Sidearms & Scatterguns Masterpeice arms 5.7x28mm in atacs camo

It's a neat round very fast very accurate. I would not get one of the
Masterpiece Mac 10 want-a-be's. They are a spray and
Pray gun .If you want a 5.7 get a fn 57 pistol they made the designed the gun
For the round
 
+1 on sticking with the FN 5.7 if you want a decent pistol setup to run the 5.7x28. They are high quality, well made pistols that as a1 said...were purpose-build around the 5.7 round.

Also, depending on your particular wants/needs for the pistol...there are MANY cheaper and arguably better alternatives to the 5.7 which can chip away at the wallet in a hurry! ;)