These are the .458 bullets used for our .450 Marlin loads: (from left) Barnes 300-gr. FN; Hornady 300-gr. HP; Sierra 300-gr. FP HP; Hornady 325-gr. FTX; and Speer 400-gr. FN.
Introduced in 2000, the .450 Marlin is almost a teenager now. It was developed for Marlin by Hornady to be a sort of "Super .45-70." There's nothing wrong with the .45-70 except that some of the guns that have been chambered for it can't stand very much working pressure, which limits performance. Bullet for bullet, the .450 Marlin is about 600 fps faster than the .45-70. That's something like 33 percent more velocity and getting close to twice the muzzle energy. Coupled with a handy lever-action rifle, the .450 Marlin makes a great brush cartridge.
How do they pack so much performance into the same-size case? Well, the .450 was developed from a belted wildcat, the .458x2 American. The .450's working pressure is about 44,000 psi, while the old .45-70 is limited to 28,000. There's no free lunch. If you want more velocity out of the same-size case, you have to push up the pressure.
Reloading for cartridges like the .450 Marlin is not quite as simple as loading for something like the .308 Winchester. The straight case requires a three-die set. The first die sizes the outside of the case and removes the primer. Now the case mouth is smaller than the bullet. When loading a bottlenecked case, the expander plug on the depriming stem resizes the case mouth as it is withdrawn from the case.
That isn't possible with a straight case, so it's necessary to use a separate expander die that opens the case mouth to the correct diameter. The expander die can be set to put a slight bell-mouth on the case to make bullet starting easier. Don't overdo this; you don't want to excessively work the case mouth at every reloading. The third die is your standard bullet seater. Our dies were from Hornady, but several companies now make them in this caliber.
Since most - if not all - the guns chambered for the .450 Marlin are tubular-magazine lever actions, there are some constraints on the bullets you can use. Tube magazines stack the ammo nose to tail, and you do not want a sharp pointed bullet seated against the primer of the round in front, so that limits your bullets to flatnose styles, with one exception.
Hornady's FTX bullet (Flex Tip Expanding) uses a soft polymer tip that won't indent primers and provides a better ballistic coefficient for somewhat flatter shooting. The improved ballistics may not be a great help at the ranges at which these guns are used, but they certainly can't hurt.
Finally, overall length and crimp are very important in tubular-magazine guns. If the overall length isn't about right, feeding problems can result. Both too long and too short can cause problems. The .450 Marlin uses a minimum overall length of 2.49 inches and a maximum of 2.55 inches. It might pay to make up a few dummy rounds to check the feeding action before you load up a lot of ammo.
A good crimp is necessary to keep the recoil action in the magazine from pushing bullets back into the case. That by itself can cause feeding problems. All the bullets I used have nice crimp grooves, and it is important set your seating die so that the crimp is made tightly into the groove.
If you do run into a problem where seating the bullet to the crimp groove results in an overall length that's too long, there's a trick you can use. Simply trim the case mouth to slightly shorten the case and overall length. This makes the cases suitable only for that one type of bullet, but that isn't a huge problem as long as you keep track of which bullets go with which cases.
The powder selection is fairly limited. Straight cases generally require powders that are at the fast end of the rifle burning-rate chart. If you want to duplicate the factory catalog performance, which drives a 325-grain bullet at a factory-advertised 2,225 fps (it averaged 2,210 in our pressure barrel), you're going to have to get the powder selection just right.
A word about accuracy. Because the lever-action guns are often used in hunting situations, where the ranges are relatively short, we used a 50-yard range for accuracy testing. A tight three-shot cluster at that distance certainly shows that this cartridge will produce good hunting accuracy. The results we saw at 100 yards were proportional.
The .450 Marlin isn't the most popular caliber on the list, but it is a very useful number for specialized conditions. It drives a large-diameter bullet to respectable velocities, and that equates to hard-hitting killing power.