For 3gun the detachable mag is going to put you in open class, so unless you have the pistol and rifle setup for open as well you are either going to be at a disadvantage or wind up spending some more money to bring those up to open level.

If the drum is an MD Arms 20 you are good to go, otherwise you may experience some problems with it.

It takes some work to get them to run reliably with a wide variety of ammo and you are probably going to want a choke solution of some kind for competition.

All that being said, I love mine and they are a ton of fun.