6 ARC Upper Build: 18” PVA Osprey (Rifle +2”) + San Tan Tactical Billet Upper + SOLGW M89 Rail

[WeR0206]

Hey man. I'm really going to need to go back through this thread and get caught up on what has changed, but am getting ready to start my weekly staff meeting.

Still, a good way to check to see if more gas will help would be to throw a good old fashioned baffle stack suppressor on it and see how it runs. A baffle stack suppressor will simulate more dwell time and increase back pressure. Just make sure you use one with an appropriate bore diameter. Not all 5.56 will do 6mm. Real gun guys are pretty helpful and most would be glad to let you take their can for a spin. If youre lucky, as I am, suppressors are pretty common at the range.

Thanks Tony. No worries, take your time catching up. I love your idea for using the suppressor but unfortunately I live in California so that’s not really an option.

TLDR version is it’s an 18 inch 6 arc barrel with rifle +2 gas system (.093” port) so it has pretty short amount of dwelltime already. I’ve tried shooting with different mags different, different bcg’s, checked gas tube/gasy key for obstructions, tried set screw & clamp on gas blocks, different uppers, different lube, etc. and never resolved failure to feed or failure to lock back and at best (factory ammo with clamp on block) have gotten intermittent ejection.

If any ideas pop into your head once you’ve had time to absorb the thread feel free to share them!

I already have a 2.5 mm bit on hand that I can use to drill out the port to around .098 inches but I’m holding off until I hear back from Josh at PVA to see if he and team may have some other things to check for or ask me to send the barrel in etc..

 

[LRRPF52]

From all I know about these types of chamberings, the RLGS+2 is more something done for 24-26” 6mm AR and 6mm ARC.

For an 18”, I would be tempted to lean on standard RLGS with a Rifle-Speed AGB or Bootleg Carrier to tune it.

I know another guy with an 18” Geissele who says the moment he switches from 108gr ELD-M to 105gr, it doesn’t cycle reliably. Runs fine with 108s, but just that tiny change results in short stroking.

This is because 6mm has very limited bore volume, so any changes made have more consequences.

If it doesn’t run factory 108gr, I’m also of the camp of opening the port, especially with such a long gas system.

Your port pressure and port volume capacity to bleed gas into the BCG are on the downside of the curve, outside of the optimal gas system PSI requirements of the AR-15.

 

[WeR0206]

From all I know about these types of chamberings, the RLGS+2 is more something done for 24-26” 6mm AR and 6mm ARC.

For an 18”, I would be tempted to lean on standard RLGS with a Rifle-Speed AGB or Bootleg Carrier to tune it.

I know another guy with an 18” Geissele who says the moment he switches from 108gr ELD-M to 105gr, it doesn’t cycle reliably. Runs fine with 108s, but just that tiny change results in short stroking.

This is because 6mm has very limited bore volume, so any changes made have more consequences.

If it doesn’t run factory 108gr, I’m also of the camp of opening the port, especially with such a long gas system.

Your port pressure and port volume capacity to bleed gas into the BCG are on the downside of the curve, outside of the optimal gas system PSI requirements of the AR-15.

Thanks for the feedback, everything you said makes sense.

Do you happen to know of best practices for drilling out a gas port? I can put a cheap cleaning rod in the barrel to protect the bore from the drill bit but since I don’t have a drill press id have to just mount the upper in a vice and do it slowly by hand using the existing hole as a pilot

 

[Rob01]

Thanks for the feedback, everything you said makes sense.

Do you happen to know of best practices for drilling out a gas port? I can put a cheap cleaning rod in the barrel to protect the bore from the drill bit but since I don’t have a drill press id have to just mount the upper in a vice and do it slowly by hand using the existing hole as a pilot

It will only be a $400 mistake if you screw it up. lol

 

[LRRPF52]

I would defer to @45-90 on that. I seem to remember something he recommended about drilling ports where you limit the amount of material burrs in the bore, but I could be misremembering.

Some use a wooden dowel in the bore and a limiter set on a drill press, so you don’t over-drill.

Lots of cutting fluid to help reduce chatter. There was some kind of technique for keeping the cut clean so you don’t introduce crap into the bore from the edges of the port.

When you install your gas block and tube, they should be super-tight with no place for gas to leak.

