Will all the stages have steel or just one like last year? If there's a lot of steel, I might use the M1 carbine because I have plenty of non-steel ammo for it (ammo for it is not cheap) - and yes, it's a rifle cartridge. Money's so tight at the moment that I don't think I can afford 250 rounds of quality .223 in that timeframe, all I have is Silver Bear, Wolf, and Tula, all of which attract a magnet.
Make some "Mexican Match" ammo. Pull the bullets, and replace them with non steel bullets.