Need some advice on gas tuning

[mccannicalbob]

I just put together a 20” .308 with rifle length gas. I have an adjustable gas block and a JP SCS. I took it out the other day for functional testing and sight in. I started with single rounds from a magazine with the gas barely open. I kept opening gas one click at a time until the bolt would lock back. It was ejecting around 3 o’clock but only about 4’. Then I loaded up the magazine. At first it wouldn’t strip another round off the mag after firing the first round, so I opened the gas some more. Then it operated fine except wouldn’t hold open on the last shot. I added more gas until it would lock back on the last round. All seems good, really accurate, but it ejects forward at about 1:30. Brass looks about the same as from my JP LRP, kind of flat primers and just a hint of an ejector mark. Should I be concerned about the ejection pattern? My JP flings brass back 15‘ - 20’ at about 4 o’clock.
If this is a problem, what can I do? Less gas it won’t hold open, more makes it worse. Heavier buffer weights or spring?

 

[bfoosh006]

Allow some more break-in... make sure it is good and wet with lube.

Double / Triple check the gas block / gas tube is aligned properly and securely attached.

What magazine ? Lancers have VERY stiff springs.

What ammo ?

 

[mccannicalbob]

Thanks for the reply. I’m running it pretty wet. I just checked gas block and gas tube alignment and it looks good. Magpul 10 round mags.
As far as break in, I’ve had this bolt/barrel/receiver together before with the same issue (if it even is an issue). It should be fairly well broken in. Maybe I’m overthinking it and ejecting at 1 o’clock isn’t a big deal.

 

[loveha]

That ejection map is useless IMO. As long as the gun functions, that is all that matters.

 

[alamo5000]

Take the charging handle and bolt carrier out of the upper. Look inside the upper and you might see some drag marks especially from the cam pin sliding against the inside of the upper. There might be other rub marks as well. The barrel might not be causing drag but rather the upper itself. It's not a concern if it's happening. Just apply lube and shoot it.

Also if you didn't put anything on the buffer spring when you installed it try putting some lubricant there as well.

Basically other than that just slowly hand cycle and see if you can feel anything dragging. Look at one thing at a time so you can isolate the problem.

 

[iktomi]

What brand adjustable gas block?

 

[Ravenworks]

Thanks for the reply. I’m running it pretty wet. I just checked gas block and gas tube alignment and it looks good. Magpul 10 round mags.
As far as break in, I’ve had this bolt/barrel/receiver together before with the same issue (if it even is an issue). It should be fairly well broken in. Maybe I’m overthinking it and ejecting at 1 o’clock isn’t a big deal.

It's going to take quite a few rounds before the BCG functions as it should.
Cleaning it during this process is IMPORTANT paying particular attention to the bolt, extractor and ejector.
I've built many large frame AR'S and I don't even think about tuning the gas block until I have 50 or better round through it.

 

[mccannicalbob]

What brand adjustable gas block?

It’s an SLR adjustable gas block.

 

[mccannicalbob]

It's going to take quite a few rounds before the BCG functions as it should.
Cleaning it during this process is IMPORTANT paying particular attention to the bolt, extractor and ejector.
I've built many large frame AR'S and I don't even think about tuning the gas block until I have 50 or better round through it.

While I just put this back together again, the bolt/carrier/barrel/receiver have been together before, with a few hundred rounds through the assembly.
It has always operated this way; it’s accuracy wasn’t what I was hoping for so I converted it to 6.5 CM. Accuracy problem was likely me. Anyway, since now I have more .308 ammo than I do 6.5, I put the 308 barrel back on. It seems my skills may have improved, but it still sends brass forward.

 

[Maxduty]

I'd try the spring and buffer that came with it and see what you end up with. Watch the lower for impact marks near the tube.

 

[mccannicalbob]

That ejection map is useless IMO. As long as the gun functions, that is all that matters.

I’ve wondered about this. I guess, if it works I shouldn’t mess with it.

 

[Runnineh]

Hand loads or factory ammo?

 

[mccannicalbob]

I'd try the spring and buffer that came with it and see what you end up with. Watch the lower for impact marks near the tube.

I built the whole thing from parts, it didn’t really “come with” anything. It has the “standard” JP SCS for 308. I’d think if that’s too light, turning down the gas should help. But one click less gas and the bolt won’t lock back after the last round.

 

[mccannicalbob]

Hand loads or factory ammo?

Factory FGMM 168 grain.

 

[iktomi]

I built the whole thing from parts, it didn’t really “come with” anything. It has the “standard” JP SCS for 308. I’d think if that’s too light, turning down the gas should help. But one click less gas and the bolt won’t lock back after the last round.

