Please bear with me for a minute. I have an M4 that I took out a few weeks ago and ran 10 rounds of high brass and 100 rounds of Federal Valuepack through it without a hitch. Finding no problems, and wanting to get the full potential out of it, I ordered a FFT full length ti tube, follower, mag/tube spring, hammer, disconecter, trigger, and 1/2" ti charging handle. I got all that installed without a hitch.
Today I went out and tested it. It ran another 100 rounds of Fed Valuepack perfectly. Then, during the last 50 rounds, it started to have some issues. It hung up about four times. The issue was that it would fire one or two rounds and then hangup. The bolt carrier would be all the way to the rear, the spent shell was ejected, and a fresh shell would be sitting on the lifter. I could get it to close by simply hitting the charging handle forward. I searched the forum and came up with this thread which describes similar issues...
So, I went out to take a look at the FCB to see if maybe I should polish the suggested parts. That's when I noticed some strange peening/dents on the frame of the FCG. It looks like the rear of the spring pin that actuates the lifter lever (don't have the official name in front of me) is hitting the frame. See...
If things work as they should, the frame never gets hit by the pin. However. If downward pressure is applied to the lifeter while the bolt moves rearward tripping the lifter lever/arm, the pin's angle changes and it hits the frame. Anyone deal with this before?
I'm guessing the problem is due to installing the new full length tube. I pulled it all apart again and I can feel a little difference between the inside diameter of the factory tube and the ID of the FFT tube. My caliper confirms this. However, my batteries are dead so I can't tell you the actual difference. With both tubes, the receiver produces a slight lip where tube flushes up to the receiver. However, I'm thinking that this slight amount of difference might be to blame for my the issues. I can get a shell to hangup on the lip when the FFT tube is installed. My guess is that the rim of the shells would eventually wear the lip caused by the receiver down. Or, I could hit it with some fine abrasive and speed up the process.
I'm not even sure if this is "lip" is the culprit. I'm thinking if something is slowing up the feeding of the new shells it might be creating pressure on the lifter. Thus, causing the lifter spring pin to hit the FCG frame which is resulting in the bolt hanging up in the open position.
Any ideas? Maybe I'm completely off? Anyone dealt with this before?