short stroking a piston operated short barrel .300blk

[Clint Oris]

I could use some help.
I have a 10.5" barrel .300blk, and I am short stroking whether I run supers or subs, suppressed or un-suppressed. This is a build. It has never cycled. I have less than 30 rounds through it. My suppressor is an OSS Helix 762 QD - so there is no back pressure from the can.
I have an Adam's Arms pistol length piston kit, with an adjustable gas block. I have Adam's Arms light weight BCG with their bolt. I an running a JP Enterprises silent captured buffer with the H2 counterweights. Here is what I have done to try to resolve this:
I have an identical build, except it has an 18" barrel, .223W, and is running a rifle length piston. It also has just the standard JP silent captured buffer.. the only difference is lighter counterweights and I believe the spring is lighter, but mine is a first run on gen 2 and they didn't color the spring to differentiate .
That gun runs like a dream. I swapped buffers to see if the weight had anything to do with it. It did not resolve the issue.
Then I purchased the alternate spring kit from JP and swapped the spring for their lightest offering of 80lbs. The other gun is running an 85lb spring. That didn't resolve it.
Then I swapped out the piston return spring which has a 9lb compressed weight for a lighter spring that is about 3lbs. No change.
In the process of changing the piston spring, the fouling around the gas port indicated that I was dead center on the port with the gas block, so I know it wasn't an alignment issue. The OE port was .0935". I bored it out with a 7/64" bit to .1094". That did not resolve.
On both guns, I have a Hiperfire Hipertouch Elite trigger, but the .223W felt like it had a lighter hammer spring, and the spring wire looked thinner. I called Hipertouch and they told me that they did lighten the hammer spring between the 2 versions (one is 2 years older than the other) and they were kind enough to mail me their current spring, which is a 4.5lb compression weight. I put a caliper on the springs to compare and the new spring was indeed .002" thinner on the wire. That has improved things to the point where I'm at least ejecting the spent casing now, where as before it was not opening enough to even stove pipe. The spent shell was being reloaded into the bore.
At this point, I'm not sure what else I can do. I don't know if my BCG is too light and doesn't have the inertia to compress the buffer all the way, or what's going on. I just don't have enough experience in this arena to know what's going on. I would greatly appreciate some feedback.

 

[Ironicaintit]

When you enlarged the gas port from .0935 to .1094, did it improve it at all?

 

[Clint Oris]

When you enlarged the gas port from .0935 to .1094, did it improve it at all?

That in of itself didn't get me cycling, but after lightening the hammer spring - in conjunction with the gas port work is when I finally started ejecting the spent casings rather than just reloading them into the chamber.

 

[mjsdwash]

not trying to be an ass but have your tried the standard gas tube with a standard buffer spring?

My 300 will cycle with 110 grain bullets down to 2000 fps with no issue, though it is a 16".

Rough chambers could be an issue, and honing is fairly easy (epoxy an empty case solid, drill center, thread cleaning rod, cover case in rouge, run with drill)

I mostly handload with H110 and Alliant 2400 and 110 grain round nose bullets. These run 2000-2150 FPS. That puts it well BELOW standard 300 range.

 

[Havok7416]

I looked at the silent capture buffer briefly. Everything I saw indicated it was optimized for 5.56, so I gave up on it. As someone who also runs Blackout, I have found I need to be careful selecting parts for my guns as many parts are designed to work with the standard 5.56 cartridge. In theory Blackout and 5.56 only have a different barrel between them, but the reality is obviously a bit different.

OP, you have a lot of stuff stacked into one gun, much of which are "special" parts. My guess is the buffer assembly is likely causing the issue, or at least the biggest one. At the high end (fast, light bullets) Blackout ammo only develops around 63% of the pressure of 5.56. This will translate to less pressure acting on the BCG. Couple that with the higher spring tension with the buffer assembly (at least i assume that system has a heavier spring) and you might not have enough oomph to cycle the action.

 

[Clint Oris]

I looked at the silent capture buffer briefly. Everything I saw indicated it was optimized for 5.56, so I gave up on it. As someone who also runs Blackout, I have found I need to be careful selecting parts for my guns as many parts are designed to work with the standard 5.56 cartridge. In theory Blackout and 5.56 only have a different barrel between them, but the reality is obviously a bit different.

