I have owned three .45-70 rifles, a .45-90, and a .45 Sharps Express. What I've learned over the years is that bore & groove diameters are all over the charts and rifling twist rates can also make a big difference in accuracy. What works nicely in one will be disappointing in another.
Adding today's prices for factory ammo at $3 and up per round, the .45-70 is a perfect example of why handloading ammo makes a lot of sense.
I have 3 molds including Lee C459-500, 459-405, and 459-340. Depending on the alloy used the 340-grain mold will produce .459" to .461" diameter and 318 to 336 grains; the 405 mold drops .458 t0 .460" and 388 to 400 grains; the 500 gas check mold casts .459 to .462" and 470 to 490 grains. The harder alloys tend to produce lighter bullets of larger diameter (less shrinkage in the mold).
My rifles vary from .459 to .463" groove diameters and different rifling twists; slower twists usually work best with longer-heavier bullets while faster twists favor the shorter-lighter bullets. A reasonable compromise, IMO, is the 405-grain bullet that seems to give reliable performance in most rifles.
I select bullets for each rifle based to match (as closely as possible) individual groove diameter and rifling twist rate. My cast bullets remain unsized after casting and I lubricate them by hand with white lithium grease (any auto supply store).
My lighter loads are similar in performance to the old BP carbine loads (.45-55-405). I have made up some heavier loads (for the stronger actions) using both smokeless and black powder, but I find them to be excessively punishing to shoot without any real noticeable difference in performance on game animals. I have found that the 340 and 405 grain bullets provide excellent results on Colorado mule deer (175-275 lbs) and elk (450-600 lbs), in the 1100-1200FPS range. A full charge 500-grain gas check bullet will penetrate a large elk from end to end and break major bones along the way.
Suggestions:
1. Slug your bore or have it done by a gunsmith. A soft lead slug is driven through the bore, then measured to determine the actual diameter needed to perform best in that rifle. Ideally the bullet diameter should be at or about 0.001" above groove diameter.
2. Refer to manufacturer's site for rifling twist. If yours is less than about 1:30" you will probably do best with bullets around 300 to 420 grains.
3. A simple home reloading set-up will cost about $200, or about the price of 3 or 4 boxes of factory ammo. The expense will be recovered pretty quickly, then you will have a regular source of moderately priced ammo. With moderate loads the modern solid-head cases can last a long, long time. The Lyman Handloading manual will take you step-by-step through the process and provide pressure-tested load data to achieve best performance in your rifle.
4. Several bullet casting companies can provide cast bullets in the best diameter for your rifle and cast with lead alloys suitable for your intended velocities, and pre-lubed with high-tech lubricant for trouble-free performance.
Have fun!