Some of the specialty ammo manufacturers like Buffalo Bore etc do make factory loads that are too hot for an ancient(or new!) trapdoor rifle....

I own an 1881 TD rifle and a 1880 TD carbine plus a more modern Browning 1886 carbine. I load all my .45-70 to trapdoor safe data. The only real issue I have is that the TD carbine has a bit larger chamber than the other two guns. The brass from the old carbine has to be full length sized to use again.

If you cast your own bullets the LEE 405gr hollow-base mold delivers a bullet that works great in any of my .45-70 rifles. I have a LEE 500gr spitzer type bullet mold too...works fine in the old trapdoor rifles but taboo for a tube feed lever gun.

I have used jacketed bullets in the old M1873 guns with fine results although purists will caution not to...Buffalo Arms sells a variety of .458/.459" bullets that work well...I use their 350gr round-nose flat point bullets for an easier recoiling load which all three of my guns like. Ammo for the Browning lever-gun require a good crimp so the bullets don't collapse into the case in the mag tube....not so much an issue with the old M1873 guns....however I crimp all my .45-70 loads more or less the same.

The old trapdoor M1873 guns I own tend to shoot well....the .45-70 is an easy cartridge to reload for. Factory ammo is easy to find which means brass is not an issue. You may encounter some Hornady brass that was loaded as 'Lever-Lution' or whatever trade name that is rather short(like about a tenth inch short) to accommodate Hornady's poly-tip spitzer bullets in lever-gun feed mechanisms. These I use with my Browning carbine and 400gr jacketed soft ponts...the Brownings have a short throat and that bullet in normal length brass cause problems in the Browning's chamber(bullets crimped in the crimp groove hit the rifling in that lever-gun).

If you don't reload then simply buy some .45-70 'cowboy' ammunition...not really cheap...but these are modest loads and save your brass because you will likely decide to take up reloading simply because of the price of the ammo!