OP, you have a lot of stuff stacked into one gun, much of which are "special" parts. My guess is the buffer assembly is likely causing the issue, or at least the biggest one. At the high end (fast, light bullets) Blackout ammo only develops around 63% of the pressure of 5.56. This will translate to less pressure acting on the BCG. Couple that with the higher spring tension with the buffer assembly (at least i assume that system has a heavier spring) and you might not have enough oomph to cycle the action.

That was my thought too... which is why I contacted them to make sure I have their lightest spring. Even with their lightest buffer mass, I'm thinkin' it's still too much to overcome. Since I have a 5 position adjustable gas block, I think my next move is to take the gas port to .120" and always have the option to dial back the gas.

 

[Havok7416]

That was my thought too... which is why I contacted them to make sure I have their lightest spring. Even with their lightest buffer mass, I'm thinkin' it's still too much to overcome. Since I have a 5 position adjustable gas block, I think my next move is to take the gas port to .120" and always have the option to dial back the gas.

I hope it works for you, but I'm doubtful. I don't think there's enough gas to begin with. I tried to open one of my gas ports when I was attempting to get subsonics to run in a carbine length barrel (I realize you have a pistol length). I ended up swapping the barrels out [ETA: for ones with pistol length gas tubes] on several Blackout guns to get them to run.

It could be that the piston setup is requiring too much pressure to operate too, but that seems unlikely given the drastic reduction in the spring weight that you already tried.

The trigger shouldn't cause any cycling issues in and of itself. On the other hand, you did get increased function by changing the spring there.

Regarding the BCG, I would think a heavier one (i.e. not a lightweight one) would make things worse, not better.

 

[mcb]

Silly question, is your adjustable gas block adjusted wide open?

 

[Clint Oris]

I hope it works for you, but I'm doubtful. I don't think there's enough gas to begin with. I tried to open one of my gas ports when I was attempting to get subsonics to run in a carbine length barrel (I realize you have a pistol length). I ended up swapping the barrels out [ETA: for ones with pistol length gas tubes] on several Blackout guns to get them to run.

It could be that the piston setup is requiring too much pressure to operate too, but that seems unlikely given the drastic reduction in the spring weight that you already tried.

The trigger shouldn't cause any cycling issues in and of itself. On the other hand, you did get increased function by changing the spring there.

Regarding the BCG, I would think a heavier one (i.e. not a lightweight one) would make things worse, not better.

I do appreciate the insight. .... I've attached a pic to show you my gas setup, and as you can see there is not a lot of anywhere for pressure to build up. The most restrictive place is the gas port, and then it blows off at the back of the block when the piston travels rearward. I still think I'm going to port it out a little. I'll let you know how that works.

 

[Clint Oris]

Silly question, is your adjustable gas block adjusted wide open?

it's not a silly question. I had to double check it myself the first time I fired it and didn't cycle.

 

[Hoser]

The first thing I would do it ditch the JP spring and weights. Slowly start getting it back to looking like a standard AR until it starts running. Then one by one add back in what you removed.

This is one of the problems with pistons. They are still kinda new and there are tons of different versions out there.

If this was a DI gun, there are a few textbook answers/fixes.

When you go down the piston road and add other variables, you just have to narrow it down one by one.

 

[Clint Oris]

The first thing I would do it ditch the JP spring and weights. Slowly start getting it back to looking like a standard AR until it starts running. Then one by one add back in what you removed.

This is one of the problems with pistons. They are still kinda new and there are tons of different versions out there.

If this was a DI gun, there are a few textbook answers/fixes.

When you go down the piston road and add other variables, you just have to narrow it down one by one.

Yeah, @Havok7416 gets the first "I told you so" on the gas port. I bored it out to .1200" which = the most restricted port in the gas block, so I have has much gas as I'm ever gonna get, and I only barely turned the brass off axis... still not enough to even stove pipe it.

I reached out to JP to see if they have anything lighter, to which they said no, and then I volunteered to be a dev guinea pig for .300blk, and I'm not surprised that I was met with crickets.

I'm checking with Sean at Wolff Springs to see if he might be able to run something down. If not, my only options are to start cutting coils, or swap out for a mil spec buffer and validate that the JP is the issue. Mil spec rout is certainly an inexpensive one, so not a big deal there.

I'll update when I have more data.

 

[mcb]

If you have that much gas in the system and the gun is still not cycling you have an issue other than springs. There has to be something rubbing or out of spec to cause enough friction to stop that BCG or the piston itself. Look for wear in the upper and the piston. Does everything move smoothly when hand cycled. Does the piston move freely in it cylinder smoothly. Take the hammer out and cycle the bolt is should be smooth with no catches or tight spots. Take the spring out of the piston and do similar.

