VSSF,
If I where to have the choice in rifles, I'd take the 6mmRem because it is a more versatile round than the .243. Don't get me wrong, the .243 is an excellent round and has far more rifle and factory ammo selections than the 6mm. Kyode's suggestion is VERY valid if you don't mind paying a little more for brass. You'll end up saving the extra brass money in powder if you shoot it much anyways. I can't speak directly to the 6mmBr, but I can the 7mmBr, and it is an extremely accurate round in a pistol. As far as the Ackley rounds go, save your money in this instance. The extra velocity attained with these rounds is almost always a function of increased pressures more than it is in increased powder capacity. I have worked with several of them in rifles in which the small increase in powder capacity would do far more than in a pistol length barrel, and it would be insignifigant except for the increase in pressure that is a result of having no reliable data at similar pressure levels as the standard chambering. The lack of trimming can still be a plus in a single shot pistol. The downside to Ackleys is that you need to buy slightly to grossly more expensive dies, and if you decide you don't want to keep it, it can be harder to sell even though you include you premium dies with it. You also have to spend one load fireforming all your brass, which could otherwise be spent developing meaningful load data.
As far as the 45-70 vs .450 Marlin debate, I'd take the .450 Marlin or a .444 Marlin. You need not determine, or guess, which level of 45-70 load data to use in this instance. Just use the .450 Marlin or .444 Marlin data that has been established by professionals using profesional grade equipment and leave it at that. All the hard work is done for you in advance. I know that part of the fun is in the experiment, but there are plenty enough variables (powder, primer, seating depth, projectile) to keep you VERY busy developing your OWN load for your OWN pistol without having to question the data available, or if it's suitable for your firearm. An added bonus of avoiding the 45-70, in this application, it's a great and accomplished cartridge, is that you will be able to buy factory ammo for the .450 that will be full power, this cannot be said of the 45-70. I don't know what your experience is with stiff recoiling handguns, but even the factory loaded 45-70's are far more than MOST handgun shooters/hunters can tolerate. Just because you can buy it doesn't mean it is the ideal tool for you. I realize the Encore has opened doors that where never concieved by Contender shooters, but you still have to be able to shoot it proficiently. If you intend to hunt with the pistol, keep in mind that a muzzle brake is not an option unless you are prepared to wear ear plugs and ear muffs to prevent permanent hearing damage in a hunting situation. I would never shoot my .284 XP-100 in the field with the KDF on it, and I would never shoot my Weatherby MkV .340 with the KDF in the field either. It takes dual hearing protection to prevent permanent hearing damage with a muzzle brake, and that is something you will not likely be using in the field, if any is used at all.
