Ahhhhhh, I C! What spine of FMJ are you going to shoot? The outside diameter of the 400 and 500's is different. There's also a slight OD difference between the black metal plate ones and the camo version, as well as a grain per inch difference. I shoot them both as well as 500 MFX's. What I did was get some of those 100 grain brass HIT inserts from 3 rivers, and use 1 full one with the insert threads forward at the front, plus cut another one down to whatever size I need to get the arrow weight and flight I want. In my case I'm shooting a 50 lb at 28" longbow, and use a 500 spine arrow with 175 grains of insert up front, (100, plus one I cut down to 75 grains by removing the threaded portion with a hacksaw), so I can use any normal 100 or 125 grain broadhead depending on whether I want the arrow to weight 575 or 600 grains total. Kind of an internal footing if you will, especially with the FMJ's which already have an external aluminum shell. Works great, the only point to remember is just like an external footing, by stiffening the arrow up front, I effectively shortened my arrow and I needed to lengthen it a bit to make up for the spine change. I think Easton's website lists the outside diameter of their newer arrows if you're looking for that figure for an external footing. happy hunting