 

[Havok7416]

Yeah, @Havok7416 gets the first "I told you so" on the gas port. I bored it out to .1200" which = the most restricted port in the gas block, so I have has much gas as I'm ever gonna get, and I only barely turned the brass off axis... still not enough to even stove pipe it.

I reached out to JP to see if they have anything lighter, to which they said no, and then I volunteered to be a dev guinea pig for .300blk, and I'm not surprised that I was met with crickets.

I'm checking with Sean at Wolff Springs to see if he might be able to run something down. If not, my only options are to start cutting coils, or swap out for a mil spec buffer and validate that the JP is the issue. Mil spec rout is certainly an inexpensive one, so not a big deal there.

I'll update when I have more data.

I certainly didn't intend my comments in an arrogant or superior fashion. I do however fully understand the aggravation of trying to get this cartridge/gun combination to run properly. In fact I just posted a fairly wide-ranging thread on that last week.

 

[Clint Oris]

I certainly didn't intend my comments in an arrogant or superior fashion. I do however fully understand the aggravation of trying to get this cartridge/gun combination to run properly. In fact I just posted a fairly wide-ranging thread on that last week.

No sweat! I never took it to be anything but constructive. I'm gonna go look at your thread and see if there's something I might not have covered. THX!

 

[Clint Oris]

If you have that much gas in the system and the gun is still not cycling you have an issue other than springs. There has to be something rubbing or out of spec to cause enough friction to stop that BCG or the piston itself. Look for wear in the upper and the piston. Does everything move smoothly when hand cycled. Does the piston move freely in it cylinder smoothly. Take the hammer out and cycle the bolt is should be smooth with no catches or tight spots. Take the spring out of the piston and do similar.

So, yeah... think I might have a headspace issue. I've checked many of your suggestions already, but I did take the hammer out and hand cycled a bit. Long story short, clean bolt goes in, dirty bolt comes out. I've swapped bolts and BCGs with another, and it does the same on the other bolt. Trying both in the other gun after cleaning, the bolts remain clean.
I can't tell if I'm fully rotating and locking the bolt in battery. Could be a reason why it's under gassed still. In any case, I don't think I have any way to adjust the headspace on an AR, do I? This is something that I need to take up with Ballistic Advantage.. exchange the barrel.

I took the bolt out of the carrier and tried just rotating it in battery and it doesn't have any snags - either bolt, either barrel. But when I have either BCG in the .300blk, they both hang up on the barrel extension, and shaving material off that is accumulating on the bolt face.

 

[Clint Oris]

However.... if it was headspacing, my brass should show bulging or stretch marks at the head - which it does not.

 

[Demi-human]

It may not be headspace, per se, but definitely a wonky chamber.

If it’s cut crooked the bolt has to overcome bending the brass through it, or shave steel to make clearance, before it can cycle.

So your rifle may not be under gassed at all, there is just an energy stealing binding point.

I’m glad you’ve found the “Ah, Ha.” Without the rifle in hand we are at such a loss.

New barrel time! An easy repair. Keep updating this thread, we want to know how it turns out!

 

[Demi-human]

Um, how straight is the barrel in the upper receiver tenon? It doesn’t look crooked or anything, right?

What are the receivers? Maybe we haven’t found the cause yet…

Perhaps the bolt carrier races are out of spec or something on the receiver. It’s easier to get these wrong than to cut a barrel extension weird.

If it shaves that much metal, one should be able to feel the difference.
(But I’ve pulled a smashed lifter through an engine head with pliers before. So, when one doesn’t know, then, one doesn’t know.)

Just some more thoughts, I want you to have the right cause. It saves time.

 

[Clint Oris]

Well... I spent a **** ton of money trying to get good quality pieces.
Any suggestions on how I can check tolerances/specs on the upper without just throwing parts at it?
Ugh.... so not looking forward to tearing this down .. such a PITD.... kinda like fishing a lifter out with a pair of pliers so you don't have to tear the GD thing down.

 

[Demi-human]

Any suggestions on how I can check tolerances/specs on the upper without just throwing parts at it?

This I’m not sure about and m going to investigate myself. Something doesn’t line up there for certain.
I would hope that it’s the barrel, that is easier to remedy.

