I've received several questions about loading big bore straight sided case particularly the 450 Marlin, 458 Winchester and the 458 Lott.
These calibers respond well to the same group of powders, specifically those developed for the 223 Remington or the 5.56x45 Nato military round.
Most of the questions ask are about "down loading" in particular the 458's. Down loading is done it two ways, using less powder or using less bullet, or both. This can be done but there are better ways to do it.
These cases are at their best with a full case of powder, a firm crimp and a magnum primer. The expansion ratio is very high (the gasses have plenty room to expand) in a straight sided case. That means as the bullet leaves the case the "rate of confinement" drops off quickly and the pressure curve drops quickly. Powders are designed to burn efficiently at a certain pressure. If pressure drops too quickly, burn stops, or is reduced, and this results in unburned powder. This is not so bad but this is always inconsistent from shot to shot. No two shots burn the same amount. This results in erratic ignition and high velocity variation. Other factors can aid or degrade this situation such as chamber and barrel dimensions.
This "rate of confinement" (RoC) is in full control of the handloader and is increased by these factors. More powder, faster burn rate of powder, more bullet (weight), heavier crimp, and seating depth, both as seating deeper in the case reduces the powder space and seating out against the lands. (this could boost RoC a lot)
Of course these RoC factors are the factors that increases or decrease pressures, and certainly burning rate of the powder is a critical factor, which brings me back to my point.
This list of powders that we want will fill the case (and still be a good, safe charge). Some will fill it with 350 grain bullets some only with 500 grain bullets.
Powders listed are of approximately the same burn rate:
H322, RL-7, AA2015, Benchmark, Vihta N133, N-201, RL-10X, and others. With 350-400 grain bullets there is only about a 4-6 grain window from good consistant load to a max load. These max loads will be slightly compressed (or better just with a long drop tube) but will cause no problems when compressed.
There is a group of slightly slower burning powders that is also used successfully: H4895, RL-15, Varget and Vihta 540. These powders when slightly compressed will not be max with the 350-400 grain bullets and that makes them good for reduced loads in these big cases.
These last four will fill the case and are useful when down loading even with 350-400 grain bullets. Basically we can use a caseful of this powder and get better ignition properties and more uniform yet lower velocities. These powders are also very good for load full power loads with 500 grain bullets and a full case as max loads.
Some have asked about using 458 Win data for the Lott, I think that is too much of a reduction in powder and will result in erratic loads. I would add about 4 grains of the faster powders for a starting load for 350-400 grain bullets for the Lott. Max load for the 458 Win is about 77 grains of AA2015 and a starting load for the 458 Lott would be about 82 grains, with 350 grain bullets. With 82.0 grains in the Lott and the 350 grain bullet is about 2500 fps which is about the same velocity in the 458 Win with 77.0 grains. So match the velocity, not the powder charge.
You might notice that any and all "ball powders" or spherical powders are conspicuously absent from my lists.
The 458 Winchester was made "possible" back in the 1950's because of a specific military ball powder developed for the then experimental military round, which became the 5.56 NATO. This powder was loaded in a compressed charge with a 500 grain bullet. It almost achieved it's advertised velocity of 2030 fps. When these heavily compacted rounds layed around in the hot sun for a while this powder clumped together and failed to ignite or partially ignited making a squib load. This failure almost brought about the demise of the 458 in the lands where giants grow because it could not be trusted for dangerous game. Another problem will arise when this or any ball powder is used in these big bores in reduced loads. It is extremely irratic with air space in the case and can result in squib loads also. And to reduce the charge there would be airspace in the case.
This powder is marketed under different banners and includes: AA2230 and W-748.
This is more of a squib load, effeciency, clean burn, consistant velocity issue and not so much of a danger, but a squib load could lodge a bullet in the barrel and if another round is fired it could result in a spontaneous disassemby of a fine rifle. Also cooler temperature can make this worse.
Have fun with the big bores.