I like 20", which is why I ordered a predator persuit upper. It's not so much the velocity loss of the 16" tube, it's the muzzle blast when shooting at a covered range. Even with plugs and ear muffs, I just don't care for the feeling of a small concussion grenaded going off in my face with each shot.
Still a couple months before my upper shows up, so we'll see how well it shoots. I would be suprised if I couldn't find a handload that will print 5 shots into 3/4" at 100 yds. I won't be suprised if I find a load that'll go into 1/2" at 100 yds.
I purchased an RRA lower with m4 stock and national match trigger a year or so back and have been debating uppers ever since. The predator persuit seemed like the perfect match of accuracy potential, and yet not to heavy to use as a carrying varmint rifle. The A4 upper is the way to go as you'll want to mount the scope forward and that's easier with the A4 upper. As my kids and wife shoot my guns, the A4 stock is more flexible, but if it was a gun for just me I'd go with a fixed stock.
