No adaptor needed. I did mod the mags and install a block of wood inside the mag spring to limit capacity since it has to hold less than 9rnds in the barrel and mag to hunt deer with in MI. The 357AR is basically a rimless 357MAX run up to 556 pressures. It gives similar performance to standard 35REM loads. 158gr @ 2400fps, 180 @ 2260fps for max loads out of an 18" barrel. The 9" retains quite a bit more than max since the majority of velocity is gained in the first 10" of barrel length. The mags need to have the bullet guides removed in order to feed the fatter bullets and still have the rounds double stack. If you don't remove them, after about 3 rounds the bases of the cartridges have dropped low enough that the bolt will no longer grab them. Pmags, theromags, and the Bulgarian mags in the pic can all have the bullet guides trimmed off of the inside. In the pic you can see whats going on between the USGI mag and a modified Pmag. The straight 20rnd mags seem to work better than the 30rnd mags which will only hold about 25rnds before the curve makes the bases drop too low to pick up.