You always check fps is under the site limit with .2g bbs. If your'e under 450fps on the .2s then all is well.
You can then empty the mag, stick your .4s in and rock and roll.
I'm sure a marshal will correct me if there is some kind of weird and wonderful DMR specific fps rule.
Just with the DMR remember you have to be locked to single shot if its an AEG. That's something you need to get checked and OK'd by the marshals before you play.