I’m fixing to trim some fat from my DPMS 20" heavy profile rifle barrel…I’m going to a 16" medium profile.
The feller who’s doing the lathe work said (since I’m doing an adjustable gb) he’d open the gp to 1/8" (.125)! That seems extremely large and would lead to seepage around the gb and even lead to undo stress on the gb itself.
I’m thinking a 1/10" (.100) gp would be sufficient, but I’d like y’alls input. I’m also planning on running a suppressor and shooting all speeds of ammo (just so y’all have a full picture). Okay, go!
I’ve never seen or heard of rifle length gas on a 16" .308 setup. From what I’ve read, when people trim a 20" 5.56/223 barrel to 16", they would typically open the gas port up to something in the .125" range. The .308 loses pressure a bit faster with barrel length than a 5.56, so off the cuff your gunsmith’s recommendation doesn’t sound unreasonable. FWIW, the FN-FAL, when running a 16" barrel with the standard gas port location, uses a very large gas port - but this is apples to oranges given that it’s a piston setup with a gas regulator, vs. a DI setup with no regulator.
I’m no expert by any measure, but my own research into .308 builds tells me that the gas pressures in .308 are very different (and much lower) than 5.56. Nearly every conversation I’ve seen regarding rifle gas on 16" has resulted in a consensus of “way too undergassed.” In working my own 18" build, I asked Rainier Arms why their ultramatch series were mid length rather than rifle, and they said it had to do with gas pressure.
In my case, I’m going to use the mid length and an adjustable gas block to tune it where I need it. That seems easier than putting an adjustable gas block on an undergassed rifle and just leaving it open all the time.
Honestly, you may be better off just getting a new barrel in the length you want. You could probably sell your current barrel on the market and use the money towards something else.
That would be my preference as well, but no one makes and sells a rifle gas 16" barrel (that I can find) besides Fulton Armory…and theirs is a lightweight design.
I fell in love with rifle gassed carbines when I worked for CMMG. Their CBR was like shooting a Cadillac! Very little recoil when compared to the carbine gas carbines.
You all make good points regarding the larger .125 size. The smith said he does .125 gp’s on adj gb’s because the gas tubes have an ID of .125, so open it to full gas then tune it with the gb. I just didn’t know if that’d cause undo stress on the gb…I’m using a Syrac LoPro if that design makes any bit of difference.
I am considering cutting down a MEGA 20" to 16". From what I have read, the port size sounds correct. I too have a adjustable gas block on a 16" Rainier Arms Ultra Match. I will be tagging this for future reference.
Does anyone happen to have a picture of a 20" rifle gas length system cut down to 16" with a 14" FF rail? I really would like to know just how far out the gas block and gas tube will hang out before I cut mine. I don’t want to change rails, I finally got the one I wanted and they are hard to find for a LR .308. I am running a Troy TRX EXTREME rail 13.8 Thanks.
Thanks Soldier, to be clear, is the Gas Block in the RIFLE, MID or CARBINE position? If it is in the RIFLE config, then it would not hang out at all. Thanks again.
I sold an Eagle Arms 10T upper I had to someone who then cut the barrel down to 18". Without opening up the gas port the only reliable way the rifle would cycle was to use an AR15 spring and buffer. IIRC, Armalite has a gas port opening of around .090. I would think your gunsmith might be correct as you have much less dwell time with the shorter barrel. Maybe a compromise of .110? I wouldn’t think that you’d put any more stress on the gas block regardless.
