I\’ve loaded quite a few of the 124 gr RMR MPR in 9mm, and I generate data from QuickLoad using the Sierra 125 gr 8125. It is about the same length as the MPR and has a profile that doesn\’t accommodate long OALs in most firearms. All my barrels will plunk the MPR at 1.075\”, some will go to 1.100\”, and a couple will handle 1.125\”. Chrono data matches the QL predictions pretty well. VV 3N37 at 6.2 gr and 1.075\” is a standard pressure load and produces over 1150 fps from a 4\” barrel. If the bullet can be seated at the longer 1.125\” length, that charge can be increased by 9% with a corresponding velocity increase of about 50 fps in a 4\” barrel. My goal with these loads is to generate enough velocity for reliable expansion. If that isn\’t a priority, I would use enough W231 or AA5 to generate 1050 – 1100 fps and call it a day. 3N37 is a good powder, but it isn\’t cheap or widely available at the moment.
Attention to the seating depth is a must for the 9mm. Small changes in OAL will result in big pressure swings. For the loads above, the difference in pressure between 1.075\” and 1.125\” using the same powder charge is about 30% (according to QL), so work up carefully.