Took a new AR I put together to the range the other day. Thought I was getting short strokes with it. Well actually I was, but not from any of the normal reasons. It seemed to start with not locking open on last round. Then went to not feeding the next round. At first I blamed the ammo. I have good ammo but only 20 rnds with me which seemed to work ok. Intended to burn through some old steel cased stuff I had from when that was the only thing around. This stuff just would not cycle. When I got home I wanted to see if everything was ok with the gun. I noticed when I racked the bolt back and locked open, the slightest jar would release the bolt. WTF could cause that??? I thought the bolt catch might be bent or just defective. But no, the bolt wasn't going far enough back for the bolt catch to engage the front of the bolt. It was just barely catching on the bolt carrier. I pulled the spring and buffer out and checked them up against another set I had. Buffer was the same length but the spring was actually shorter, I think that was just from compression cycling in the buffer tube. Same amount of coils. So whats left? The buffer tube! It was 1/8" shorter... inside depth! Outside was the same as another I had. Right down to the machining marks, they looked identical. Just the depth was different. Swapped the old tube out for a new one and now it locks back fine. Who'da thunk it! I didn't know that the buffer came that close to being totally compressed. Not much in the way of wiggle room.

 Wont know if the crap steel case stuff added to the mix or not till the next trip to the range.

 

 

There are actually two lengths for the action spring depending on if you have a rifle length or carbine length reveiver extension.

 

With each action spring type - carbine or rifle length, there is an acceptable range.

 

Make sure you have the correct spring for the receiver extension you're using. Then make sure it is within the acceptable range.

 

SmittyMHS, I had a similar issue with an AR. I was using some spacers to make the stock a bit longer and it turned out I had swapped screws and grabbed one that was about 1/4" (maybe less) too long. This was long enough to prevent the bolt carrier from locking back and was causing intermittent failures exactly as you describe. I had only swapped the stock so didn't perform a function check before heading to the range. Good times.

There are actually two lengths for the action spring depending on if you have a rifle length or carbine length reveiver extension.

 

With each action spring type - carbine or rifle length, there is an acceptable range.

 

Make sure you have the correct spring for the receiver extension you're using. Then make sure it is within the acceptable range.

 

I'm gonna measure the spring against this. I always get them as a set carbine length tube spring and buffer but who knows. Coulda slipped past. 

It happens.

 

I marked my work bench so I can lay them down on the table real quick and gauge them.

Just measured 2 springs that I have. Both are just about 10 1/2" Well with in spec. for carbine length. Had to be the buffer tube depth. Something else to check against I guess.

Smitty- can you explain the i/o tube depth a bit, I'm not very savvy with the nuances of this platform and my sons AR was doing what sounds the same, not picking up a rnd after firing. He too did some swapping of parts so maybe I can help figure it out with him. All we could do at the range was mix up the mags but the issue remained.  Maybe a replaced bolt could cause this too? 

Smitty- can you explain the i/o tube depth a bit, I'm not very savvy with the nuances of this platform and my sons AR was doing what sounds the same, not picking up a rnd after firing. He too did some swapping of parts so maybe I can help figure it out with him. All we could do at the range was mix up the mags but the issue remained.  Maybe a replaced bolt could cause this too? 

Can he rack the slide and lock the bolt open? And is the bolt catch engaged in front of the bolt or behind it on the carrier? The buffer tube that was giving me fits measured 6 3/4" deep. (where the spring goes in)The new one is 6 7/8" so the bolt goes back that 1/8" further and now locks back. Weather that solves my problem I wont no till I get back to the range. But at least now it locks open.