The RAP has a very wide/thick beavertail, though. Where the 320 cuts in, the RAP stays the same width. I'm one of the people whose fingers are just short enough with the medium grip size backstrap that I get my thumb knuckle...er RAPped when I fire the gun
Mine is the .45 version, so it doesn't have a small backstrap. It also has a 4.5" barrel which adds a bit of weight.
It's no big deal. A slow bit of sanding on the frame fixed the issue for the most part. It's still more hand-filling (or seems to feel that way) than the 320.
Also, I would suggest not taking the chassis out of the RAP unless you either: A) Have to or B) are mechanically inclined and sitting at a work bench. It's not as "snapped together" as the SIG. I think the Ruger is probably a beefier FCS compared to the SIG, and I think either of them would be easy enough to work on if need be.
I just had a mini heart attack when I popped out the RAP chassis willy-nilly and a bar came free, the trigger fell off, and a tiny spring on the rear popped off the back. It's not as contained as the SIG. I still really like the Ruger. It's a to-hell-and-back gun. I just shoot my 9mm SIG 320 full size better.