purpose of saving money and effort, do yourself a favor and sell the mid-length barrel and buy your desired length in a known-good gas configuration. If you must have a screw-on suppressor (which isn’t really done in this platform), just use the adaptor sleeve.

17

It’s not exactly form over function. The function is to reduce the overall length for maneuverability and so it fits in my carry bag without removing the suppressor. Specifically moving the cut to inside the rails is indeed form over function, but it’s a difference of .5-1.5 inches. Figured while I’m shortening it for function, I may as well make it look good, too.

A silencer inside the handguard gets hot real fast on a rifle.

I’d thought of that, but was thinking a mere inch inside the rails might not be too much of an issue. Might reconsider to within a half inch or less instead. I won’t really be doing mag dumps, but I know it heats quickly regardless.

The OP hasn’t specified where exactly he wants to chop it.

I’m wanting to cut it to somewhere between just in front of the gas block and an inch inside the rails

Without getting exact measurements, I estimated this would make the barrel between 12 and 13". The gas block is about 2" inside the rails, putting it roughly at around 11.5" (so an 11.5" length was out anyway).

I just realized he wants to cut a middy barrel. Ooops. Bad idea.

You and others have offered similar synopsis. However, no offense, but a “that ain’t gonna work” with no explanation of why or offer to a solution is, personally, counterproductive and lazy. It doesn’t attempt to answer the questions I posed and is simply negative.

Is dwell time the only real issue with cutting the barrel?
Is dwell time simply a function of back pressure?
Wouldn’t a suppressor increase the back pressure, and by definition, the dwell time?
Would a roughly 6" .30 suppressor increase the dwell time sufficiently to compensate for a roughly 4" barrel reduction?

If yes, what issues would remain with a dedicated suppressed system?

If no, would this be something that could be in range to be tweaked with a buffer (perhaps the A5 system)?

Also, while it is initially an endeavor to get a shorter barrel while saving money, as stated, the initial places I’ve looked for barrels (Daniel and Noveske) are out of stock. If stock continues to be unavailable by the time my paperwork goes through, it will be more than financial as there won’t be many other options (cut it or deal with it long and cumbersome).

18

Ignore the advice of those wiser than you at your peril.

That said, i wish you luck. Should be a learning experience we can all share in so please update us on your progress.

19

Before calling a mod lazy, you might want to read back up a few posts.
Clint answered the why and he was simply agreeing.
I dont want to sound like a jerk here but you dont appear to have a clear understanding of the system at all. I would just get a factory sbr barrel since you considered doing so at some point anyway.
Rainier has a Noveske 10.5 barrel in stock:
https://www.rainierarms.com/?page=shop/detail&product_id=3964
You can just shave down the FSB.
As well as Centurion’s 12.5:
https://www.rainierarms.com/?page=shop/detail&product_id=3430
BCM has all theirs in stock too:
http://www.bravocompanyusa.com/11-M4-Barrel-s/41.htm

20

Ok, I’m pretty sure I’ve seen a setup like this. I think you can get it to function with, and ONLY with a suppressor. I don’t know if there would be a difference between .30 cal and .223, but I’ve seen it with .223.

It doesn’t make any real sense to do this for any reason other than to just do it. As other posters have mentioned you can buy a quality barrel with the correct length to gas port ratio, and get yourself a rifle that will be reliable suppressed or not.