Need help tuning AR15 gas system for 358 Wildcat please :)

[lar45]

Hi everybody, I'm working on a new AR15 rifle and have run into a snag with getting it to cycle and lock back.
The cartridge is a 358 Winchester short x 1.71", just short enough to fit an AR15 magazine. I'm loading Hornady 200 FTX to 2471 and Speer 180s to 2599fps with CFE-BLK.
The barrel is from DeadShot Barrels in Oregon, it has a mid length gas system.
They cut the chamber with a standard 358 Win reamer and set the headspace off of a case I sent to them with the bolt.
For loading dies, I just cut about .3" off the base of Lee dies and they seem to be working great.
For brass I'm using Winchester 308 Win once fired cases, cutting them off at the shoulder, then lube and run through the shortened sizing die, then trim to length at 1.71"

My first loads were with 1680. I started with a CAGB adjustable gas block. It came with the gas port drilled to .086" and would not lock the bolt back. I drilled the hole to .096" and now it locked back on the largest setting. I set the gas block back one notch and it wouldn't lock back, so I set it back to all the way open and thought I was good.
I tried some cast loads with the Lee 358-200 and they barely hit the target, so I gave up on that idea for now and went back to jacketed.
During load developement, I decided to switch to CFE-BLK as it's slightly slower burning and I got better velocities.
Now the gun started to not lock back. I went back to 1680 and it still wouldn't lock back on the last round.
I cleaned the inside of the upper and reoiled the BCG, but no change.
I then drilled the gas port to .116", but no change.
I oiled the recoil spring and buffer, no change.
I wondered if maybe part of a gas check got stuck in the gas block, so I replaced it with a standard gas block, no change.
I'm at a max load with CFE-BLK and I'm getting great accuracy and velocity, so I can't load any hotter.
My other thought is to open the gas port even more, but I think something else must be wrong as it used to cycle when I was first breaking it in after the gas port was at .096".
I am open to any and all suggestions.
Thanks
Glenn.
http://www.lsstuff.com/pics/guns/358razorback/358r-04.jpg

http://www.lsstuff.com/pics/guns/358razorback/358r-010.jpg

http://www.lsstuff.com/pics/guns/358razorback/358r-011.jpg

[cwtebay]

What buffer weight are you using?

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[lar45]

I'm not sure on the buffer weight. I'll go weigh it.
2.9 oz

[Barry54]

I like the looks of that cartridge!

For testing, take the weights out of your buffer and see if it locks back on an empty magazine.

I bought an ultralight buffer for a lightweight build. I’ll post a link when I find it.

https://www.tacticallink.com/Taccom-Ultra-Lightweight-Carbine-Buffer-for-AR15.html#mz-expanded-view-1022275889370

Someone smarter than me will probably know, but I suspect you need a shorter length gas system. Do you have a lightweight bolt carrier you could try as well?

[imashooter2]

Is the gas port in the block aligned with the barrel? Gas key tight?

[Fitz]

I run a 358 Yeti and run a J&P captured spring , they come with 3 different weight springs to tune with and sell different weights for the buffer, have you verified that the gas block is set with the barrel gas port some barrel makers have the block set back around .030 from the wall.

[cwtebay]

I run a 358 Yeti and run a J&P captured spring , they come with 3 different weight springs to tune with and sell different weights for the buffer, have you verified that the gas block is set with the barrel gas port some barrel makers have the block set back around .030 from the wall.This^^^^^

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[lar45]

I'll see if I can run a bore scope down and check.
My cartridge should be very close to the Yeti. I was just too late to the Yeti party. I Don't think the barrels and bolts are available anymore, unless you find one on the used market...

[lar45]

The gas key is tight.

[DDriller]

On my .30 HRT I opened my port to 0.125 (full opening of my gas block hole) to get it to work with a piston system on a 12" barrel. With the adjustable I wouldn't worry about opening it up if changing to a lighter buffer doesn't fix your problem.

[Barry54]

On my .30 HRT I opened my port to 0.125 (full opening of my gas block hole) to get it to work with a piston system on a 12" barrel. With the adjustable I wouldn't worry about opening it up if changing to a lighter buffer doesn't fix your problem.

I opened a 300 blackout to 1/8” as well. That size hole seems big in a 30 caliber barrel but percentage wise, isn’t as much in 35 caliber. Hopefully it will be enough for the OP to make the new rifle run.

[Outer Rondacker]

Sorry I am short on time this morning but will Check back later. I recently had a bad buffer tube give me the same trouble in a 300blk. If the lower is new check the travel on the bolt. Just a suggestion. The buffer I had was not machined deep enough to allow the buffer and the bolt to achieve full travel.

[lar45]

The lower is new as well. I will try it on a different lower to see if it runs there.

[Moleman-]

What is your gas port pressure for the loads you're using?

[lar45]

What is your gas port pressure for the loads you're using?

As close as I can tell with Quickload, the gas port pressure at 9" is between 10-12kpsi for the 200s and 180s.

[lar45]

Now I can't find my bore Scope :( It only worked with my phone from 5 years ago...
Got on Amazon and ordered a new one...

[Moleman-]

As close as I can tell with Quickload, the gas port pressure at 9" is between 10-12kpsi for the 200s and 180s.

I'd think you should be fine with a smaller gas port then. I've made a few midlength 450BM barrels and used a .081" port on them which has a lower port pressure. Are you getting ejector swipes?

