I have the AR 15 Bushmaster. While at the ragne today, I had a failure to feed after about 100 rounds (using 20 round Adventureline mags).  Seems I had 2 rounds tryng to get chambered at the same time, one on top of the other.  I cleared the weapon, and proceeded to fire another 150 rounds from my 30 round OKAY mags without a hitch.  This is the only time that this has happened in many firing sessions at the range, but still disconcerting (old Vietnam days). I must admit, the weapon was not as clean as it should be.  I   should have marked that 20 round mag and tested it further.  None of my mags have the green followers. I wonder if I should rebuild with new springs and green followers for better reliability (they are 35 years old). Anyone have similar incidents? ideas or thoughts?

Dump the adventureline mags.  Stay with the OKAY's. Problem solved.

Yes. Magazine failure. The previous advice to dump that mag and others of that brand is correct. If you're mule-headed, you can try to tweak the mags to work right, although it just ain't worth the time and trouble now that they're a dime a dozen. Use only new, government-issue magazines.

I also vote for mag problem. I had the same issues with old GI mags. Rebuiled them and they run 100%.

With 35 years on your mags, I think rebuilding is worth it.  I would recommend SAW's enhanced (red) magazine springs and green followers.

Look at your mags carefully. Check the rear of the feed lips for cracks and alignment. Look for proper alignment when it was assembled. File and or grind it into decent shape, if necessary. If you ruin it, so what, it's already crap.

I had a POS USA steel 30, and for grins, I put a USGI spring, MagPul coyote brown follower and a ranger plate on it. I loaded it with mixed ammo and had a friend, who could bumpfire, shoot it out. It bumped well and it has performed flawlessly since then. It's the only one I have, but I'm eyeballing a 40 rounder to try out. They aren't going into my BOB, but may be good for some fun at the range.