Yeah the typical 45 recoil, especially in a polymer frame is more of a shove than a kick. The 45ACP is a low pressure load, about half that of the 40S&W. Not saying the 40 is bad, though - I have three Glocks in that caliber.
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