If you want the most power, go with the MVVS .21 However, it is a little more expensive and can be a little tempermental to run. Also, K&B and Fuji make a .21 size. You can get the Fuji from www.mecoa.com I have not run either of those though.

OS and TT make a .25 size, which I think is legal for some classes of combat I think. Magnum makes a .25, and sometimes I see Magnum .21's for sale on Ebay, but I never see a Magnum .21 in any ads or catalogs, so I not sure what the deal is with that.

I too would like to get into scale combat and I would like to use a Magnum. They are cheaper than MVVS, run as well as OS or TT. I have had durability problems with OS (peeling cylinder wall) and carburator problems with TT's. I have had a couple MVVS engines and they pull like a monster! Maybe someone can report on the K&B's and Fuji engines?