Another update...
I opened the buffer back up and replaced one more tungsten weight with a steel one, bringing it down to 4.5oz with 1 tungsten and 3 steel weights. Took it to the range with the last 126 rounds of independence XM193 I had left. Loaded 25 in each type of magazine I had.
First up was a non-window M3 PMAG. The whole mag fed, fired, ejected, and fed again like clockwork.
Next up was a Lancer L5AWM. The first 2 fired, ejected, and fed, but after the 2nd round, the 3rd round stayed in the magazine. I kept trying, but the next 5 or so rounds would fire and eject, but the next round would never get picked up.
Then I tried the same with a window M3 PMAG, then with a USGI mag, with basically the same results. Occasionally I was have 2 or maybe even 3 fire back to back, but nothing consistant. Then there was the rare round getting jammed at an angle as it was being fed into the chamber.
I emptied about 10 rounds from one mag and loaded them into the original PMAG that fired all 25 rounds, but this time I had the same problem I was having with all the other mags.
So what do yall think?
1) Was it just a fluke that the first mag functioned flawlessly the first time?
2) Could the problem have something to do with the heat from firing the first 25 rounds? (I didn't fire them particularly fast or anything. In fact, took a short break half way to walk down range and back)
3) Is Independence XM193 really so weak that it still needs an even lighter buffer while Lake City M855 worked fine with the 5.3oz buffer?
Most likely from here I'm going to have to order a lighter buffer from VLTOR (or at least one of those special steel weights with the spring attached).

