300BlkTalk

All things 300 AAC BLACKOUT
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PostPosted: Sat May 26, 2012 10:35 pm 
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Silent Operator

Joined: Sun May 13, 2012 11:38 pm
Posts: 55
Seems I'm not allowed to post pics or at least can't figure it out. Unless someone would like to school me. In any event, no gun porn.
Specs:
SAA 300 blk 16" stainless h-bar barrel, carbine length gas
Aero Precision ar15 upper receiver
YHM railed gas block
DPMS ejection port cover kit
DPMS delta ring kit
Colt sp-1 lower with factory h-buffer
Colt factory bcg and bolt
Colt factory charging handle


Assembled upper Monday. Tested. Found bolt would not lock back after firing a single round from 30 round pmag. Also no ejection. PNW 155gr match and PNW 125gr ballistic tip ammo. Both supersonic. Removing gas block revealed misalignment of gas block and gas port. Maybe this block was meant for a free float handgaurd not delta ring style hand guard. Anyway, gas block port and was well forward of barrel port causing about half of barrel port to be covered and ineffectual. Ported gas block to allow full access between ports (kind of like porting cylinder heads). Testing shows barrel gas port perfectly centered in gas block port now.
Testing showed continued failure to feed and erratic failure to eject. Failed more often than not.
Found gas port in barrel measured .093-.094. Internet forum research regarding similar issues specified Midway's ARStoner barrel gas port at .104. Misaligned gas port explained original issues but upper still seemed undergassed. And research supported larger gas port needed at barrel. Opened gas port to .107 on drill press. Very carefully. Pretty scary drilling a hole in a $200 barrel.
Soaked bolt in CLP (run it wet). Sometimes still fails to lock back on empty mag. Feeds and ejects perfectly every time.
Mags are pretty new. Feed and lock back 223 rounds flawlessly. Bolt functions perfectly in sp-1 upper. Colt sp-1 rifle was purchased new in February and zero failures thus far, so interchanged parts should be gtg.
All indications are upper is still slightly undergassed. Concerned about opening barrel port anymore and necessitating an adjustable gas block to regulate an overgassed upper. I'm presently only concerned with running supersonic ammo for hunting purposes, although may someday ATF up and get a suppressor for subs.
Any advice?


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PostPosted: Sun May 27, 2012 12:39 am 
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Silent Operator

Joined: Sun May 13, 2012 11:38 pm
Posts: 55
On my "smart phone" now and having trouble editing my previous post. Nothing works right on these foums with phone internet. At least i can read posts with no problems.
I'm finding a correlation between gas system length, buffer and spring, and gas port size (difficult to find someone giving specs for this. Proprietary info?). Of course this all makes perfect sense.
So, logic leads me to conclude either lighten my buffer or increase barrel port. I don't want to change my buffer as the lower is donated from my colt rifle and will still be used for that. This leaves me with increasing the barrel gas port again. Worst case I'll have to fit this upper with an adjustable gas block. Thanks for the faq rsilvers. I think this provided the last relevant piece of info needed to wrap my head around this. Of course someone could just publish a chart documenting gas system length (i know this forum faq states mid and rifle don't work) vs gas port size vs buffer type.

I am 100% certain I'll make this upper run flawlessly. And it will be worth the trouble given that accuracy has been outstanding and only DSA beat the price for an assembled unit (with a who knows how long wait). I'll wait a couple days for replies and research in case my logic falls thru. Really hoping the 300 blk god will give me the magic number for the gas port with an h buffer. Seriously, no sarcasm intended. I love this cartridge and am already obsessively attached to this upper. Thanks all.


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PostPosted: Sun May 27, 2012 3:06 pm 
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New Member

Joined: Sun Mar 11, 2012 10:14 am
Posts: 7
Location: Fort Worth, TX
crs7861 wrote:
I don't want to change my buffer as the lower is donated from my colt rifle and will still be used for that.


The buffer is a 10 second change out and is temporary. I would look at doing the buffer prior to drilling out a bigger gas hole.


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PostPosted: Tue May 29, 2012 11:25 am 
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Industry Professional

Joined: Wed Aug 23, 2006 2:48 pm
Posts: 481
Location: OR
16" or 14.5" is as short as you can really go with carbine gas, depending on if you want subs to cycle reliably without a silencer. The gas tube is .116" ID.

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Todd


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PostPosted: Tue May 29, 2012 11:06 pm 
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Silent Operator

Joined: Sun May 13, 2012 11:38 pm
Posts: 55
soncorn wrote:
crs7861 wrote:
I don't want to change my buffer as the lower is donated from my colt rifle and will still be used for that.


The buffer is a 10 second change out and is temporary. I would look at doing the buffer prior to drilling out a bigger gas hole.



Something to consider I suppose. I don't own a carbine or standard buffer though. Also rsilvers says in the faq here h-buffer for carbine gas and h2-buffer for pistol length gas. So I ask myself what's different?

-k- wrote:
16" or 14.5" is as short as you can really go with carbine gas, depending on if you want subs to cycle reliably without a silencer. The gas tube is .116" ID.


So a longer barrel would keep pressure on the gas port longer. So far only supersonic rounds. Seems these should definitely cycle. Thanks for the gas tube spec. Tells me I should be able to open the gas port all the way to .116 before further opening begins to lose effectiveness as far as increasing gas to operate bolt.
Is it just me, or is this little bit of info (gas port size) some kind of secret. I can see manufacturers have done the research and developement and therefore paid the price to acquire this info. Hope I'm not being rude by asking industry professionals for help with non-name brand build. Thanks for the responses thus far.


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PostPosted: Sat Jun 02, 2012 6:56 am 
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Joined: Sun Apr 08, 2012 3:10 pm
Posts: 8
Change the buffer!

I began a build with a tungsten buffer(extra heavy) and it would fail to lock back on occasion, extra lube helped at first but after ~50+/- rounds, it presented again.

I run a 16" barrel and shoot super only. Ammo is PNW 147 range.

happy shooting!


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