Optimal gas port size, 8.5" barrel?

Moderators:gds, bakerjw, renegade

Post Reply
Ape Factory
Member
Posts:10
Joined:Wed Sep 08, 2021 8:31 pm
Optimal gas port size, 8.5" barrel?

Post by Ape Factory » Sat Oct 02, 2021 12:37 am

So I switched from a 10.5" Faxon match barrel to a Rainier Arms 8.5" match grade barrel recently. Both barrels seem to have the exact same port size, about a .109 in diameter.

My first outing, even with the gas block fully open, it would not cycle factory loaded subs.

With the 10.5" barrel, it'd cycle anything I threw at it, super or subs. Didn't matter what the bullet grain weight was, I could dial in the gas in restriction mode as well as bleed off mode and it would cycle without fault.

I use an SA adjustable gas block, a lightweight titanium BCG, an adjustable Odin Works adjustable buffer on the light side of things. I double checked gas block alignment and it seemed to be fine. The only variable is the bolt and barrel length. I switched to a JP enhanced over a Rubber City Armory enhanced bolt. I'm wondering if their gas ring design has an effect on cycling subs.

So I'm looking at either enlarging the gas port and/or switching back to the RCA bolt. Anyone have similar issues with the JP bolt? Is the gas port size appropriate or should I look at drilling it out?

BulletFlight for Android
User avatar
dellet
Silent But Deadly
Posts:6674
Joined:Fri Nov 30, 2012 1:25 pm

Re: Optimal gas port size, 8.5" barrel?

Post by dellet » Sat Oct 02, 2021 11:35 am

The short answer is the port is too big already. If increasing the size solves the issue, it’s only a bandaid.

Honestly you’ve put so much ultra light whiz bang stuff into this assembly, it wouldn’t surprise me if you were over gassed and hyper cycling with ammo designed to function in something with an H2 buffer, standard spring, and full auto BCG. In essence you may have created a problem by assembling a rifle that may not work with SAAMI spec ammo.

Swapping the bolt or carrier group is easy enough to try.
Sometimes a new assembly is too tight and cycling it a couple hundred times by hand to break it in helps.

The only way the barrel would be an issue is if the bore is to large causing a leak. Too tight would not likely cause a cycling problem without a really low velocity showing up.

Check carefully for leaks showing up as a carbon track where one does not belong.

Whhat is the ammo for reference?
300 Blackout, not just for sub-sonics.

Ape Factory
Member
Posts:10
Joined:Wed Sep 08, 2021 8:31 pm

Re: Optimal gas port size, 8.5" barrel?

Post by Ape Factory » Wed Oct 06, 2021 12:50 am

Ok I took the rifle back out after reassembly and visually checking to make sure the gas block was aligned by both measuring and with a bore scope.

Here's a photo of the barrel after removing the gas block. Seems fairly normal.
Image

The SA gas block always seems to get a bit stiff to adjust up to the point where I almost can't turn it. I have two, one is on my 5.56 upper and I don't have that issue with that particular SA block but I'm also not shooting subs out of the 5.56 upper.

So far I've run a variety of grain weights for both super and sub. My "plan" is to have two loads, 110gr. supers (Hornady VMAX) and 190gr. Hornady Sub-X subsonic rounds. LC brass, CCI 400 or 41 primers for the supers, Tula small rifle for the subs. 100% suppressed with an OSS 7.62Ti suppressor.

I also have Berry's 220gr. "plinker" rounds for fun and function tests and they're just inexpensive in comparison.

I've tried everything from factory Hornady 190gr. sub-x rounds (SD of 4 over a 10 shot spread out of the 8.5" barrel) to Hornady Black 208gr. to S&B 200 gr. rounds.

For the 8.5" barrel, the Hornady Black 220gr. did not lock the bolt back on an empty mag. I don't think it's a hyper cycling issue.

I went out again on Monday after a bad day on Sunday (long story) and was able to mostly get the bolt to lock back with Hornady sub-x factory loads with the SA gas block wide open. I had three FTF's but it would otherwise cycle subs and supers even though the bolt wouldn't always lock back on an empty mag. I closed the gas block back down to 3 turns from fully closed for supers and it would not lock the bolt back on an empty mag, had to run it wide open. It would do so with the 10.5" barrel and even with subs, I don't think I had it fully open (4 turns from fully closed) with the 10.5" barrel.

