Well, I've been there, done that, so my opinion might have some weight. In fact, I've been thru the U.S. Army's Basic Combat Training twice. (I was out for 5 years, so when I went back in, I had to redo BCT.) Of course, you can always disregard my advice, afterall, you aren't paying for it.
In my experience, those people who haven't shot a rifle before and who pay attention to what the Drill Sgts tell them, will do better than those who have previously shot a rifle, developed bad habits, and are too stubborn to listen to what the Drill Sgts are trying to teach them. Now, those with some weapons familiarity who haven't developed a lot of bad habits and are willing to learn from those who know more than they do, are going to most likely excell at Basic Rifle Marksmanship.
If you're dead-set on getting a jump-start on BRM, do it right. Get ahold of the proper Army Field Manuals and find yourself an Army vet who is both willing AND capable of teaching you the RIGHT way. I say again, capable of teaching you the RIGHT way! Just because your neighbor down the street spent a couple years in the Guard and now plays Airsoft on the weekends, he may not be capable of teaching you.
When I first went to Ft. Benning as an 11X, I thought I knew everything there was to know about shooting. Heck, I'd hunted deer in Montana with my old .30-30 and had even handled an M16A2 at a DEP function once. Boy, was I in for a rude awakening. One thing that sticks in my mind was the day at the zero range, when I went to wrap my sling around my forearm as a way to steady the rifle. Drill Sgt. Harris was quick to tell me to stop what I was trying to do. Using a sling in such a way is great in competition, but it's not a good thing on an Army qualification range. Think about it, you're training to use your rifle in a combat situation, you most likely won't have time to wrap the sling around. You can't tell Hajji to stop shooting them RPG's so you can get your proper shootng stance.
Besides, unless you have access to a Kevlar and Load-Bearing vest, you won't even be really replicating the way the you'll be shooting in Basic. Also, you'll most likely get some strange looks when you start digging a hole at the firing line of your local range in order to practice for the foxhole-supported position.
In short, if you're willing to forget everything you think you know when you get on the plane to Ft. Benning, and to shut your pie-hole and open your ears once you get to Sand Hill, go ahead and practice now. (Although, I wouldn't recommend it.)
However, if you know you're the type of person to assume you've already learned the right way, so what could that 10-15 year veteran with 2 combat tours teach you, then don't do it.
Frank
PS - What others above have said about practicing PT instead of BRM is great advice. The Drill Sgts will be much happier having a PT stud around than having a expert rifleman around.
