[DarkAngel]

So really, it has nothing to do with the ammo weight itself, but rather the fact that the hop up is not holding the round in the chamber without the hopup.

Try putting retention around the hop up rubber to see if thats enough friction to hold it in the chamber without the hop up.

Easiest way to find the issue is to remove the hop up/ib assembly from the outter barrel and put in a round by hand. Then see if the bb is retained. It should retain as is.

If not, then you have an issue.
Best at that point to take an unmodified TM hop up assembly and compare the two for differences.