The Hodgdon web site lists several 300 BLK loads including the Sierra 125. I loaded 50 and shot them this past weekend. I spent most of my time with 220 grain Sierra loads. Hodgdon recommends COL of 2.050 for the 125 Nosler.
Leupold recommends sighting in at 50 yards with the 220 grainers and then you should be close at 200 yards. I only had 150 yards to shoot, but was on paper. It looked like I might be a little low at 200 but it will take some trigger time to be sure. All function well with a Gemtech NVT suppressor.
One surprising thing for me, out of an 8" Noveske, all the loads were faster than advertised. I've read that may be due to lack of friction in a short barrel.
My load was:
17.5 Lil' Gun
125 Sierra BT
Remington 7 1/2 primers
LC brass from Brad's
Average 2239 with 17.5 Lil’ Gunhttp://data.hodgdon.com/cartridge_load.asp