It's neato to see a couple of folks with .45-90 rifles here on the Forum.
I have both a Springfield Model 1884 "Trapdoor" .45-70 and a Winchester Model 1886 .45-90. I've used the Trapdoor from a bench rest out to 400 yards with excellent accuracy. The Model '86 45-90 has been shot at targets of opportunity overlooking the length of a section of river. Repeat hits on relatively small rocks and logs seen up and down this section of river were not difficult to achieve after a few ranging shots. Neither of these old-time cartridges are slouches if one's rifle is sighted for distance. It does require a keen ability to estimate range along with knowing how to properly use and adjust one's sights.
Too much is made of flat-shooting rifles though a "stretched string" trajectory can be an aid when shooting at unknown distances. I did some comparisons of the .30-06 and the .270 Weatherby some years ago using information gleaned from the tables in the back of the 1978 Sierra manual. A now ex-brother-in-law used to poo-poo my .30-06 and wag his Weatherby Mark V bolt action at me in derision. Now the .270 Weatherby must be considered a very flat-shooting cartridge but the useful difference in trajectory between it and the .30-06 at all realistic and prudent game-shooting ranges is absolutely nil.
The .30-06 is a flat shooting round and provides a useful trajectory for 99% of distances that a prudent hunter is likely to engage his quarry. A .270 Weatherby Magnum using a 130 grain bullet with a muzzle velocity of 3300 feet per second has long been considered an extremely flat shooting big game round. When sighted to be dead on at 300 yards, this .270 Weatherby Magnum cartridge/bullet combination drops an additional 8 1/2 inches at 400 yards. A .30-06 with a 150 grain bullet and an initial velocity of 3000 feet per second sighted to be dead on at 300 yards drops and additional 11 inches at 400 yards. If a rifleman isn’t skillful enough to compensate for this piddly 2 1/2 inch difference in bullet drop at the extreme range that 400 yards represents, then he should have enough respect for game animals to refrain from taking long shots with any round. Another way to consider the relative trajectory of the .270 Weatherby Magnum and the .30-06 is that if each cartridge/bullet weight mentioned is sighted for 300 yards, the .270 Weatherby Magnum will be 3 inches high at 100 yards and 3 2/3 inches high at 200 yards, while the .30-06 will be 3 3/4 inches high at 100 yards and 4 2/3 inches high at 200 yards.
A .220 Swift comes closest to the "stretched string" trajectory of any cartridge I have and it whops up badly on the .223. But even the Swift bullet is a gasping flea-weight when employed at extreme range where its blistering velocity advantages have fallen away and all that one is left with is a light, small-diameter bullet.