Piston AR build - problems

[Thegeek]

First trip to the range with it last Saturday, and I've got a cycling problem. The Adam's piston system has 3 settings. Single, suppressed, and wide open. I'm bought it with the light weight carrier they recommended. With the silencer on, and in the wide open setting, it just had enough gas to barely kick out the spent casing and reset the trigger. Wasn't going back enough to load the next round. Without the silencer, it would move the carrier enough to pop the dust cover but wouldn't reset the trigger or eject the case.

I'm using a Wilson Combat .300BLK barrel 11.3". The factory gas port is .073". I took it apart last night to check it out. The port on the gas block is about 1/8". This left a nice black circle on the barrel. I could see that a sliver of the port was covered. I know it's sensitive, but like this I'd imagine it would have cycled suppressed with the setting on full. I contacted Adam's Arms about 10p last night. They responded first thing this morning with a suggestion to correct the alignment (no surprise there). But, the one thing that raised an eyebrow is that they said their recommended gas port for .300BLK is 0.125". That sounds HUGE. I'm not sure if I'd want to drill my expensive barrel out like that. So, I've contacted WC for their input as well.

For the record, I was shooting Sig Sauer Match grade 220gr subsonic.

 

[Foxfire]

Back when I was messing with the 300 they were called the whisper.
That was a few years back so things may have changed.

What i recall is, if you want to shoot subsonic suppressed and cycle you have to have a pistol length gas system and a 12" barrel minimum. Also that was a DI system.

I may be way off on this, if so someone will correct me.

 

[Thegeek]

Wilson replied with the most political answer I think I've ever seen. "We designed the barrel to function on a DI system. That being said, if they recommend a .125" gas port you may do so at your own risk. It you do choose to go that route, it voids any warranty we have on that barrel." All I asked is if it would be safe to do so.... they never said either way.

Like it or not, it looks like I'm going to have to drill the gas port. Other than ruining a rather expensive barrel, I guess I need to look up how to prevent having a burr in barrel when I'm done. I'll probably step it up to around .090" first, then in small increments until it runs right.

 

[Thegeek]

Did quite a bit more research on gas port sizes. I found a site (micromoa.com) that had a lot of good information. Most of which is for DI gas systems. There was no number for an 11.3" barrel and the closest were the 10.5" and 9.5" with pistol length gas systems. The 9.5" was .093" and the numbers got smaller the longer the barrel. So, I drilled mine to .093" and I'm going to try and get to the range to test.

 

[Huntinfool]

Ammo shouldn't be the issue (as that's what I shoot suppressed). Have you tried full speed ammo? What weight buffer and spring ?
does the carrier slide smoothly, when you cycle it by hand ? If there is any resistance try running the upper really wet.

 

[Thegeek]

I have not tried any supersonic ammo. The buffer and spring are out of the stock that will go on once my Form1 is approved. It's a BCM Gunfighter Carbine buffer and spring. I don't know the actual weight of the spring and buffer. There is no drag or binding in the system. I provided all this information to Adam's Arms when I contacted their tech support. They're pretty confident that it's not enough gas.

 

[Thegeek]

Went to the range this morning to test the 0.093" gas port. I had very little improvement if any. Unsupressed and it still wouldn't extract. Supressed and the spent case would start to eject then get the mouth would lodge on the end of the barrel. So, what the hell, I came home and opened it up to the recommended 0.125". I really don't want to drive back to the range an pay again to fire a couple rounds. I wonder if the neighbors would be too upset if I fired a couple out back....

 

[Huntinfool]

Really should try some higher power loads to see if there is any chance that it might cycle with those. That way you may know if your chasing a mechanical issue or gas pressure.

Maybe you have a bad piston assembly. I really don't think it just a gas problem, on the pressure side.

 

[Thegeek]

I don't disagree. The piston assembly is pretty simple and I've checked all the tolerances. I really don't see how it could be mechanical, but I could be wrong. Looking at some load data, the 125gr data is around 40k CUP and the 220 subsonics are around 30k CUP. Even if the hotter ammo works, it doesn't solve my problem since I want to keep this subsonic. This is when having a private range and a chronograph would be monumentally helpful....

 

[Thegeek]

4 test rounds in the back yard with the silencer. In the suppressed setting, half way extracts. Full gas and it kicks the brass out, and comes back far enough to drag the next round out of the magazine. Not enough to feed or lock open on empty. Disappointing still, but now I need to find some specs on this ammo. I'm going to need to raise the pressure a bit. Question is, can I do that and keep it subsonic?

