Adjustable Gas Block and Buffer Weight

[jrhtx]

I have a SLR7 on my 6.8 SPCII build. Been working on this for a for several weeks and not making progress.
Now I see my adjustment screw will not completely close so I need to take apart and clean.

Is there a rule of thumb on AGB vs buffer weight vs working up load data? I am using a standard buffer, standard spring right now. I think I have all the weights in my parts, so swapping isn't an issue.

Trying to conserve components to get some hog loads worked up and save my factory ammo.

Added: I am trying to push closer to SPCII specs. 120 gr SST is listed at 27.2 gr A2200 max (SPC spec, NOT SPCII). I can't get over 26.6 on 1x fired without destroying brass. New S&B primed brass can't get to 26.6.

Thanks

 

[CaptArab]

My superlative adjustable gas blocks have a bleed off mode and a restriction mode. The bleed off mode doesn't bleed off enough gas to prevent overgassing with a light buffer.

If I use the JP H2 or H3 buffer (tungsten inserts) I can get a really nice ejection pattern using purely bleed off mode. This produces way cleaner brass and much less gas to the face in my experience.

I'm sure any old h2 buffer would do, just describing my setup.

I'm not sure how working up load data factors into all this. Generally my load data doesn't account for the buffer/gas system.

 

[bfoosh006]

No rule of thumb that I am aware of... gas port sizes vary to much, barrel lengths.. etc.

I'd give the H2 version a try... after you get your SLR to cooperate.

What kind of progress is lacking ?

And... make double, triple sure the Adj. GB is centered over the gas port.... if your barrel has a shoulder by the gas port, and you are running a free float tube... typically the GB has to be off that barrel shoulder. About the width of a USGI handguard front retainer bracket.

 

[shax2lex]

I think the key is get an adjustable gas block. Then try upping your buffer weight.

 

[jrhtx]

No rule of thumb that I am aware of... gas port sizes vary to much, barrel lengths.. etc.

I'd give the H2 version a try... after you get your SLR to cooperate.

What kind of progress is lacking ?

And... make double, triple sure the Adj. GB is centered over the gas port.... if your barrel has a shoulder by the gas port, and you are running a free float tube... typically the GB has to be off that barrel shoulder. About the width of a USGI handguard front retainer bracket.

By progress I mean I keep dropping powder and still see signs of pressure. Heavy ejector and extractor marks. After I can clean the block, I need to try another ladder. My last velocity at 26.6 gr is 50FPS blow factory Hornady loads with brass damage.

The block is not on the shoulder. I haven't measured it but wild eyeball guess is about 0.040" off the shoulder.

Thought this posted last night and I guess I didn’t hit post.

 

[Yondering]

Your "pressure signs" may likely be signs of excess headspace rather than pressure. Do you know how to verify shoulder bump and are you doing it right? If you aren't, that can definitely be a source of ejector and extractor marks.
Also related to this, and something I've experienced with the 6.8, is the tendency of certain brass and barrel combinations to set back the shoulder on chambering, causing the same effect as bumping the shoulder back too far during sizing. Pull a few chambered rounds without firing and compare to sized dimension, if you've set the shoulder properly to start with.

For the most part, if your gas block can be infinitely adjusted, you don't need to adjust buffer weight as well. In my experience with a lot of handloads and 12-13 different AR cartridges, a carbine buffer is fine most of the time, IF gas is adjusted correctly. A lot of people disagree and really like installing heavy buffers, but I've rarely found them necessary or helpful, with a few exceptions.

Also - the comment about gas blocks needing to be off the barrel shoulder is not universally true; in fact not even most of the time. It only really applies if you've got a barrel that was made for clamshell handguards, and the gas block was as well. In that case there should be a gap if the front retainer bracket isn't there. For most aftermarket AR barrels though and typical low profile gas blocks, right up against the shoulder is the correct location for alignment.

 

[jrhtx]

I am bumping shoulders .004" measured from fired cases. Measurements with Hornady gages. Been doing that a long time.
I have not measured a chambered case but can tomorrow.
Are you talking using new or sized?
New brass is .008-.010 (not exact, not looking at my notes) less than fired brass.
memory
new - x.x46 - Hornady and S&B new brass are very close
fired - x.x55
bump - x.x51

The barrel is ARP.
I have checked the headspace twice since building with PTG 6.8 gages.

 

[Yondering]

Sounds like you’re doing it right then. I’d be interested to hear how much your chambered cases change.

What you’re describing is exactly the same problem I had, also with an ARP barrel. Never did get that one right; ended up building my own barrel in essentially the same chambering and the problem went away, using all the same loading techniques and components. I never did figure out exactly why. Good luck.