I have a number of vintage .45 caliber rifles including Trapdoor Springfields in .45-70, Winchester 1886 in .45-70, and a Sharps .45 Express rifle that functions well using .45-90 Winchester brass (although the original chambering was a bit longer).
I have used Lee molds exclusively: #457-340F, #457-405F, and #C457-500F. Bullet metal is 50% wheelweights (old) and 50% Linotype metal. Results: 322 grain RNFP, 380 grain RNFP, and 470 grain RNFP-Gas Check. Bullets are lubricated with Alox, left unsized (as cast diameters are all in the .4575 to .4585" range with the metal used, which shrinks much less than softer metals after casting).
.45-70 loads I have used:
#457-340F (322 grains), R-P brass, large rifle primer, IMR-4198 28 grains, 2-grains loose Dacron fiber filler. This shoots well in the Trapdoors, and is very low in recoil even in the lighter carbine models.
#457-405F (380 Grains), R-P brass, large rifle primer, IMR-4198 28 grains, 2-grains loose Dacron fiber filler. The 1886 Winchester likes this load (2 and 3" groups at 100 yards are easy to do) and has taken mule deer and elk with it on several occasions. Second shots have never been required.
#C457-500F (470 grains, Lyman gas check), R-P brass, large rifle primer, FFFg 70 grains. This pretty much duplicates the rifle "service" load of the late 19th Century. Recoil is brisk, even in the full-size Trapdoor rifles. Accuracy is quite good with 4" groups at 100 yards easy to do from a bench rest, sitting or prone position. Metallic silhouette targets are a lot of fun out to 400 yards, huge cloud of white sulphurous smoke followed by a noticeable waiting period before the loud "clank" of bullet impact on steel. Used this load in the 1884 Springfield rifle to take a bull elk at just under 100 yards, uphill and quartering away, bullet entered just behind the rib cage, passed through the lungs, and broke the off-side front shoulder (about 4 feet of penetration and a fist-sized exit wound).
Some general rules that I follow:
1. Rifle bore must be absolutely free of jacketed bullet residue to perform reliably with cast bullets. This cannot be stressed enough. Good old Hoppe's, swabbed liberally and allowed to sit overnight, followed by a thorough scrubbing with a good bronze bore brush; 2 or 3 repetitions should remove any jacket fouling completely.
2. Cast bullets must be hard and strong, requiring a metal alloy that will stand up to the stresses involved. Cast bullet diameter must match groove diameter closely, and 0.001" oversize is better than any undersize diameter.
3. I never mix brass that has been used with black powder with cases to be used with smokeless powder. Cases fired with black powder are cleaned in boiling water with dishwasher soap, soaking overnight followed by several rinses with boiling water.
4. Smokeless powder loads that take up only a portion of the cartridge case capacity benefit from loose Dacron fiber filler, keeping the powder charge back against the primer rather than letting it settle as it might when the loaded rifle is handled prior to firing (consistent and complete powder ignition).
With the exception of the Winchester 1886 (made in 1916) all of my rifles are over 100 years old. All were well used before coming into my possession. None have perfect bores (the Winchester especially has some fairly heavy scaling and throat erosion, probably from corrosive primers). However, all shoot remarkably well with cast bullets that fill the groove diameter and are hard and strong enough to withstand the pressures involved (not particularly high by modern standards, but nothing to be sneezed at when accelerating a thumb-sized cast bullet from 0 to 1200FPS or so, forcing it through rifled bores from 24" to 32" in length).
I would expect that your Browning 78, with much closer tolerances in manufacturing and far better steels than anything available a century ago, would do very well with properly selected cast bullets. All of the loads I have given are considered safe in the weaker Trapdoor rifles, so your B78 should be able to digest some heavier loads if you (1) feel the need, (2) follow the well-researched advice in manuals like the Lyman Reloading Handbooks, and (3) exercise caution and good sense in developing your loads.
Have fun!
