[pmulcahy11b]

When I was in the Army, I bought an AR-15 just for that reason. When I first enlisted and got out Basic, I had marksmanship scores that were barely enough to graduate, but worked my way up to expert with the help of that rifle. These days, there are zillions of AR-15 clones out there, so there is a lot of choice; you'll be able to shop around that way and possibly find a deal. If possible, get an AR-15A2; if you can afford it, get an A3 (which has a MIL-STD-1913 rail atop the receiver instead of a carrying handle) and a Trijicon ACOG, since almost every soldier is issued on these days. Shoot with and without it; my nephew, who went to basic over last summer, says they didn't have ACOGs when they were shooting for record in Basic.

Another possibility, especially for BRM practice, is an AR-15 "Clone" that is chambered for .22 Long Rifle instead of .223/5.56mm. This will give you some of the "muscle memory" at a lesser cost, and let you practice basic marksmanship.