hey guys..

ive been trolling for some yrs now, from home in PA, and now in Europe.. and wanted to say a big thanks for all the info that has lead me in the right direction all these yrs...  

but now i need to reach out for specific help with my setup..

ive tried several different weights and springs to accomplish a reliable SBR F/A platform using my personal NFA lower that is now using a law tactical folder and the upper off my issued S/A rifle 11.5".. i say now using cause it has proved more reliable with it then without it.. the added bolt weight must have helped..

both rifles are 100% reliable on its own upper, mine with the 14.5" and the S/A 11.5.. but when switched, the SBR on my lower is tempermental.

the upper is agency issue and swapped every few rotations, so i cant really change the gas block without raising flags why i want to change it and have a duty weapon signed out to me..

BUT... the problem ive been having are all too common, Ive searched, short stroking with occassional FTF and stove pipes,  OVER stroking and the bolt slamming the receiver extension, and more so contradicting effects from each setup when using the agency issue birdcage quick mount CQB50..

but 99% my problems arise under full auto fire with an occassional on semi auto, cold or hot barrel doesnt matter, 109, 855, 193, nato issue LC, hornady hunting, 55-62-70-77 grain ammo doesnt matter, with or without suppressor doesnt matter..

ive been battling this issue for 3 yrs now and i am now looking into the captured spring systems but they arent cheap to be testing diff mfgs to no avail..

so im asking if anyone has any insight on this..

lower specs: Colt with hiperfire EDC F/A, very reliable..
upper specs: Unknown mfg, M4 extension, CHF, Nato 1/7, nitrate 11.3" bolt face to crown..
NEW gen 3 pmags
ammo as listed above..

and a box of springs, buffers, custom lead weights, 2 new BCGs with no luck thus far..

Welcome, I can’t link from what I am using at the moment, but there is another subsection under armory that had specific threads for machine guns.   There might be more experience in that sub forum.  Go back to the discussion drop down, and instead of going to ar15, go to armory.

Edit: Here you go.
https://www.ar15.com/forums/Armory/Machineguns-MG-/22/
https://www.ar15.com/forums/Armory/Short-Barreled-Rifles-SBR-/51/

A spare full auto bolt carrier group with an adjustable gas key (Rubber City Armory) will let you throttle gas at the rear end of the gas tube.  You will want to use that on a spare BCG because the adjustable gas key is less convenient to adjust on the fly versus an adjustable gas block.

The other adjustable BCG is by Bootleg and that reduces bolt velocity by venting with an accessible 4-position screw. Left-hand shooters may find it spits gas in their face.  It’s more expensive, has limited range of adjustment, and only 4 settings, but is a turnkey solution.

If you have machine shop access you could make an adjustable gas key. It’s just a drilled and tapped hole and tandem set screws.

BRT sells throttled gas tubes as well.  More hit & miss and only one setting.

Don’t assume it is over gassed or under gassed. Each barrel  has its own gas port size. I had a 10.5 that was crazy under gassed. Make sure gas block is lined up correctly and not moving. I use adjustable gas blocks on many guns and tune them. Next you can try an adjustable BCG like the rubber city armory one. I have an old Gemtech adjustable BCG and it works as advertised. Next I use a H3 buffer and it helps tremendously. There are even H4 and other heavy buffers. Again, test each configuration. If you want a rough estimate on how fast the bolt is moving, try FA fire and measure ROF with a shot timer. I have had my M16 from 1400 rpm to 700 rpm. It seems happy at 750 rpm. That’s with a 14.5 barrel. I even have 750 on a setup with my 10.5 and SureFire Suppressor. It’s all about testing each configuration. Good luck!