The one included in the box with the pistol works okay but could be better. I also got one from Excess Arms that works pretty well but it is not inexpensive ($50!). I also slightly modified a RAEIND speed loader for the Ruger 57 to work with the S&W magazines. So there are some choices available. I would like to find some mag extension base pads for the S&W 57 but have been unable to find any. Anyone know of such?
The mags are a tight fit so you may need to lightly file the inside of the loader. I also found that if I didn't quite seat the mag into the loader all the way, it worked better. About 1/8 to 1/4 inch seemed about right.
The Bullet Buddy is available for compatibility with multiple caliber rounds including 5.56, .223, 300 Blackout, 7.62, .308, 6.5 Creedmoor and 6.5 Grendel.