I converted a stagger feed blind magazine Savage 12FV to 223AI and would like to shoot the long 75gr Amax to the lands. The mag length won't allow me to get where I need without single feeding. I noticed the existing mag has, for lack of a better term, a spacer which should give me about 3/8" or more length if it was not there. The Savage seems to have a long short action and was wondering if a 243 box will work? Any other suggestions?