I've been having some trouble with my M4A3 lately.  It was basically jamming up after the first round was fired. The bolt was half way closed as it rested against a round in the mag. The round was half way out of the mag. I was thinking that I had a problem with the recoil spring so at the last gun show in the area I bought a new spring.

I took apart the collapsible stock and removed the old spring and to my surprise it was much longer then the replacement that I'd bought. During some research on the web, it would seem that someone installed the longer spring that was made for the normal butt stocks on the AR-15.

Anyway, I put the new spring in and went to the range. At first it seemed like my problems were gone. But while into my second mag the bolt jammed half way back like it had done in the past, except this time it was jammed solid. I had to pull the pins to free it up. Once freed, (ok maybe this was dumb on my part) I tried a few more rounds and again it did the same thing.

So this time I fully took apart the upper and lower and tried to find out what the problem was. Yes, I know, I should of done it the first time. Well, I'm not sure what it's called but the first thing I notice is that the spring and the metal piece that inserts into it is past the point it usually sits and is projecting into the lower. I then notice a spring and another pin in the lower. It's the pin that sits right before the opening to the collapsible stock. Not sure what that pin is called. Guess I found my problem.

Here's my question though. It looks like the pin somehow got jammed with the bolt. The bolt has some scratches and a few slight grooves where the pin must of been lodged. Anyone think that will cause me a problem once I put everything back together? Also, what is the name of that pin? I'm definitely going to need a new one. Maybe even a new spring. Not to mention I think I'll need a new stock as the groves to the one I have seems to have taken a beating by the metal piece that in the spring.

I can fix it though, I have the technology! I think.

  You would know better than anyone if you are qualified to fix it.  As you seem to have identified the problem, fix it only if you are comfortable with what you are doing.  I have never had that issue, so I can't give first hand advice.  But, it seems like it could be worthy of a look by a good gunsmith if you shot it like that for a while.

Let's see if I decipher this right.

The buffer and recoil spring were protruding from the receiver extension (buffer tube) into the lower, because the buffer retainer pin and it's spring came loose, which also jammed up your bolt/carrier.

Sound about right?

Likely the cause of the buffer retaining pin and spring coming out is that the receiver extention (buffer tube) was not screwed in enough to retain the pin.

Since you found the recoil spring to be a longer one than the replacement you bought, it's possible that at one time your rifle had a full length stock, buffer and spring originally. Someone changed the stock and buffer, but used the old spring and failed to screw the extension in far enough to retain the pin.



Maybe I'm off base, but sight unseen, and from the description you gave, this seems like a likely scenario to me.

From my Armchair Gunsmith Position



Lonny

 Great, informative reply.  Thats why I love this site!  


Seems like the right diagnosis based on what he said.