The SR40 should be a good gun but I haven't fired one yet so can't say first hand.
My son in law and I were comparing his SR9 and my Glock 23 and both are pretty darn good. No reason to expect the SR40 wouldn't be good, as most Rugers are.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek