I have a stock 1911A1 pistol. It's fairly well used, but in good shape. I've installed a few aftermarket parts (extended safety, new hammer, grip safety), but it's mostly original. I also had to put a new slide stop on it because the old one was worn and would pop out under recoil. The new one works fine with most mags, but I have two Chip McCormick mags (one 8-rd. stainless match grade and an 8-rd. shooter grade) that give me some last round feed failures and do not activate the slide stop on the last round. If I disassemble the pistol and insert the mag, it engages the slide stop just fine. However, when shooting, the follower can jump over the slide stop without engaging it. This can happen without a feed failure, or the last round can jam vertically with the case head still down in the mag.
I have another shooter grade McCormick mag that never gives me this problem. Since the problem only occurs with the last round, I would think it's a weak mag spring, but these are new mags. Any ideas?