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Need advice on a "single shot" build


chevyoneton

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I started gathering parts a couple of years back and finally got around to putting together a new 3 or 2 gun rifle.  It feeds, but is a single shot.  The BCG is coming back barely far enough to cock the hammer but that is it.  It is not extracting or ejecting the spent case and certainly not feeding another one.  Works fine when you work the bolt by hand.  So, probably way, way under-gassed.  Upper is a Stag 18" stainless fluted rifle length gas barrel, Yankee Hill gas block and melonited tube, AIM BCG. 

I have an idea of a course of action but would like to hear suggestions.  I don't mind trying a new gas block (maybe adjustable), and low-mass BCG.  If I have to pull the handguard, which I will, I might as well at least change the gas block.  Opening the gas port in the barrel is a last resort, but I might get there eventually.  What say you? 

 

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I recently help a buddy of mine who had a very simular situation. Everything was fine as assembly goes...except for one small thing. He had installed the gas tube up side down! So it was getting zero gas to the BCG. Flipped the gas tube over and voila...a running AR-15!

just something to check.

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don't have my AR handy but I do recall measuring from the front of the shoulder to gas port and then from the back of the block to the port on my gas block I remember reading that some blocks are drilled forward of where they should be due to using the Mil-Spec location which take into account the hand-guard spacer,now omitted due to a rail system, and that if you put the block flush against the shoulder when installing it can impede gas flow and cause short stroking/non functioning

I also can't recall if I had to space it forward or not, sorry 

J
 

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Thanks for the replies.  I thoroughly checked over the BCG last night and saw nothing amiss.  Key was tight and pipe cleaner went through.  The rings on bolt did not have their slots aligned.  BCG moved freely with light finger presser back and forth with action open.  I did decide to try the cheapest and easiest thing first, I swapped on a different lower.  This is the lower off my "old" 3G rifle.  It is a rifle buttstock instead of the carbine I had on the build.  It also has a better ($$$$) trigger and was really the set-up I was going to run anyway.  I don't know if it will make any difference but it should be interesting to see.  Will also try some more ammo, although the two types I tried are what I have run in the past and what I wanted to use.  I suspect I will end up at a gas block issue after all though.

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1 hour ago, Amerflyer48 said:

don't have my AR handy but I do recall measuring from the front of the shoulder to gas port and then from the back of the block to the port on my gas block I remember reading that some blocks are drilled forward of where they should be due to using the Mil-Spec location which take into account the hand-guard spacer,now omitted due to a rail system, and that if you put the block flush against the shoulder when installing it can impede gas flow and cause short stroking/non functioning

I also can't recall if I had to space it forward or not, sorry 

J
 

I usually slip a credit card between the gas block and the step in the barrel and then tighten it down.  Spaces it out just right.

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