The carrier weight is about 7.9 oz and it's a standard weight enhanced bolt. One thing I didn't try was swapping back to the JP bolt over the RCA bolt, to see if that had an effect. The JP bolt will not pass the inverted test, meaning it'll slide out of the carrier if you turn it vertically. I don't know whether or not the design uses gas pressure more effectively to seal as I haven't examined the JP rings or setup under a magnifier to see if there's any wizardry there. All I know is my second time out, switching bolts, I had more success with the RCA bolt.

The buffer is an Odin adjustable so I can change the weight if necessary but I want it to function with both the 300 blackout and 5.56 upper. Both use titanium carriers and are within .1 oz of one and other.

At this point I'm thinking the 10.5" barrel, the new replacement one, from Faxon is going to be a better choice from a functional standpoint. The chamber does not appear to be as smooth, nor the feed ramps, but I can probably fix that myself or have someone experienced touch it up a bit.

Rainier 8.5"...
Image

Faxon 10.5"
Image

User avatar
dellet
Silent But Deadly
Posts:6674
Joined:Fri Nov 30, 2012 1:25 pm

Re: Optimal gas port size, 8.5" barrel?

Post by dellet » Wed Oct 06, 2021 9:22 am

I wouldn’t call that normal at all.
Looks like a big part of your problem.the discoloring dome size ring with the clean spot where the gas is blowing out at 11:00 is a pretty clear indication of poor sealing.

Then there is the comet trail at 6:00 right where the barrel steps down.

Here’s more what it should look like.
Image

Part of the problem is the satin/blasted finish of the barrel. The area of the gas block should have been taped off. Best you can do is try some lock tite or high temp silicone under the block.

A clamp style block will seal better than a set screw type so that’s a choice.

Depending on your handgaurd, you can probably feel the gas blowing out when you shoot if you hold your hand over the gas block area. The problem is that Some leak enough to cut skin. But if you glove up and hold a piece of clean paper between your hand and the gaurs, it will probably leave a mark on the paper.
300 Blackout, not just for sub-sonics.

Ape Factory
Member
Posts:10
Joined:Wed Sep 08, 2021 8:31 pm

Re: Optimal gas port size, 8.5" barrel?

Post by Ape Factory » Thu Oct 07, 2021 1:35 am

The Superlative Arms gas block can run in restrictive or bleed mode. The comet tail is from me trying to run it in bleed mode where gas is ejected out the front. I shot a few in that mode just to see if it'd work. It is a set screw type and not a clamp-on. I'll see if I can torque it down more and help solve that problem. Might be a solution! Thanks for spotting that.

In comparison to your photo, my gas "ring" is much larger. I figured it would seal with carbon over time. You could be right about the bead blasted finish. should have opted for polished. Strange that Rainier would even offer that finish if it'd cause issues. I've got some time this week so I'll micromesh that area and I may just slap the new replacement Faxon back on there. Debating whether I should have someone experienced work on the Faxon barrel extension area, polishing the feed ramps and chamber to remove all the CNC marks, smoothing everything over, before reinstalling.

User avatar
dellet
Silent But Deadly
Posts:6674
Joined:Fri Nov 30, 2012 1:25 pm

Re: Optimal gas port size, 8.5" barrel?

Post by dellet » Fri Oct 08, 2021 9:40 am

You might also measure the barrel and gas block diameters. If using a set screw type block, I want to have to basically force the block into place.

If it will slide on by its own weight, one or the other is the wrong diameter.
300 Blackout, not just for sub-sonics.

Ape Factory
Member
Posts:10
Joined:Wed Sep 08, 2021 8:31 pm

Re: Optimal gas port size, 8.5" barrel?

Post by Ape Factory » Mon Oct 11, 2021 12:02 am

The barrel is .750 and the gas block is .750 (theoretically). It'll slide on easily. It's definitely not an interference fit. I have to say I do vaguely remember my other SA gas block being slightly "tight" when fitting it on to the 5.56 barrel.

Here's a photo of the SA block installed. Notice the vent port on the front and corresponding gas "jet" mark on the barrel.

