45.2 grs of IMR 4198/Rem 9 1/2 primers/Win cases/Hornady 300 gr JHP's gives me 1840 fps out of my Handi. It has been my favourite deer load here in the Thick woods of central PA for a lot of years. Same accuracy as you state. Same components, except
51.2 grs of same powder gives me 2173 fps. Accuracy is still at the 1" mark. Too much meat damage at the ranges I shoot at in the woods, but that might not be the problem with the Noslers.
42.2 would give you somewhere around 1650 fps, which is what most 300 grain factory loads used to be like 13 years ago when I first started shooting the 45/70. They said 1880 fps for the Win/Federal loads, but lied as usual. Killed a couple of deer with those loads with no problem, until I started handloading. I mean....300 gr might seem light to us, but that's downright heavy to most folks.