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  #1  
Old 07-06-2018, 1:05 PM
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Default AR pistol .300 blackout will not reliably cycle heavy 190-220 grain loads

I have an AR pistol 8.5 with brake pinned on, lighter/hotter ammo cycles fine but the heavy stuff 190-220 grain from very reputable ammo suppliers is not reliably cycling. It will cycle 95% of the time but NOT enough gas to lock back on an empty mag.

I'm running pistol length tube spikes upper standard bolt and barrel to my knowledge. I attempted to fix this by clipping the recoil spring WAY short but there are two problems. A: hotter ammo is feels like its battering the tube excessively and B: its VERY easy to ride the bolt home on the last 9 round and sit out of full battery with the weak spring. Right now I'm keeping a standard and "clipped" powered springs for both ammo but, is there a better way to fix this problem? I looked into adjustable gas carriers and they all seem to only LOWER gas pressure. Is there an adjustable gas block out there than runs HOT? all I'm finding are things the reduced gas pressure or control by reducing I'd be looking for a gas block that has a "fouled" setting if something exists, or am I stuck resizing the gas port and working backwards?
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Old 07-06-2018, 6:15 PM
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The gas port is likely small.
Clipping your operating spring actually increases the spring rate because it reduces the preload.
Go back to a standard operating spring.
Open the gas port, then use an adjustable gas block to tune for specific loads.
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Old 07-08-2018, 3:29 PM
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Quote:
Originally Posted by ar15barrels View Post
The gas port is likely small.
Clipping your operating spring actually increases the spring rate because it reduces the preload.
Go back to a standard operating spring.
Open the gas port, then use an adjustable gas block to tune for specific loads.
Thank you, I'm going to check and resize if necessary. I have an adjustable gas block in the mail in addition I ordered up a JP captured spring system. I was told its very easy to tune the buffer weight with these. Also Ian and Karl from InrangeTV recommended it so I figured why not.

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Old 07-08-2018, 4:31 PM
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The existing gas port fits a 3/32 bit (that micrometerd @ .0932) fits VERY snug should I open it up to .120?

I have no machinist Gage pin set, I would most likely be using a 7/64 bit as the next step up which is .109 actual dia. Which should put me around .115-.120 actual port size imho.

Stolen from http://www.tacticalmachining.com/lea...ort-sizes.html

AR-15 Platform 300 BLK Conversion Subsonic Ammo
16" car 300 BLK UNSUPRESSED 0.12
16" car 300 BLK UNSUPRESSED 0.125
16" car 300 BLK SUPRESSED 0.106
16" car 300 BLK SUPRESSED 0.11
16" pistol 300 BLK UNSUPRESSED 0.067
16" pistol 300 BLK SUPPRESSED 0.086
9.5" pistol 300 BLK UNSUPRESSED 0.093
9.5" pistol 300 BLK SUPPRESSED 0.093
8.1" pistol 300 BLK UNSUPRESSED 0.104
8.1" pistol 300 BLK UNSUPRESSED 0.12
8.1" pistol 300 BLK SUPRESSED 0.086
8.1" pistol 300 BLK SUPRESSED 0.093
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Old 07-08-2018, 10:36 PM
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Those numbers are laughable.
Sounds like you are in over your head.
My recommendation is to use a standard buffer and spring.
I see many more problems with non-standard buffer systems.
Open up the gas port one wire gauge drill bit size at a time until proper ejection and bolt stop actuation is achieved.
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Old 07-09-2018, 12:31 AM
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Quote:
Originally Posted by ar15barrels View Post
Those numbers are laughable.
Sounds like you are in over your head.
My recommendation is to use a standard buffer and spring.
I see many more problems with non-standard buffer systems.
Open up the gas port one wire gauge drill bit size at a time until proper ejection and bolt stop actuation is achieved.
When you say laughable you mean the .109 bit is going to create a diameter way bigger than .120 actual bore?

Sink or swim or I'll post post results. My goal is to be able to switch gas settings for loads with a single hand tool on the fly.
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Old 07-09-2018, 4:28 AM
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Get you some # size Drills instead of the fraction size, that way you won't over drill.
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Old 07-09-2018, 6:16 AM
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Quote:
Originally Posted by repoman1984 View Post
When you say laughable you mean the .109 bit is going to create a diameter way bigger than .120 actual bore?
A .109 drill bit makes a 0.109 hole unless you are egging the hole out because you are using a hand drill.

The laughable part is how wrong the listed gas port sizes are.
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Old 07-09-2018, 11:59 AM
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Quote:
Originally Posted by repoman1984 View Post
When you say laughable you mean the .109 bit is going to create a diameter way bigger than .120 actual bore?

