Systema 455mm X 6.04mm in my CA36
Deep Fire 455mm X 6.04mm in E1 AK
M14 had a Prometheus 650mm X 6.03mm in it, but I put it back to stock and sent the barrel Slackers way.
I use Excel .23's in my AEG's and have had great results without a single jam in any of them.
In my Bar10, which has a stock barrel right now, I am currently using KSC .30's which seem to do the trick. I would like to try some .28's though, the place I ordered from were out of stock.
I use Excel .20's in my M9... and anything in my ICS M203.