I bed or press-fit gas blocks, and bed tubes into the back of the block to fight that leakage problem.

 

[WeR0206]

It will only be a $400 mistake if you screw it up. lol

Haha yep....I'll going to wait and see what the PVA guys say before doing anything. They may very well ask me to just send the barrel in (or complete upper) so they can take a look 1st hand and I'd be happy to oblige as I'm not in any sort of time crunch....just want the gun to cycle b/c the accuracy is lights out.

 

[LRRPF52]

Don’t be skeered!

 

[WeR0206]

Don’t be skeered!

Haah! Out of curiosity I was looking on YT to see if there were any videos on drilling out gas ports and came across the below video...your pic reminded me of his setup. I guess his method works for him but I was cringing the whole time he was wobbling the barrel around while drilling

 

[LRRPF52]

I would have used cutting fluid.

 

[Rob01]

Note to self. Don’t buy AR from Dixie Gunworks lol

 

[LRRPF52]

Note to self. Don’t buy AR from Dixie Gunworks lol

C’mom man! He’s already got a pilot hole and a floating head drill.

The reason for opening up the port is to run Wolf anyway since it’s a 16” MLGS, so no harm, no foul. He could blow through the other side and as long as he doesn’t go all the way through the barrel wall, Cleatus won’t notice.

 

[WeR0206]

C’mom man! He’s already got a pilot hole and a floating head drill.

The reason for opening up the port is to run Wolf anyway since it’s a 16” MLGS, so no harm, no foul. He could blow through the other side and as long as he doesn’t go all the way through the barrel wall, Cleatus won’t notice.

Cleatus is loving it

 

[357Max]

@WeR0206 - Before you drill anything please post a picture showing where the gas tube is sitting in your upper.

We need to make sure you have the correct +2" gas tube for Josh's barrel.

Here's why. You bought the tube from WOA correct? What dimension did @bohem use for the + 2"???? Does it match Craddock or Rainier?
Rainier gas block shoulder to receiver face is 14 - 1/4"
Craddock gas block shoulder to receiver face is 14 - 5/8"

There is more than 1 version of +2" & if that WOA tube is for the shorter + 2" & Josh's port location is for the long you'll have short stroke issues for sure.

In the below pictures your looking at Two 6 ARC barrels. Both are 22" + 2" RL. Notice anything odd.

Here are the 2 gas blocks/tubes that go with these 2 +2" barrels.

 

[WeR0206]

@WeR0206 - Before you drill anything please post a picture showing where the gas tube is sitting in your upper.

We need to make sure you have the correct +2" gas tube for Josh's barrel.

Here's why. You bought the tube from WOA correct? What dimension did @bohem use for the + 2"???? Does it match Craddock or Rainier?
Rainier gas block shoulder to receiver face is 14 - 1/4"
Craddock gas block shoulder to receiver face is 14 - 5/8"

There is more than 1 version of +2" & if that WOA tube is for the shorter + 2" & Josh's port location is for the long you'll have short stroke issues for sure.

In the below pictures your looking at Two 6 ARC barrels. Both are 22" + 2" RL. Notice anything odd.
View attachment 8752559

Here are the 2 gas blocks/tubes that go with these 2 +2" barrels.

View attachment 8752560

Great question. Yes its a WOA +2 tube. From shoulder of the barrel to receiver face im measuring 14 5/8”. Re the gas tube, From the port of the tube to the end is about 17”.

It Seems to have the same protrusion into the upper as my 556 builds with appropriate tube lengths.

Also if this helps to see how everything fits together here is a pic of how far the tube protrudes into the receiver:

 

[357Max]

Great question. Yes its a WOA +2 tube. From shoulder of the barrel to receiver face im measuring 14 5/8”. Re the gas tube, From the port of the tube to the end is about 17”.

It Seems to have the same protrusion into the upper as my 556 builds with appropriate tube lengths.

Also if this helps to see how everything fits together here is a pic of how far the tube protrudes into the receiver:
View attachment 8752571

That looks right. It's probably Rainier that has an abomination of the 2" length & that barrel shipped with the tube.

FWIW - My 6 ARC has always had a low mass carrier with std silent capture set up. I had to drill the Craddock port about .003" to get it not short stroking even with the low mass carrier.

Also Lever make a considerable amount more port pressure than any other powder I've tried. Still it needs to run with factory ammo.