I went ahead and micro-meshed the barrel where the gas block interfaces with the gas port. There was an ever-so-slight raised portion around the gas port which I knocked down too. Results were a bit better but had a different snafu which shut my testing down early. BUT I was getting the bolt to lock back most of the time.

But great suggestion on the beaded finish. Mad props as I did not even think about that. I'll put some calipers on the barrel and gas block to make sure they're within specification.

Image
Image

Ape Factory
Member
Posts:10
Joined:Wed Sep 08, 2021 8:31 pm

Re: Optimal gas port size, 8.5" barrel?

Post by Ape Factory » Thu Oct 28, 2021 11:41 pm

The gas block journal on the barrel measured .748. After putting everything back together I went and shot a box of supers (painful due to cost!) and then moved on to shoot about 90 subs, no FTF's and it mostly locked the bolt back on an empty mag.

Had issues with a Lantac 10 round mag that would not feed the first round. The 30 round mags worked fine, both Lantac and Magpul. Shot out to 300 yards on steel and was easily able to hit man-sized targets and smaller. Quite the delay with subs out to 300 yards. No complaints with the Primary Arms 1-6 ACSS reticle, it was pretty much spot-on to 300 yards.

So I feel like I'm getting somewhere and I have a lot more confidence in the rifle. Still not 100% happy with being so over-gassed with supers as I'm forced to run the gas block wide open for subs. I also have an issue with the gas tube hitting the inside of the barrel. Smoke Composites is actually sending me another handguard but after looking at some of my photos, he feels the gas tube needs to be tweaked slightly for better fitment. I did tweak it a bit and made sure an empty carrier would slide on to the gas tube with nothing more than gravity. Took me a good hour and was super tedious. I may order a few and see if some fit better than others.

I ordered and now have a bunch plinker 125gr. FMJ's and I'm going to shoot a few mags of those before cleaning the barrel and moving towards load development. In the meantime, I'm taking the 10.5" Faxon barrel to have it looked at to see if the CNC marks on the throat can be removed and have the feed ramps polished. From the data I've seen, I'm not going to gain much in velocity though so we'll see what I end up doing.

Took the lower off and slapped on a 16" 223 Wylde upper and it functioned flawlessly.

Image
Image

Ape Factory
Member
Posts:10
Joined:Wed Sep 08, 2021 8:31 pm

Re: Optimal gas port size, 8.5" barrel?

Post by Ape Factory » Sat Oct 30, 2021 10:19 pm

Smoke composites sent me a new hand guard. I ended up disassembling the upper, short of barrel removal, and the new hand guard fit much better. Made a bunch of detail changes and I ended up tweaking the gas tube a bit more. It's no longer hitting the inside of the hand guard but it isn't free float in the upper receiver's gas tube port, it leans to the right more than the left and I can't seem to get it "just right" so it's floating without hitting one side or the other. Top to bottom it's floating. None-the-less, the carrier, a lightweight titanium carrier at that, slides right on with no resistance under its own weight with no gas tube movement so that's good.

With that experience, I disassembled my POF Revolution DI to perform the same test and it was absolutely spot-on. I think my rifle must have been built on a good day because everything on it is just spot-on.

Surprisingly, when I took the gas block off the new 8.5" barrel, no gas ring. It was actually stuck on there a bit so shooting some supers through it seemed to help seal everything up. Headed to the range tomorrow to shoot a few mags of supers and make sure the barrel is broken in and test the JP enhanced bolt back to back with the RCA bolt. Will then move back to developing a load with supers and subs. I have 8 pounds of CFE Blk and H110 so I have to make those powders work, LOL.

Debating going with a JP SCS system for the sound of silence but not sure I can make it light enough to work with the Ti BCG.

User avatar
dellet
Silent But Deadly
Posts:6674
Joined:Fri Nov 30, 2012 1:25 pm

Re: Optimal gas port size, 8.5" barrel?

Post by dellet » Sun Oct 31, 2021 10:22 pm

Good to hear it’s working out for you.

With the spring and weight kit there’s should be no problem getting the JCS set up working.

If you shoot enough of the CFE Black, it will blow enough carbon and garbage through the gas system to seal up any leaks. That’s probably it’s best quality.
300 Blackout, not just for sub-sonics.

Post Reply