WARNING: THE FOLLOWING POST CONTAINS OUTSIDE THE BOX IDEAS AND LOAD DATA THAT DIFFER FROM TRADITIONAL RELOADING PRACTICES. USE AT YOUR OWN RISK
First up is initial sizing. This Jamison 45-90 has thick tapered walls that expand too much with the Lee expander rod, it will not chamber once resized. Simply size without the rod, chambers smooth and gives us an inside neck diameter of .444. Can't ask for better than that as our bullets are .446 to .447. After forming, trim to length you desire. Leaving the brass as is 45-90 length causes the mouths to crimp excessively on bullet seating even with the die backed out.
The original scope of this how to was to utilize a shop press with a custom machined die (pictured below) to swage the A base design into the 45-90. The die is perfect, but my press has too much slop and presses crooked every time. I scrapped the idea after 4 ruined shells. I will try again after shimming my press or just using a 1 ton arbor press. It swages quite easily.
My remedy to allow the flat base 45-90 rim to chamber properly and sit flat against the bolt was simple. Remove a little material from the ejector tip. It still has plenty of protrusion when pushed forward on extraction to kick the shell into orbit. It still works with proper A bases too. I don't particularly care for this fix. But.....this picture is in the retracted position and gives us a flat face for normal cases. THIS RIFLE IS A MIXMASTER AND NO VALUE WAS LOST HERE. If your rifle is a matching piece or you just prefer to mod another part, you can buy ejectors from Old Western Scrounger for around $12 on ebay and Gunbroker.
Now onto loading and fire forming. I experimented with 2 powders and various charge weights of each. The pictures below illustrate my findings. Your chamber may vary. These were all fired with 405 grain bullets. The H4198 worked great at all charge levels of 28 to 30 grains. I will be settling on 30.5 to 31 for my future range loads. They formed well except the main body area did not fully expand, this is an eye sore for me. The light loads of unique expanded the main bodies even less. I then thought, we're dealing with low pressures here. Why not fully anneal the whole damn thing head and all? So I did, and 14g unique gave 100% expansion. The last 1/4" of the head is solid brass and will never form.
That's it folks, pretty easy. I do recommend the full case anneal as some have had splitting issues. The Jamison is the cheapest .45-90 at $1 each but is by no means cheap, it has been great stuff to work with. I don't really like modding the rifle, i always prefer to mod the brass or equipment first. In this case no value was lost and I gained a lot of flexibility in being able to shoot the heck out of it which I fully intend to.