Reducing gas drops overall cycle energy.

This is why you're not getting lock-back on last round.

If you increase the mass, you change the overall system balance.

There is more inertia in the reciprocating system.

Usually, it will require more gas to unlock and eject, the extraction will be slightly delayed, and the cyclic rate will be slower.

Optimal tuning for large frames can be a bit more involved than tuning 223/556 builds.

The mass of the reciprocating system plays a big role the timing of the unlock.

In my experience, heavier reciprocating systems have larger tuning 'sweet spots' for felt recoil and reliable performance.

If you have access to (a) tungsten ring(s) for your SCS, you might find it calms your ejection (easier on brass), and reduces recoil impulse.

 

[Maxduty]

I built the whole thing from parts, it didn’t really “come with” anything. It has the “standard” JP SCS for 308. I’d think if that’s too light, turning down the gas should help. But one click less gas and the bolt won’t lock back after the last round.

I'd add more weight or more spring at that point.

 

[mccannicalbob]

I'd add more weight or more spring at that point.

That's my plan, though I won't get to the range for a couple weeks. I'll report back.

 

[Evintos]

Check to see if there's any burs on the barrel extension feed ramp corners and locking lugs. Make sure your carrier gas key is aligned properly with the gas tube and that the bolt carrier (without bolt) slides freely inside the receiver. I see you're reusing the bolt that you were using with the 6.5 barrel for your 308. Your bolt may have worn in with the barrel extension of the 6.5 barrel. Switching barrels like that without switching bolts your screw with your rifle's precision capabilities (in my experience). The parts mate together after a certain number of rounds.

Every time you remove the gas block, the first 50-100rds or so you're gonna have to adjust the gas block fairly regularly if you want to maintain the 3:30-4 o'clock ejection as your gas block starts fouling with carbon to seal up any gaps. You'll be able to see residue around your gas block. Depending on tolerances, this could take more rounds and you'll initially go from lock back under gassed to over gassed and finally properly adjusted.

If you find that the clicks on the gas block are too gross and not fine enough (1 click brings ejection too far in one direction or another) after fouling up the gas block, your options at that point are either switching to a different gas block with finer adjustments or play with the buffer mass, or get the JP spring kit and use a slightly stronger spring.

 

[MarshallDodge]

That ejection map is useless IMO. As long as the gun functions, that is all that matters.

Agreed.

 

[mccannicalbob]

Check to see if there's any burs on the barrel extension feed ramp corners and locking lugs. Make sure your carrier gas key is aligned properly with the gas tube and that the bolt carrier (without bolt) slides freely inside the receiver. I see you're reusing the bolt that you were using with the 6.5 barrel for your 308. Your bolt may have worn in with the barrel extension of the 6.5 barrel. Switching barrels like that without switching bolts your screw with your rifle's precision capabilities (in my experience). The parts mate together after a certain number of rounds.

Every time you remove the gas block, the first 50-100rds or so you're gonna have to adjust the gas block fairly regularly if you want to maintain the 3:30-4 o'clock ejection as your gas block starts fouling with carbon to seal up any gaps. You'll be able to see residue around your gas block. Depending on tolerances, this could take more rounds and you'll initially go from lock back under gassed to over gassed and finally properly adjusted.

If you find that the clicks on the gas block are too gross and not fine enough (1 click brings ejection too far in one direction or another) after fouling up the gas block, your options at that point are either switching to a different gas block with finer adjustments or play with the buffer mass, or get the JP spring kit and use a slightly stronger spring.

Thanks. There hasn't been any intermingling of bolts and barrels. This bolt has only been used with the 308 barrel. When I had the 6.5 barrel on, I used a different bolt.
Good to know about the gas block mating surfaces sealing up.
I plan to try a stiffer spring and/or heavier buffer weights. Springs are cheaper. I'll probably do that first.

 

[Cynical]

The ejection pattern (clock time) of a 5.56 doesn’t compare apples to apples to a 308. Function is the key not location/time clock ejection pattern.

 

[Yondering]

The ejection pattern (clock time) of a 5.56 doesn’t compare apples to apples to a 308. Function is the key not location/time clock ejection pattern.

This (and acknowledging it was already said previously as well). Different cartridges eject to different locations. 300 Blk for example follows a completely different ejection pattern than 5.56, even with the same carrier and upper receiver; the case shape and weight play a big part in ejection pattern, and location of the shell deflector does as well.

Tune it for function and forget about where it ejects.