Non-suppressed Piston pistol cycling issues.

Discussion about rifles in 300 AAC BLACKOUT (7.62x35mm), hosted by the creator of the cartridge.

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Duade
New Member
Posts: 1
Joined: Thu Sep 13, 2018 7:36 am

Non-suppressed Piston pistol cycling issues.

Post by Duade »

Good morning gentlemen, I have been lurking around the forum and finally decided to join. I get more useful info from your site than anywhere else, so thank you very much for all of your previous insights. So good to join you.

I have built several DI pistols in 300 BLK, and decided to make the jump to piston drive. I understand the issues folks have, with the added cost, the added weight, etc, but I like having a clean-running gun. great discussion for another day. What brings be here is cycling issues. Here's what I have;

Daniel Defense upper/lower receiver. 10.5” Rosco Manufacturing 300 Blackout pistol-length barrel, gas port micrometer measurement is about .062". Adams Arms Piston Kit - P Series Micro Block with low-mass carrier. Standard carbine length buffer spring, with both H and H-2 buffers in the mix. I do not wish to go the suppressor route; I have a Precision Armament M4-72 Severe-Duty Compensator out front, and I am using 300 AAC Blackout, 110 gr VMAX rounds by Hornaday. If I have left out any other pertinent details, I will update as requested.

I have consistent issues with short-stroking. Failure to feed, failure to eject, no bolt lock-back on an empty magazine, a complete cornucopia of crap. And not consistent failures, either. All parts are brand-new. I verified and re-verified the gas block positioning. I swapped out the H and H-2 buffers. I have used other supersonic loads with different grain and manufacturer mixes, really didn't see a difference. I have all indications that it is under-gassed. Before I drill out the gas port, I would love to hear some feedback about the following;

1.) Is rifle gunpowder "hotter" than handgun powder? I am not a reloader, and have no experience in this area. I have read that the 300 BLK powder is not true rifle gunpowder, and do not know if this has any bearing on my issues.

2.) I have also read many testimonials that all 300 BLK cycling issues go away when a suppressor is attached. The original military contract called for a close-quarters combat round that would be used suppressed. The dwell time is extended by the additional back-pressure provided by the can. Is there a way to compensate for the shortened dwell time, sans suppressor? Lighter springs, lighter buffer, hotter loads, heavier bullets, I am open to any reasonable suggestions.

3.) Has anyone seen any data for drilling out 300 BLK gas ports? I am of the mindset that the best path is to drill it out too large, and get an adjustable gas block to tune it down. I definitely solicit your input on this, if you have done this before.

Thanx a bunch for reading this far, I didn't mean to make such a long post, but it's not a simple issue. I live in Texas, DFW area, and I like to haunt Shoot Smart and Shoot Point Blank. I'm very grateful for your tips and suggestions!
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Flatliner
Silent But Deadly
Posts: 266
Joined: Tue Nov 17, 2015 4:43 pm

Re: Non-suppressed Piston pistol cycling issues.

Post by Flatliner »

This is a very well written post and I wish I could give you some constructive advice. I am curious what responses you will get.

I will say that your post got me thinking about a piston upper and I may be headed down another project rat hole. My wife thanks you...
kilibreaux
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Posts: 46
Joined: Tue Jun 08, 2010 12:37 pm

Re: Non-suppressed Piston pistol cycling issues.

Post by kilibreaux »

I can tell you this: I started out building 8" and 8.5" blackouts with "expensive" CMMG barrels. As an early adopter, I also had to go through the process of figuring out the loads that would cycle, and, like many I am sure I wanted to cobble up a perfect subsonic load! All the issues you stated I had. I quickly discovered early factory dies weren't made to set the shoulder back far enough which meant a lot of rounds that simply wouldn't chamber - even though they did fit in my chamber gauge! Next was the fact that anything less than 180 grains simply lacks the mass to cause the bolt system to cycle at subsonic speeds....with every powder I tried. At that time, Blackout ammo was selling for $3 a round due to price gouging and component prices weren't a lot better. I finally found a deal on Hornady 208 grainers and loaded them up, only to find they still wouldn't cycle those short barrels with pistol-length tubes! Then I added a "Krinkov" brake which is in reality a muzzle "booster" built to return reliable function to AK's with short barrels. With a properly adjusted and modified Krink booster on the nose, the shorty Blackouts would cycle with 208 grain subsonics! WooHoo! Well, by then I'd pretty much lost interest, combined with California making "pistol" AR's basically illegal which meant I certainly wouldn't be taking any out to a range for testing!

Finally I bit the bullet and bought a Troy PAR-15 pump-action 300 AAC and that is absolutely the BEST platform for the round! I love it! Mind you, even here in California we can still own AR's - and I do, but a manual action 300 AAC is perfect for a round with so much weight variation. Now I can chamber and shoot everything. From 110 grain factory Noslers, to 125 grain Speer TNT's, 208 grain Hornady's, and 220 grain hard cast subsonics by Grizzly, the pump-action accepts all equally and works superbly! The rifle groups into 1/4" at 25 yards, and will easily go MOA with the right sights and ammo. As for those who think owning a pump-action AR pattern rifle is some sort of compromise, please think again....it's actually the BETTER platform for all purposes other than simply standing up for the right to own a semiautomatic AR - of which I am most definitely a supporter, however, just because one should be able to own something doesn't make it automatically better, and factually speaking, a pump-action "AR" that uses a rotating bolt for lock-up, and accepts the same mags from five to 100 rounds, is really the "better" choice. I don't have to chase brass...the brass isn't ejected into the next county - right past my fancy brass-catcher setup, and it comes out clean, without dings and dents, and the rifle is as clean as the day I bought it! Imagine that! An "AR" that doesn't need cleaning! Sounds like a Vietnam story, except in this case it's true!

So, there's my answer...I still own four AR's in 300 AAC, but they're all sitting out there on the shelf while my pump-gun is going to the range!
cdl
Senior Silent Operator
Posts: 113
Joined: Thu Sep 15, 2016 2:30 am

Re: Non-suppressed Piston pistol cycling issues.

Post by cdl »

Here is some DI gas data. It might make you feel better about opening the port up a bit... or not...

http://www.tacticalmachining.com/learn/ ... sizes.html

I went from 0.093 to 0.101 in 3 cuts on my 8.3" BA barrel with DI gas. I really like it now. Course the gun was plenty broken in.
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