To fill in some of the details people have been asking about...
My rifle is a 100% stock Bushmaster "post-ban"...aftermarket parts on the rifle itself and it is not a parts gun. It is low-miles, maybe 2-3000 rds or so through it, so the buffer spring could be a little tight yet. I lube it very lightly with Break-free. I have good luck with just about any 30 rd mags that I use in it, especially since I have put Magpuls in all of them. Also, I do lightly lube the buffer spring with a little litheum grease. Perhaps this is an issue, but it hasn't been with other mags/ammo in the past.
Larry, if you really want me to I may trim the spring on a mag but I kinda hate to do that because all of the mags you sent me loaded just fine to 20 rds, so I don't see how it can help.
The first thing that I want to do is try these mags with a different ammo. Give me a few days, because it's don't nothing but rain here in WI for about a week now, and it's pouring again today. I'll re-post with results when the weather breaks...(gives me an excuse to tell the wife I need to go to the range anyway).