betheglock and drekkerd, I wouldn't worry about your new Gen 4s. I also had trouble getting the slide back on my Gen 4 G23 before I fired it. But it's not the RSA getting caught, it's the dimple on the trigger bar hitting the firing pin safety. If you hold the slide vertically with the muzzle down and press in the firing pin safety the firing pin should drop and protrude through the hole. If not give the sear a little push to get it to drop. Release the safety and it will remain depressed and the firing pin still protruding. Now your slide will go on much easier.
Once you get 50-100 rounds through the gun that won't be necessary anymore.
betheglock, I agree with the others who said the marks on your frame are cosmetic. Shoot the gun and keep an eye on that area over time. If it doesn't get worse I wouldn't worry.
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