The price of copper projectiles made me try cast, but I'm not keen on casting them myself. With a gas check you can drive them at normal 458WM velocities, and 458Lott velocities as you point out. There is one company over here (Westcastings) that make a hard cast, coated lead projectile that is the correct diameter for my rifle - 0.459". The others are 0.458" which doesn't wotk in my rifle. Westcasting make two grades of projectile coating - one rated for 1700fps max and the other for 2300fps. The 1700fps version are more commonly available since this is all you need for western action style shooting with the 45/70.
If I got the 2300fps rated version I could use full house loads with the 500gns and slightly reduced with the 400gn.
Trying to come up with a load below 1700fps for the 400gn was a major headache - in effect I was trying to produce a reduced reduced load. The staight walled 458 cast and long throat of my CZ550 made ignition very erratic and the odd projectile got stuck in the barrel. Only a mid-range pistol powder worked (with a filler) but I was warned off using them by a fellow that explained the "ringed" chamber phenomina. Trailboss has about the same burn rate as the pistol powder I was using but almost fills the case up thereby preventing the chamber ringing issue. Anyway they are super cheap loads, accurate, heaps of fun (still kick up heaps of dust) and good for off-hand practice.
It wasn't my original intention to produce light loads but cheap loads. Although happen with what I ended up with.
I know what they mean when they say that the larger bores are very versitile - I have three categories of loads now: sub-sonic lead (good for ?), mid-range lead (500gn/1600fps) (good for pigs maybe?) and full house loads (good for dangerous big game).
