Mus, Actually, the stock 40 cal spring works just fine with the 357 Sig round as well as the 40. Normally, the only thing you will need is the barrel, and an occasional tuning of the mags. By the way, I'd recommend only the factory mags with this setup, as I've found that the Mec-Gars don't work as well in the 40 cal guns. If you're looking for a good barrel, Bar-Sto makes one, and KKM Precision makes a nice SS barrel as well. Good luck, it's a nicer shooting gun to my mind than the 40 caliber round.