I changed out my old M16A1 buttstock for a 6-position M4 type. Following the lower assembly guide on this site because it had been years since I worked on my Bushmaster, I completed the rather simple job in a few minutes. Cocking action was smooth and everything function-checked. The next day I picked it up and started to draw back the charging handle and it jammed. Remember, the gun has not been fired since putting the stock on, just static-checked. Naturally, I did not try to force it, and eventually got the action open. Not seeing anything wrong, I closed it and tried again, and it worked. Not being satisfied with that, of course, I tried it again and it jammed again. This time I had to bump the butt on the floor to get it unstuck so I could open the action. I noticed gouges in the face of the buffer, so deep I could feel them, and small flakes of metal on the lower sharp edge of the bolt carrier. Eventually, I realized that the three flattened edges of the buffer had 'casting flash' which broke off as the buffer moved, until a piece got wedged between the buffer edge and buffer tube. After taking it all apart, cleaning the tube out and stoning the buffer edges just enough to break the sharp edges, function is perfect. Just to be sure, I loaded up a magazine with some dummy rounds I had made for this purpose, and manual loading and extraction is running fine. I just never knew you had to smooth out the damn buffer before dropping it in, but its always good to check parts for that kind of problem before installing them.  

Hadn't heard or seen that before, but I'm always in for something to keep in my mind's toolbox for troubleshooting.

Thanks for the tip.

Quoted: Hadn't heard or seen that before, but I'm always in for something to keep in my mind's toolbox for troubleshooting. Thanks for the tip.

same here!  thanks I'll defenatly keep an eye out for that when building from now on...