Sink or swim or I'll post post results. My goal is to be able to switch gas settings for loads with a single hand tool on the fly.
Going to a larger gas port with adjustable gas block is the option if you want to run supersonic and subsonic loads.
A heavier buffer helps my set-up for subsonic loads and if you're not already doing so, run the action wet.
IMHO, I'd just run subsonic ammo on that short barrel. Good luck.
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Old 07-10-2018, 6:12 PM
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Ty, yes the action is coated in hoppes gun oil and I gave everything a visual inspection and solvent bath and scrub as usual.

I've been searching a bit and I'm hearing conflicting opinions on buffer weight/total reciprocating mass carrier+buffer, some say the heavier is better as its going to collect more inertia and therefore will do better with a standard pressure spring and an underpowered load. I believe your way of thinking on this is correct but, I won't know till I install the new block and buffer assembly.
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Old 07-10-2018, 6:17 PM
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I think this is an ammo problem. The heavy bullets are for subsonic use with a suppressor. I'm guessing you are not suppressed? I don't think I would change anything until I confirmed it can't shoot that load suppressed.
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Old 07-10-2018, 7:03 PM
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I have the opposite problem. On a 10.5, pistol length, I'm overgassed with a standard spring/buffer even with subs. With supers it throws the brass to 5 oclock with a wicked dent from the deflector! I've got an H2 to put in it but haven't had it out yet.
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Old 07-10-2018, 9:23 PM
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Vltor A5H2 with a G super gas block on my Rosco barrel runs awesome with 220's both ways. The Vltor A5 system is the only way to go.

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Old 07-12-2018, 9:08 PM
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Virgin .125 bit was spun on a lathe against a stone till it micromered @ .119



http://oi63.tinypic.com/osslty.jpg

Rolled up wad of masking tape was lodged in barrel under the gas port to protect opposite side of barrel from over-drilling and collect chips.



started with a .109 then moved up to .119 lot of cutting oil and the bit was run at high rpm with lowest pressure. The bit had a BIG tendency to bite randomly, would not recommend this with a hand drill. The burr on the barrel exterior was stoned flat, a borescope revealed a nasty burr on the inside of the barrel which was polished down with 220 grit paper followed by a copper brush run over it 4-500 passes till the burr was no longer visible.

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Old 07-12-2018, 9:43 PM
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Quote:
Originally Posted by repoman1984 View Post
Virgin .125 bit was spun on a lathe against a stone till it micromered @ .119

Why not just use a 3mm or a #31 drill bit?
A properly made drill bit will cut cleanly and leave a much smaller burr.
A sharp drill bit leaves almost no burr.

By re-sizing a drill bit smaller, you are making it really dull where it would normally be sharp.
That's why it was not cutting.
It was pushing and pushing and then FINALLY taking a big bite as you were drilling.
The big bite was the "catching" you were feeling.
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Last edited by ar15barrels; 07-12-2018 at 9:46 PM..
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Old 07-12-2018, 9:59 PM
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I had the exact same effect cutting with the virgin .109, the metal distinctly felt case hardened, very soft between surfaces, (I've drilled 100 or so case hardened floor safes and felt the exact same feel) The front cutting edge was not modified only the outside diameter.

If I had to do it all over again Yes, I would have definitely used a solid carbide 3mm bit and saved at the very least some the effort of de-burring.


ALSO: I don't know why I called the barrel a 8.5" originally. It was marked "300BLK 7" I noticed after the hand guard was removed.
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Old 07-12-2018, 10:04 PM
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Quote:
Originally Posted by repoman1984 View Post
I had the exact same effect cutting with the virgin .109, the metal distinctly felt case hardened, very soft between surfaces, (I've drilled 100 or so case hardened floor safes and felt the exact same feel)
When you are drilling and feel the grabbing, that's a chip forming after the drill bit had been spinning without cutting and just creating heat.

When that happens, you are probably spinning the drill bit too fast and not applying enough cutting force to properly form a chip.
Doing that dulls a drill bit almost immediately and puts you into a loop of non-cutting and then grabbing.

If you are drilling harder material, you use a harder drill bit to get it to cut.

Smearing the metal by using a dull drill bit that's going too fast and not forming a chip is doing nobody any good.
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Old 07-13-2018, 3:04 PM
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gas block installed and JP spring after dialing to zero and reversed open up again about 20% per attempt found the sweet spot around 75% gassed, main spring is nice and strong bolt is locking open last round as it should. I ran out of time but I need to set a little "gas dope" list for ammo and loads but other than that it appears everything will adjust on the fly for low pressure and high pressure loads.
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