Glass bed the action. Relieve the areas behind the trigger guard and the rear shoulder and tang of the action. This will prevent stock splitting. Accurate Arms 2230 and Ramshot Exterminator will give you 2100 fps with a 22 inch tube without compression. Hodgdon's H 335 and whatever you boys call Benchmark are also good .458 Winchester powders. A lot of tyro's are disappointed with the accuracy they get from lighter slugs. The .458 Winchester has approximately 7 tenths of an inch of freebore. This leads to short bullets tilting before engaging the rifling and as a consequence, poor accuracy. Stick with 500 grain conventional bullets when first figuring out accuracy and point of impact. These heavy slugs do cause point of impact change due to barrel vibration when you are playing with loads.These big boys are not varmit rifles. Free floating only exacerbates the barrel harmonics. I haven't seen the underside of the forend, but many big bores have a bedding block on the underside of the barrel under the front sight. This should be glass bedded as well.

Because there is no case neck as with a bottleneck cartridge, neck tension is tricky. I do not flare the case neck to accommodate bullet seating, instead I use a Lyman (the cheapest) 50 calibre VLD inside chamfering tool. I have never crumpled a case. Use a Lee Factory Crimp die to crimp the bullet. Trying to seat and put a heavy crimp on simultaneously leads to many crumpled cases. Many bulged cases will not clamp onto the bullet to give sufficient neck tension. Annealing cases is recommended.

Once you are onto to them, these big boys are fun.