The BDMs will not function with the Better Mag adapter.
You need the SW MP 15-22 25 round mags...They are really nice mags. Though we have two BDM AR22 drums and love those. But no last shot hold open with those and the adapter.
Also you can get a better price on the BDM drums at JoeBobs. I think they are 49.00 right now and if you buy two you get free shipping yet I think..
EDIT: I figured it out. The better mag adapter is made by Boonie. Never knew that and I have one
They SW 5-22 drums with the SW tower are more expensive than the AR22 ones at 69.00 a pop. But the last shot hold open probably will not work for those either. But you could leave the adapter in place maybe. And then just use SW 15-22 25 rounders without having to remove the adpater like I have to when I use the BDM AR22 drums.
Hope you are not more confused than before. I do that to people sometimes
http://www.joeboboutfitters.com/Blackdog_S_W_M_P15_22_50R_Drum_p/bdm-sw%20mp15.22-50r-drum.htm