Dedicated Suppressed SBRs – Some questions

General Firearms NFA
1

I’m in the process of assembling two SBR uppers/complete guns that will be dedicated suppressor hosts. I believe I’ve largely figured out the component and set up details but wanted to float them here. I’m looking for a sanity check to ensure my thinking is sound and for answers to a couple of outstanding questions.

Some details. Both uppers will be assembled with high quality parts (mil-spec uppers, BCM BCGs, URX rails, and probably BCM barrels, etc). FWIW, the lowers have DD LPKs, mil-spec RE, standard springs, and H buffers (for now). Both barrels will be 11.5” long and wear muzzle brakes. One upper will host a SF SOCOM RC (still pending) and the other will host a Gemtech G5 (had for a few years).

My plan is to run these uppers/guns suppressed 90-100% of the time. If/when unsuppressed, they’ll get fed quality M193 ammo. I may use some weaker stuff (i.e. PMC Bronze) with the cans on. I’m a civilian so these are not work guns. They will get used for carbine classes/training and one of them may eventually turn into an HD gun.

Bottom line, I would prefer that these function unsuppressed (I’d drop down to a carbine buffer if needed) but it is more important that they run without issue and without unnecessary wear (not overgassed) when suppressed.

I’ve read a number of M4C threads about gas port sizing and adjustable gas blocks, including the one about Grant’s dedicated suppressed gun. I’ve come to the preliminary conclusion that I’d prefer a small gas port barrel vs. the other options. I’m currently thinking about grabbing a couple of carbine gassed 16” BCM barrels and having them cut to 11.5”. Assuming the barrels are in spec, that should leave me with gas ports that are approx 0.063”.

So my questions are:

  1. Is cutting 16” carbine barrels down to 11.5” the most efficient way to get the right gas port size for this application, and is 0.063” about the right size?

  2. Does anyone with a set up like this regret not getting an adjustable gas block instead?

  3. My limited understanding of suppressors and back pressure suggests that the SOCOM cans generate less back pressure than most (the Gemtech G5 in my case) cans. Should this be a consideration for me (i.e. should the SOCOM host get a relatively smaller gas port)?

Thank you.

2

After doing all of the same research as you, I eventually decided to go with the insertable gas port from Black River, on a standard sbr barrel. These allow you to fine tune your gas system, but are a simpler, and cheaper, option that the adjustable gas blocks. I’m waiting on my stamp, so I don’t know how well my personal setup works, but there’s a thread around here somewhere with some good results.

3

Yes,

No,

Probably not enough to worry about. There is no practical way to get a smaller port and you can just run an H3 buffer or A5 setup if you want to slow it down.

4

I appreciate the responses so far.

Revrogue - I may give the BRT inserts a try if my plan doesn’t work well or possibly to salvage another one of my barrels.

Todd - Thanks for chiming in. Really appreciate the input from an experienced industry guy.

MicroMOA - I may come around to the idea of using an adjustable gas block at some point. I’ll probably reconsider once I see people get a lot of rounds through some of the existing offerings.

5

Your getting an 11.5 which is right IMHO. Just buy a BCM, LMT, Noveske 11.5 and run a Vltor A5.

Don’t buy a 16 and cut.

I have several of the above with zero problems. I have seen no need for a adjustable gas port. They work for people though.

Good luck.

6

This is my “dedicated suppressed” SBR.

DD 16" carbine length barrel cut to 10.3". Gas port remained the same. I’ve heard its anywhere from 0.058-0.063"

It also has a VLTOR A5.

My thoughts:

It definitely has less felt recoil, less “gas face” and less violent extraction than my 12.5" BCM w/ standard gas port. It feels extremely smooth. Function is 100% over several years and 10,000 rounds.

With that said, if I were to make another 10.3"-11.5" SBR I would buy a factory DD/LMT/BCM barrel and call it a day or grab a switchblock. The difference is just not great enough to justify having a single shot rifle with the suppressor removed.

To answer your questions directly:

  1. Yes. Absolutely.

  2. I don’t regret it, but I wouldn’t own more than 1.

7

Thanks. Perhaps I’ll assemble just one and then reassess after getting some rounds downrange.

8

I would cut down the barrel.

If you run the right parts, (A5 system with standard 5.1 oz buffer, and LMT Enhanced carrier) you can get the gun to lock back with the can off.

9

Maybe I need the LMT carrier. I remember reading grant talking about it a few years ago but I didn’t want to spend the $200 if it didn’t work.

I’m very close. My gun will eject spent rounds, it just needs a few more millimeters to pick up the next round. I’m not too concerned with the bolt locking back although that would be nice.

