I don't know how many folks know this, but DPMS changed the top height of there AP4 uppers some time early this year. I had an older AP4 upper with a DD middy handguard, I was designing a gas piston gas block to interface with the rail, after I got my project finished and working I decided to build another upper using the same height gas block and DD rail, WHAT!! the new AP4 upper was about .050 lower than my older one. Some searching around I found a post that someone confirmed what I had found. Today a customer came in with a new AP4 and a Tray rail, and sure enough the upper was about .050 lower, it looked like s--t, told him I didn't know what he should do. I think the rest of the manufactures of other DPMS type uppers are sticking with the original height. Can or does anyone else have any knowledge of this ??