Yeah, I used a JP Enterprises Grade 1 24 inch upper for a rifle I built up. It shoots pretty darned well, I recommend getting a catalog from JP Enterprises as for every configuration they show on their webpage there is a configuration that they aren't showing but you get to see when you have a catalog shipped to you.
Head to this link to see my rifle I built up using a JP Enterprises upper, [url]http://home.bak.rr.com/varmintcong/jpar.html[/url]
This is my main squirrel rifle that I use for nailing the squirrels out to 300-400 yards and up til now only educating them at 500 and beyond but I have gotten close to them at the 450-550 yard ranges.
With a good load shooting at 100 yards this rifle will keep 10 shots within a group the size of a nickle. It's a pretty heavy rifle though at 17 pounds(I used not so typical parts in the build that make it heavier) but the good thing about it is that it has ZERO recoil and through the scope you can watch all the bullets impacting the target as they chew it up.
Using this rifle one of the girls I took out shooting managed to put 2 shots right through a penny at 100 yards within 5 shots, the penny is on her key ring now.