Yep, just get a 357 Sig barrel of the same length (actually you can use the longer one if you don't mind 1/2" sticking out). And use your 40 mags too.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together. You'll remember the quality of a gun long after you forget how much you paid for it.
US Army 1966-69, VFW Life Member, Retired Geek