Exuma said
Does anyone know where I might just buy cast bullets that are 300gr .458?

I usually buy from Matt’s bullets, but it seems he doesn’t have 300gr. I also buy from Montana bullet works which is now out of business.

I don’t cast, and I would prefer cast bullets if possible

  

Your rifle will probably be happy with a 300-350 grain bullet. To be totally honest the 45-70 and 45-90 are the reasons I cast bullets. Lyman’s 457122 (Gould design) HP would be historically accurate. A gas check is not necessary for loads I’d fire in your 1886. Mike Venturino was a big fan of 5744 powder but unless you share his apparent love of bore scrubbing I’d try 4198 using a published load for BP level loads. Lyman’s Cast Bullet Handbook has a wider variety of loads, Mike Venturino was very much involved with that publication as well. There are some very good commercial bullet casters out there but awhile back I spent hundreds of dollars looking for them, the first bullets I cast were better than anything I was able to find at the time. 

Mike

Chuck said
Sorry when I saw 45-70 I assumed it was a 45-70.  TR is correct you should use a 300 grain bullet.  I load these using 3031, 4759 and 4198.  Each of my loads shoot about 1,550 fps or a little less.  My data is from a highwall with a 30″ barrel.  Mike still has the data in his book.

  

Chuck –

Which of these powders do you use a filler with (in the .45-70)?

By the way, I get the lever resistance thing.  In my experience, Model 1892’s are generally smoother than Model 1894’s.  I have one M1892 .38-40 with an exceptionally smooth action.  It is so smooth, you can’t believe you just cycled it.  I had a friend shoot it and he had the same reaction.