10

Here is Grant’s epic thread
https://www.m4carbine.net/showthread.php?t=33743

11

I read Grant’s thread and have a question about the LMT enhanced carrier. The thread does not specify whether it is the semi or full auto carrier? Which one do I need? Is there a difference?

Please educate me. Thanks.

Cole

12

Sounds like a lot of work to stay away from an adjustable gas block.

Why not go with a Noveske Switchblock?

13

Fair point.

I decided against the Switchblock because of the lack of rail choices. A 7" rail doesn’t accommodate my grip or accessories and the SWS rails are too heavy for me. If Noveske offered NSR-railed Switchblock uppers I probably would have gone that route.

Re: adjustable gas blocks, I would say that gas port erosion on mil-spec SBRs is a known quantity. However, there isn’t enough data (or I haven’t yet found it) on how some of current adj. gas block offerings hold up on suppressed SBRs over 5k-10k rounds.

14

So I finally got one of these uppers assembled and headed off to the range to do some initial testing.

Quick list of the critical parts:

Barrel: Standard 16” carbine gas BCM with a gas port of .063” cut down by ADCO to 11.5”
BCG: An older Denny’s Super Duty FA (basically a non-HPT Stag with some minor upgrades)
Receiver Extension: Standard mil-spec RE with standard carbine spring and carbine buffer.

Here is a quick pic of the set up:

I hadn’t reread Grant’s thread in a while but tried to approach the testing in a similar way. Definitely have more work to do on this front.

I began with some of my hotter ammo and loaded a known quantity PMag 1 round at a time. 5 shots of Prvi M193, all successfully locked back on the empty mag. 5 shots of Prvi M855, same result. Then I took 5 rounds of PMC Bronze 55gr FMJ. None of the 5 attempts locked back on an empty mag. To satisfy a curiosity, I took another 5 rounds of PMC and repeated this with a USGI mag (Magpul follower). All five attempts DID lock back on the empty mag.

I then loaded up the PMag with 10 rounds of PMC Bronze and installed the suppressor (Gemtech G5 until my SF SOCOM RC clears). All 10 rounds cycled/fed without issue but ejection was markedly forward of 3 o’clock. I realize this is an imperfect way to measure proper gassing but kept track of it as A data point.

I kept the suppressor on, switched to an H buffer and ran another 10 rounds through without issue. Ejection was closer to 2 o’clock this time.

Lastly, I ran another 10 rounds of PMC with the can on and an H2 buffer this time. Same result, slightly “better” ejection but still a bit forward of 3 o’clock.

Importantly, the “gas in the face” aspect felt much like a non-suppressed set up. Next time out I have a few more things to test, including running suppressed with an H3 and more testing without the can (running more than 1 shot at a time to test for proper cycling, etc).

I appreciated all the help in this thread and thought it would be useful to post my results. Also wanted to thank Grant for the initial idea.

15

FWIW:

I’m now running two LMT MRP chassis with their original 14.5" barrels cut down to 11.5" and un-modified gas ports. I’m also using the Vltor A5, and the LMT enhanced carrier.

With my limited testing so far (about 150 rounds), this setup has functioned 100% with both 62 grain Wolf, and 62 grain Lake City M855…with, and without a suppressor attached.

I used to have a Noveske switch-block upper…and it does work as advertised. I decided to move away from it mostly because I like the MRP’s so much, and because I prefer simple gas systems with no moving parts to break, get dirty, or stuck.

I have no regrets thus far.

16

I have an 11.5 inch upper that is sometimes suppressed. Not sure exactly what size the gas port is, Adco sized it when they cut the barrel. I did not tell them it would be run suppressed.

I put a WAR adjustable upper on it. It’s pretty cool, it does exactly what they say it does. It’s very smooth and recoils mildly when suppressed. The only ‘downside’ I can see is that it has one additional non-standard part, and the gas tube is modified from standard. If I’m not mistaken, you can make a gas tube easily from a standard gas tube.

If you’re not familiar with these, it just moves the ‘switch block’ to the upper receiver. The advantage being that it never gets too hot to touch (at least mine hasn’t).

http://www.innovativearms.com/id13.html

17

I am currently working with two WAR uppers on 16’ guns… sofar nothing negative to say. As you mentioned the gas tube is different but on the other hand it’s not taking any wear from the carrier key so should last indefinitely.

IA tells me it’s designed for standard 16"-barreled carbines but your experience seems to indicate some leeway on that, which I suspected.

18

My calculations show that the target suppressed gas port sizes arent much different for carbines with barrel lengths 10"-16".