HC Just some thoughts and ideas. Do you plan on using 500gr lead gas checked projectiles? When I built my 45-70 Mosin that was my goal too. Cast my own Lee 500gr and gas checked them. Wasn't happy with the rainbow trajectory and tried everything from 300gr to 500gr. Finally settled on 405gr with 60-63gr of reloader#7 a hot load @ 2150 fps. The 400-405gr seemed to make the best use of case capacity/power/velocity for MY NEEDS. Think of it like a 308. You can shoot any thing from 110 to 220 but they excell with 155-175. No one shoots 220s out of a 308 gas gun (it beats em up). One of my buddies re-chambered an AR10 in 300WSM. He gets fantastic long range accuracy with 210gr but has to shoot it like a single shot with the gas turned off. Why cuz just opening the gas valve a crack slams the crap out of his action." He can shoot 175s with no slamming. Where is willie going with this and get to the point." The point is with such a broad range of projectile weights not only is orifice size but port location along the length of the barrel worth considering. When I built my 500 ended up with a tungsten carbide piston (added mass) to slow things down. I too have your disease of no replacement for displacement fever. One of the main reasons I like GUNCO is everyone has ideas, tips and just cool toys! If you need some gas checks I have a few to spare.