Buffer Weight Tuning

MarkLeupold

MarkLeupold

On board the .25 train
Full Member
Minuteman
Feb 10, 2017
557
371
#1
First AR-15 build went to its first range session this weekend.

Criterion CORE 14.5" midlength
Badger TDX gas block
Brownell's M16 BCG
H2 buffer, standard buffer spring

Had consistent 4:00 ejection, about 7 feet from the ejection port. Some failure to feeds, never locked back on an empty magazine. I was shooting 77gr S&B, which seemed pretty spicy (had intermittent ejector marks).

Just want to make sure I'm thinking through this correctly: it's "undergassed" and so my next course of action would be to decrease buffer weight? Drop to an H buffer or all the way to a CAR buffer?
 
#3
H2 is pretty hefty for that setup.

Every little part of the rifle is going to add up to a different experience, like trying on shoes. The barrel gas port size, the bullet weight and powder charge, the length of the barrel. Ideally, you will have everything working with a carbine (H0) buffer. You add weights if you need to, compensating for something (like adding a suppressor).

People ask me all the time, "what buffer weight should I use," and it is just not that easy. I would start with carbine (3 steel weights), and move up as needed. Yes, ideally, you want an ejection somewhere around 4:00. Sometimes 3:00 works fine. If you are getting jams or failures to load, then you probably have too much resistance.

I would try on different buffers -- FOR THAT AMMO -- and see what fits best. Most AR guys have two or three different buffers. We also sell individual tungsten weights for not much money. I like those. You pop the pin and take the weights out and mix and match. CAR = 3x Steel Weight, an H1 is 1x TUNGSTEN and 2x STEEL, and so the progression goes until you have H3 (you guessed it, 3x TUNGSTEN).

Good luck
 
#4
Probably the opposite...over-gassed. If you're getting ejection, failure to lock back and failure to feed are usually symptoms of over-gassing. Failure to eject is usually under-gassing. Also, don't discount bad/un-friendly magazines...always have two or three different mags. I have surplus GI Colt metal magazines that I always test with...if it won't feed with these, it won't feed with anything.
 
#5
Do you have one of either of the options? If you have one try whichever you have. I bet it runs an h buffer personally

When I tune an agb if I turn Gas down it will do just that, first it stops locking back every time, then it stops locking back at all, then it starts failing feeding the next round, then it starts failing to eject.

Trying other mags is also a good idea imo
 
#6

Okay, sounds like I'll be trying a CAR buffer next. Probably going to look into an adjustable or just changing the weights in my current buffer


No, I don't have any other buffers right now. It might run the S&B with an H, but I'd prefer it a little overgassed with it since it seems on the hot side. I'd like to be able to feed it a wide variety of ammo with minimal issue if at all possible.

I tried both a Magpul 20 and 30 round PMag, no dice for either. I'll pick up some aluminum mags next time I make an order up just so I have a little more variety
 
#7
I have a midlength BA 14.5" barrel that runs an H2 just fine with PMC which is usually the 'weakest' ammo I find/run. I know the gas port size will vary, but that seems like a lot of failures.

I would remove the gas block and verify it was reasonably centered. There should be a light soot ring around the barrel's gas port.

Do you have another BCG to try? I'd try a known BCG, and if no change than go down in buffer weight. I had a BCG that would do what you describe. I wasn't able to gage it and find the problem, but other BCGs ran fine. The vendor warrantied it and it's run fine since.
 
#8

I believe there should be two weights in your buffer, both tungsten. You can get to them by punching out the roll pin. You can always try it with just one weight and see if it will lock back, one round per mag, and if it ejects earlier on the clock.

EDIT: DUH, see post below. That's what I get for trying to do the math in my head to get to 3 oz., lol
 
Last edited:
#9

Carbine = 3 weights
A5 = 4 weights
Rifle = 5 weights
H# = count of tungsten weight(s)

If you want to go even lighter, there are aluminum weights out there. Or just run it empty. Or run a lightweight carrier.

Running with fewer total weights will cause dynamic oddities as the remaining weights move around a bunch.
 
#10
I had seen plenty of people doing H2 stuff on midlength gas recently, so I figured it would be fine. Should've pin gauged my gas port just for fun before I installed

I'm reasonably certain the gas block is centered on the port and the fit between the gas block and journal was really good. There may be a small indication of gas loss where the gas tube enters the block, but it probably needs more rounds before that makes itself apparent. Currently seems pretty well sealed and gas efficient.

While I am reasonably certain the gas block is aligned correctly, the real reason I don't want to check unless I have to is the BCM handguard is gonna be a bitch and a half to try to remove. It wasn't cake going on and it'll be even less so the other way.

No alternate BCG at this time, though that's another *slight* issue I saw this first range trip: the buffer impacts the buffer retaining pin instead of being stopped by the carrier. Either my lower is out of spec at the retaining pin hole, or the carrier is slightly short. Just more learning happening for me on my first build.

Yeah, I'll probably order two extra steel weights, swap those in and see what happens. Minimal pain in my neck and a relatively cheap test

Yeah, I'll get weights to put back in it. I'm sure the recoil impulse would be the oddest thing with extra sliding room in the buffer, lol.
 
#11
While I'm mentioning this, I might as well since people will probably ask, here's the important stuff:

Upper: DSA forged mil-spec
Lower: 17 Design and Manufacturing forged
BCG: Brownell's M16 mil spec
Buffer: Aero carbine 6-pos buffer tube, carbine spring, H2 buffer
Barrel: Criterion 14.5" CORE
Gas Block: Badger TDX
Handguard: BCM MCMR-13

 
#12
I would try a standard CAR or H1 based on your post. I think it's really hard to guess what you will need without knowing exactly how you're barrel was made. I've got a Criterion DCM barrel with rifle length has that requires a Tubb spring and H2 to behave; Criterion recommended a standard spring and H3. I've got a mid length 14.5" URGI that requires a Wolff extra power spring and H2 unsuppressed and it's still overgassed with a can. I wouldn't have expected to need the solution I did with either.
 
#18
Yep, went ahead and ordered a CAR this afternoon. We'll try that and if I need an H1, I'll just swap one of the tungsten from my H2 over to it.

Only 50 rounds right now.

Thanks for the offer, but I just went ahead and bought a CAR, this way I'll have options if I need to "build" an H1 for it
 
#22
Little update: dropped an H0/CAR buffer in the rifle, shot it today. Smooth recoil impulse, locked back on empty mag, no failures to feed, 4:00 ejection.

Seems just about perfect
 
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