There are only three ways to boost the speed of a bullet. One is to use more powder another other is to use lighter and lighter bullets or use a longer barrel (which at some point will have diminishing returns) to burn more powder. The most powder that you can burn is 150 grains. I wouldn' t even think about using a bullet that the companies used to get their advertised velosites on elk. The longest in-line that I am aware of is the omega with a 28" barrel. So basically using a bullet that is designed for elk isn' t going to give you a 200 yard gun.

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Bigbulls

You only listed 2 ways instead of three -- but I have the third.... Triple Seven Synthetic (or) Swiss Blackpowder. Both will generate around 15% more energy & velocity than Pyrodex Select/RS.

Elk bullets & short 22" barrels deliver way beyond 200 yards. Go read some of the threads on White rifles over at huntamerica.com. Using a 45 cal M-98 or 91 series, there' s plenty of posts of how 460 grained Powerpunch conicals have lifted deer right off the ground at 200+ yards. The 26-1/2" Omega barrel will do the same. On most new model inlines nowadays, you do not count the approximate 1-1/2" of the QLA System at the muzzle. There are no lands or grooves there so it doesn' t assist your bullet prior to exiting the barrel after ignition.
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Whok

Buy the 50 caliber! Most 45 cals are fussy little creatures. They usually take three times as long to develop a load & bullets & accessories for the 45 caliber are usually very limited in gun stores. When hunting game bigger than deer, the 50/54 calibers will perform better overall.