I bought a new 450 XMLR last year and have reloaded and shot hundreds of different rounds with it. I bought it with ten boxes of shells, 7 were 350 fp and the rest are 325 FTX and I decided to keep most of the factory stuff in the box so maybe this is not the way one should go after brass but I live in Winnemucca, NV and we have one of the best shooting ranges I have seen over the 50 years of shooting, but one can pick up belted mag brass most every time you go to the range. I have used 7mm mag, 284 win mag, 300 win mag, 300 H&H mag, 338 win mag for 450 brass. It's quite simple but cutting off the neck and trimming the case then running them thru my dies I have found a free source of brass, now granted the other cases do seat a little deeper when chambered vs factory brass so given that I have had to pull the hammer back twice to get it to fire, but that has been once with about five boxes of my home made 450 stuff....now I don't shoot any other mag's and I am aware that this could chamber in guns not intended for so I am careful with it but it's just plinkers for me. I have loaded 300 gr. HP, 350 fnsp, 325 ftx and 400 gr speer fp using H-322 or H-1895, I din't know how many other have done this but it works for me.