So I have a mint 1886 extra light on lay away and am starting to get set up to load for it. I plan to only shoot lead in this one just bc. I currently have some 300gr cast from a friend and will be buying some hard cast from beartooth bullets probably their 350gr piledriver jrs. My question for those who load for their miroku 1886’s is what diameter lead worked best for you : .458, .459 or .460?
I have the same gun. All the Marlin 45/70s Ive owned liked .460 dia bullets so thats the diameter i tried first. I was pleasantly surprised to find out my 86 shoots amazingly well with that diameter bullet. I got a 350 grain Ranch Dog mold from NOE and it is a great bullet that functions well through the 86 action, and shoots good to boot. Most of the newer 86s have short throats, and some cast bullets wont work, so just be aware of that.
Congrats on your purchase. IMO the EL was one of the best configurations for hunting that Winchester released.
I’ve slugged a few Miroku 45/70 barrels and they came out at 0.457” - a bit tighter than Marlins tend to be.
I’d be more concerned about the bullet diameter ahead of the crimp groove and whether the bullet design has a front drive band. A thin band may work but probably best to have none at all and a bore riding design. The throating is non-existent as per SAAMI spec. In my Browning 1886, which also has the same throat treatment, I can load and cycle lead pills with an OAL of 2.8”, but to do that I need to use a bore riding design. A Lee FCD will help you fit a bullet to the throat if they are not quite right.
I started out with a Lyman mold # 457102, 445 grains, gas checked sized .459, there about. I've use Alox or what ever lube I could find.then settled on LBT Blue. I've used most any alloy, wheel seems to be the most common. Then I found a mold throwing, maybe NEI, 350 grain gas checked when I Decided I was not going to run into any Cape Buffalos or American Bison here in Virginia. I've found that IMR 4064 and IMR 3031 do well for full power loads. When you fire a hundred rounds of 445 grsbullets in an after noon,you have done a days work.
Before you load up a batch, make sure the bullet you choose will chamber. When you get up to the .460 diameter, it may be a tight fit.