The gas systems are a little weird. Too low and too high port pressure can mimic the same result of not cycling. The ID of the gas tube is around .116"-.118" depending on who made them. Once you hit about .090" gas port size you're getting into deminishing returns as opening the port further has less and less effect as just so much gas at "X" pressure is going down a .116"-.118" tube. I'd really check the gas port alignment. If your barrel isn't dimpled you can take the rear set screw out of the gas block then install the gas block upside down centering the rear setscrew hole over the port and measuring the gas block/barrel shoulder gap with feeler gauges to know how much to stand it off when putting it back together. also check the passage in the block to gas tube alignment as I've had that be off as well. Might also check gas tube drag/alignment by removing the bolt and seeing if the carrier key is rubbing on the gas tube.

[lar45]

Thank you very much for the detailed information.
I'll check all of that and try again.

[Rapier]

The AR system is totally about timing. If you are going to change loads you really do need an adjustable gas block. Then you full close it, then open 1/4 turn, try, open a click at a time, until you get full function. Do not change more than one thing at a time, ever. The AR system does exactly the same thing UP and OP, so you change three things and end up right where you started.
Been doing this since 62. Had 384 ARs at one time.
Last wonder guns I got in, new in the box, would not even function subs or super factory ammo, they were so messed up, so no deal, even the big makers set up ARs that will not function.
The 358s and larger bore guns went to tungsten extra heavy buffers to get MORE weight. Counter intuitive, slow the BCGs down to drop the pressure so the bolt can open at the proper time on the pressure curve. So go back and think slow down, not speed up.

I shoot my two 358 AR-15s with the adjustable gas block, near full closed when at 2,500 with a 200. I use 1680
This is the round, based on the 6.8 SPC. Resembles a rimless 357 Herrett.

[lar45]

324485
Ok, the new borescope came in.
I put the adjustable gas block on and it's lined up with the port.

[lar45]

Rapier- Is that the 358MGP that you are running?
Is that the Lee 358-200 powder coated in the picture?
How fast can you run the PC Lee bullet and still get good accuracy?
I gave it a quick try, but couldn't barely keep them on the backer at 25 yds. I think I was pushing it too fast.
Thanks.

[lar45]

It locks back now!
I switched magazines and it locked back. So I took the first mag apart and stretched the spring and it locks back on that magazine also.
Once ir was running reliably, I dialed the gas block down one step and it still locked back. I turned it down 2 steps and it didn't so I'm down 1 step from wide open.

[lar45]

324518
It shoots reasonably well, but I think it wants to do better, lol.
I picked up some shim stock and I'm considering swimming the barrel extension or bedding it with loctite 620...

[cwtebay]

324518
It shoots reasonably well, but I think it wants to do better, lol.
I picked up some shim stock and I'm considering swimming the barrel extension or bedding it with loctite 620...Nice! Great outcome!!
I never fail to learn something from this sort of discussion. I was honed in on buffer weight - as always a little education makes me realize that I need to stop being so miopic!

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[popper]

shimming the barrel extension -- never made any sense to me (assume proper barrel nut - old style timing gas tube). Nor the locketite or 'squaring' the upper.

[Texas by God]

I’m glad that you got it running right.
Load up some cheap plated bullets to break it in and have fun.


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[BK7saum]

I am glad you have it working. Makes me want to start load development on my 358 Winchester WC-10. It is a little over 6 lb (rifle only) with lightweight billet upper/lower, carbon fiber butt/tube and carbon fiber handguard, with 16" Wilson threaded barrel.

Been dragging my feet because I don't have a 35 cal suppressor. Need to remedy that as well.

[lar45]

I used a Tromix Linear Comp. It directs the muzzle gasses forwards. I've got them on my 7.62x39, 450bushmaster and now the 358 Razorback.

6# is great for an AR10 sized platform.
I didn't know that you could build them that light.

[BK7saum]

I used a Tromix Linear Comp. It directs the muzzle gasses forwards. I've got them on my 7.62x39, 450bushmaster and now the 358 Razorback.

6# is great for an AR10 sized platform.
I didn't know that you could build them that light.

I need to doublecheck that weight, it might be closer to 7 lbs, Getting it confused with my 6x45 AR15 with similar furniture, but it is really light for a 10.

Nope. Weighs 6 lbs 8 oz with standard BCG. With my low mass BCG, it should weigh about 6lb 5 oz.

Any other smaller parts like titanium takedown pins were not used due to minimal weight savings for the increased cost.

[Rapier]

Sorry, did not get back until now. It is the 358 MGP that mike came up with. I have two, a 12 twist and a 20 twist with 16" McGowan barrels. That is the RCBS 200 grain plain base, double powder coated, no GC at 2,500 fps that shoots .43" at 100. I make a very hard alloy 70-20-10 (WW, Lino, mag shot) water dropped. Dried, PCed , baked, PCed and baked again at 400 for 20 minutes then sized to groove diameter with case lube on a pad using an RCBS Luber Sizer. I use Federal 6.8 brass from Nellis, die formed, trimmed, then fire formed. A very hot load, with a near closed gas block.
Makes a great hog gun. Whatever you shoot with the 200s stays shot.
Here is a picture with 180 power pistol bullets and the rimless 30 Herrett or TAC 30.