I know we buy these semiautos for "fast repeat shots", but....

I have a "Colt" (stamped) 5-round I bought years ago for turkey hunting that still protrudes about 3/4" beyond the magazine well. Don't know if those are still available or not (knowing "Colt", probably not). There's a lot of spring and mag follower that has to fit into all of that, I'm not sure you'll find a true "flush" 5-round.

Something else you might consider, especially since you apparently expect some accuracy from your rifle (I have a RR too, super rifle). A friend shoots black rifle competitively and saw him loading some really looooong .223 ammo into his rifle one day (much longer than would fit into my magazines). Found out it's a single-loader, basically a block that fits into the mag well, holds the bolt open after each shot, allows him to use 80-grain bullets in competition.

Here's the link. They're a bit spendy, but you'll note that you can saw it off at the bottom to fit flush, and the ability to load ammo with heavier bullets might be an advantage if you're varmint hunting or target shooting.

http://www.dctechs.com/obspage.htm