Howdy
A few things.
It is not recoil that can set off rounds in a tubular magazine. The effects of the magazine follower slamming the column of cartridges back every time the carrier strips a round out of the magazine are much more violent than what cartridges see under recoil. Every time the lever is worked, and a round is fed out of the magazine onto the carrier, the magazine spring slams the entire column of bullets back pretty roughly. With a full load in the magazine, the spring is more highly compressed than with less rounds in the magazine, so those first few rounds the magazine spring is slamming them back pretty violently. Much more jostling than the rounds see with recoil. That is why you always want to use bullets with a 'flat point' in a lever gun with a tubular magazine.
All of these cartridges have Round Nosed, Flat Point bullets. Or Flat Point, Round Nose if you prefer. Left to right they are 45 Colt, 45 Schofield, 44-40, 44 Russian, 38-40, and 45-70. These are my Black Powder loads and they all have some sort of flat point on them. I shoot these 44-40 and 38-40 rounds in my lever guns all the time. I do not own a lever gun chambered for 45 Colt, if I did, that 45 Colt round would be perfect.
If you don't have a loading manual that specifies different bullet weights and shapes, you need to get one. All the powder manufacturers put out excellent reloading manuals. I'm talking about an actual book, not something you find on line, and not the freebies the powder companies give away. If you are going to be loading for your new rifle, you should have at least one manual that specifies many different bullets shapes and weights and the appropriate powder charges for each bullet. Off the top of my head, the books by Lee, Hornady, Speer, and Lyman are excellent. Do yourself a favor and buy one, it is well worth it.
Based upon my internet research, I've learned that with the 1873, higher pressures are preferable (to lower pressures) because at higher pressures the bullet case expands in the chamber and creates a seal that forces the gas and unburnt powder (soot) out the muzzle. But with lower pressure cartridges, gas and debris (soot) flow back into the action, gumming up the works.
OK, here's the deal with that. What you are talking about is Blow By. If the case does not expand fully before the bullet leaves the chamber, some of the expanding gasses will blow past the rear of the case and blow into the action. I have been shooting lever guns for a long, long time. A little bit of powder residue in the action is not going to 'gum up' the action. You would have to shoot a whole lot, I mean hundreds and hundreds of rounds, before enough fouling accumulated in the action to seriously impact how well the rifle functions. In Cowboy Action, we have guys who are always on a quest for the 'cleanest burning' powder, so their guns will not bind up and fail to function properly. We are not talking about fussy semi-autos here. I'm here to tell you that a little bit of fouling will not bind up a lever gun. Besides, the '73 is the easiest to clean of all the lever guns. One screw and the side plates come off, revealing the entire guts of the mechanism. Super easy to clean. Much easier than for instance a Model 1892, which is a pain in the butt to disassemble.
Here is a photo of my Uberti 44-40 1873 with one side plate removed, showing the internal mechanism. Super easy to remove the plates and clean the entire thing out from the inside.
Here is the inside of my 38-40 original Winchester Model 1873.
As the bullet leaves the case, if enough pressure has not built up to seal the case in the chamber you can get Blow by. This is particularly true of 45 Colt, because the brass is relatively thick. The brass at the case mouth of 45 Colt usually runs around .012 thick. 44-40 and 38-40, on the other hand only run around .007 thick at the case mouth. Yes, I have measured them many times. So the same amount of pressure that will seal the relatively thin case mouth of 44-40 or 38-40 may not work to seal the thicker bass at the case mouth of 45 Colt. That is why 44-40 is the darling of us Black Powder cartridge shooters. The case seals the chamber so well at the relatively low pressure generated by Black Powder that all the fouling goes down the bore, none of it leaks into the action.
I strongly disagree that you should be shooting maximum loads through a Model 1873. Very strongly. All you need to do is shoot mid level loads. The load I mentioned earlier, 7.5 grains of Unique under a 250 grain lead, hard cast, round nosed flat point bullet will work just fine in a 45 Colt lever gun. This is no powder puff load, it is a bit under maximum, but it should work just fine.
Also, use a heavy bullet. A 250 grain, or 255 grain bullet will delay exiting from the case ever so slightly longer than a 200 grain or less bullet. So pressure will build up a little bit more before the bullet leaves the case. Of course, use the appropriate data for the bullet weight you have selected. Do not use 200 grain data with 250 grain bullets.
Yes, ammunition sold as 'Cowboy' loads are generally loaded down a bit, but really not all that much. The guys in the winner's circle in CAS are loading their ammo so a 45 Colt recoils about like a light 38 Special. Commercial 'Cowboy' ammo is not loaded that light. There is no industry standard for 'Cowboy' ammunition, it is what ever each manufacturer chooses. I have not bought any 45 Colt in many years, I have been loading all mine myself for many years. I did win a box of 45 Colt 'Cowboy' ammo in a raffle a few years ago. Nothing to write home about, they certainly were not what we refer to as 'mouse farts'. Recoil was mild, but respectable.
Anyway, if you can find some 45 Colt 'Cowboy' ammo, that would be fine in your '73.
Plus, with modern powders and jacketed/plated bullets, cleaning is easier.
You Smokeless guys kill me. A little bit of soot or lead in the barrel and you think cleaning is difficult.
Be sure to buy a loading manual.
Both were thru and thru with minimal meat damage. Round balls from muzzle loaders have been efficiently taking game for a long time. I don’t see cast bullets being any different. Good luck on your quest for an 1873, they are fine rifles.