 

[Demi-human]

Sorry it took so long for me to say that I didn’t find anything.
There are many military papers with measurements, but no easy button besides putting the thing together and looking for interference, like you did.

Any progress with it?

 

[Sgt_Mike1]

I have a 10.5" barrel .300blk, and I am short stroking whether I run supers or subs, suppressed or un-suppressed. This is a build. It has never cycled. I have less than 30 rounds through it. My suppressor is an OSS Helix 762 QD - so there is no back pressure from the can.
I have an Adam's Arms pistol length piston kit, with an adjustable gas block. I have Adam's Arms light weight BCG with their bolt. I an running a JP Enterprises silent captured buffer with the H2 counterweights.

I don't think it is a headspace issue at all. Or a receiver or anything else outta spec here is my thoughts and I maybe completely wrong.

Not a new issue matter of fact many users simply can't get a .300 Black to run with a Adams piston set up especially in a SBR configuration.

I would recommend that you shift it to a DI configuration and test. If that cures it then preview the following attached links if you desire to get the piston system working while getting in contact with Adams in one of the links I noticed a direct email address. Or just leave well enough alone with the DI system and use the gas piston on another project.

what is the issue??? simple the .300 black is based on a .221 Remington case coupled with a larger bore of a 30 caliber vs a .22 bore and a larger case doesn't produce the required gas volume that a .223 does. Dwell Time of a pistol vs carbine length is also working against you as well. The .300 black simply doesn't get you to pressure /volume to cycle right outta the box in the .300 Black without some adjustments in a piston setup from reports of others never attempted it myself. Usually with a 10.5 setup like you mentioned I done DI ( adjustable gas block) with mil spec components in pistol format run like a top right off the bat in both super & sub sonic loads.

Now can you get it to work?? more than likely I think you could ....will it require tinkering yea I think so.

The brass flakes you are seeing on the bolt I've seen that before with lower pressure / volume in both Semi auto's and bolt guns.
What your NOT seeing is where the rim base is contacting the bolt face it looks unfired and signs of brass flakes being compressed by bolt thrust. The case is malleable acts as a gasket when firing it starts to relax / contract /retract once the bullet passes the port. Usually in low pressure /volume you will see soot around the neck and shoulder area of the fired case almost if not to the rim. It will be dirty in the chamber and carrier as the case has NOT expanded to make a full seal. If a Case fully seals you might see a little soot around the edge of the neck. now the more you fire the chamber will dirty up with carbon but the first 20 or 30 will come out pretty clean from a clean chamber provided it has enough volume/pressure.

The DI is a overgassed setup for reliability having been around the M16/M4/ Ar15 series for over 36 years kind of versed in the setup I have only seen 1 failure a gastube had burned hole in it took less than 15 min to repair. Am I biased yes i think that the gas piston in a AR is a solution to a non existing problem. but you chose it for your own reasons which is good enough for me and I wish you the best of luck. looking over the FAQ from Adams in the "what Calibers does it work with" I noticed they had a special kit for the .300 black is that the one you have I don't know.



https://www.300blktalk.com/forum/viewtopic.php?t=95132
https://www.ar15.com/forums/ar-15/Adams_Arms_Not_Cycling_/126-690690/?
https://www.snipershide.com/shooting/threads/ar-cycling-problems-with-piston-kit.87001/
https://www.snipershide.com/shooting/threads/adams-arms-gas-piston-review.4664/
https://www.ar15.com/forums/ar-15/Adams-Arms-carbine-piston-kit-not-cycling/66-746930/
https://www.ar15.com/forums/ar-15/Adams-Arms-piston-install-question/126-763799/
https://adamsarms.net/faq/#1606556908598-09bf334f-c517
.300 black specific kit sku FGAA-03300
https://adamsarms.net/product/300blk-piston-kit-p-series-adjustable-micro-block/

 

[jamesgs4]

I had the same issue and used this, it solved the problem, 7.5" 300 blk

https://nemoarms.com/product/300-blackout-custom-carbine-buffer-spring/

 

[N555]

If you like the gas piston run the gas piston.
I say put a standard 3oz buffer and normal spring in there.
I bet you will get a little more stoke. Might even run normal now that you punched out the gas port.

I punched out the gas port on my mini14 so it would cycle my very accurate H322 start loads. Took it from about a 0.06'' hole to a 0.072'' and that was plenty. Glad I have the adjustable gas block.