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Old January 16, 2020, 22:06   #1
Bubacus
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.300 Blackout Pistol Buffer/Spring Question

Neighbor called me up and let me know his new .300 Blackout 10" pistol build wasn't working. Not cycling correctly. He let me know that the gun fires, ejects, strips a new round and pushes it 90% into the chamber then stops. He has to bump the forward assist to get it fully seated. I'm assuming it is a buffer and or spring issue. He has a carbine length buffer tube and stock spring and buffer that came out of a misc box of AR parts. What buffer/ spring set up should he be using? I have zero experience with AR pistols so I can't weigh in on them much.

Thanks
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Old January 16, 2020, 22:26   #2
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STD carbine buffer and spring associated works fine for me.

Have you personally seen the 300BO barrel that he is using ?

Correct feed ramps ?
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Old January 16, 2020, 22:58   #3
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I saw it once after he received it from Primary Arms. Was a Radical Arms 10.5 pistol upper chambered for 300 Blackout. I believe it was this one...only way I remember was the price seemed pretty low:

https://www.primaryarms.com/radical-...n-3-m-lok-rail
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Old January 17, 2020, 10:19   #4
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"is it plugged in?"

"Did you put gas in it?"

AR version... Did he clean the chamber before use?

Is it properly lubed?

Once that's done new AR builds sometimes just need to be broken in. Anytime I beat one together I lube it up and cycle the action a few hundred times while sitting in front of the TV. Often there is a mild clearance issue between the upper and the cam pin, or the BCG is rough from the parkerizing and needs to be worn in, or a chamber is on the tight side and just needs breaking in. I've cured a lot of issues with new builds by doing that 1 simple thing. Cycle it a bunch, break it down, clean it, reassemble and relube.
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Old January 17, 2020, 10:32   #5
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If it's a pistol should it not have a pistol buffer tube rather than carbine buffer tube? Unsure if that might define it as an unregistered SBR but I worry too much. Maybe a cheap KAK buffer tube kit at $39.99 might be an idea or at least a $14.99 KAK pistol buffer tube. My guess is the carbine buffer tube is too long and buffer is not rebounding properly especially if used a pistol buffer spring. Cheapest fix may be to put a spacer in rear of tube to shorten it's internal length for buffer travel. If that doesn't fix it then it's the usual check for proper buffer weight, proper gas flow whether gas block is misaligned or port in barrel needs to be opened. I would start with the $39 KAK pistol buffer kit.
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Old January 17, 2020, 10:56   #6
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Those "less expensive" uppers seem to have a bunch of issues to keep one busy!

I would first swap the lower out and see if a known working lower will produce the results you are looking for. If it works, its the spring/buffer if not........

I would check to make sure the gas block is lined up right and in general the gas system. I.E. is the tube properly installed in the gas block and etc.
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