 

[Huntinfool]

Is this your 1st AR? or lots of experience?

Do you have different lower that you could swap out the buffer and recoil spring. Since you already have the lite BCG maybe its at the back end.

Try one with the rounds running the gun "wet"

 

[Thegeek]

First piston AR. Never had these problems on DI guns. Running wet didn't change a thing. Wet on this thing is in the piston rod and gas selector. The problem is I don't know what the spring rate is for the BCM buffer spring, nor do I know what the spring rate is for my S&W AR. Without that information, it's kind of a wasted exercise. I've been in contact with Adams and I'm going to see what they say to try next.

 

[Huntinfool]

While I understand you may not know the spring rate of weight of the other buffer (from a different working AR). But it cost you nothing more than a round of ammo to try. If it cycles better then you know which end to look towards. If you have the same
results. That's a quick and easy problem elemanated.

Also I would still try lots of lube in the upper receiver and on BCG. That is still where most all of the friction is, wether the AR is piston or DI. Your piston and rod-assembly will always run dry quickly, due to the extreme heat of the gases. Any lube in that area will be quickly burnt or blown away.

It it sounds as if you have another AR. Just switch out the lower and see if the problem stays with the piston upper. And if the piston lower now works. Again not much more cost than a couple of rounds.

 

[Huntinfool]

Also it's a know fact that some subsonic 300's are contrary. But you can tinker with one and figure it out. I have been told that is why some of the ammo companies don't run sub-sonic for sale

 

[Thegeek]

Can't exactly do that. The upper is only 11.3". I'll swap out the buffer spring and see what she does. And the BCG wasn't dripping, but does is properly lubed.

 

[Huntinfool]

Let us know what swaping out spring & buffer does.

 

[Thegeek]

Did the swap. The free length of the spring out of my S&W AR is about an inch shorter. With full gas and the silencer, it cycled well and locked open. With it in the suppressed selection, it would extract far enough to jam the brass onto the barrel lugs. The buffers themselves were identical (I did weigh them). I did a cursory search of Midway and Brownells and saw this: .300 BLACKOUT CARBINE BUFFER SPRING | Brownells

Just found this too: http://www.davidtubb.com/ar15-tubb-parts/ar15-buffer-spring-stainless

 

[Huntinfool]

Ok, so with a ton of gas, everything cycled properly. (at least went backwards and forwards through the full range of required motion). And when the gas pressure was reduced, it didn't. So we should be alble to rule out any mechanical issues with the back section of the rifle. And we know the gas port is large enough now. So that really only leaves the piston assembly or push-rod. That's going with the assumption that the gas-block is now lined up correctly. But with then being .125 you really shouldn't need perfection, and still have enough flow.

If it were mine I would still run some hot ammo through it, and see what it does with that. (just curious)

Definatly can try one of the springs you listed. But I thnk like a lot of things AR - its a answer chasing a problem that really doesn't exist. Not saying you can't improve an AR, but usually you really don't need to. I know there is several benefits with a flat wire spring. But I find in very odd to say that a lot of guns start to fail in a little as 500 cycles. I have and do a lot of training where we fire upwards of that in a day, and still have yet to "need to" replace a spring due to cycling issues. With that said - I like to waste my money on upgrades just as much as the next guy or.

Sounds like your at least getting closer to pin-pointing the problem.

 

[Thegeek]

I think after 500 rounds there's a measurable difference in the the free length of the spring. That spring that worked had probably 600-700 rounds on it. It's obviously not worn out, and I think Tubb is using that as a marketing tool. Still, he's not the only one marketing a softer spring and specifically calling out loads like the subsonic .300blk and they're all talking about DI guns. I've emailed the the particulars and asked what they think.

I'm open to suggestions on things that could be wrong with the piston components, but it's such a simple design that I don't see what could be wrong. It's not obstructed and the push rod isn't binding anywhere. I checked the run out on the rod and it's straight. And the piston to metering assembly clearances are correct. The more I look at load data, the more I'm thinking these rounds just don't have the pressure required. I'm going to buy a box of a different manufacturer with lighter bullets. Those should have about 30% more pressure.

This is fun, ain't it?

 

[M67]

Is you barrel a carbine gas system or a pistol? I didn't see that posted anywhere (could have missed it)

If it's a carbine length, you'll probably keep having issues, that's why Wilson probably said .125" is recommended. By the time it hits the gas port on a carbine system the pressure has dropped a lot

The ammo is fine, I've shot it and cycled perfectly with a can.

Have you tried the suppressor setting? My piston guns, with a can, require the setting to be in suppressed mode to function properly.