As it was explained to me by John King & others, the difficulty w/ re-chambering a .45-2.1" (AKA .45-70) to a longer cartridge is the set-up. First of all, the original chamber --hopefully-- will be in line w/ the bore. If it's not, that will only add difficulty to a seemingly easy task. The longer chambering reamer pilot must then fit the bore to perfectly & concentrically cut the new chamber. It is a job that requires some time, the right reamer, & the right touch. It's real easy to cut the chamber too long....
As for the throat, lead, etc., I'd discuss that w/ whomever is doing the work & then get 1or 2 more opinions. If you do have it re-chambered, have the gunsmith make a chamber cast & measure the bore so you can get the right case length & bullet dia. Will definitely save time & frustration down the road.
If you are contemplating re-chambering a rifle to a larger chambering, give the overall wt. of the rifle vis-a-vis the calibre some serious consideration.
I have a John King re-built 34" barrel, 14# Borchardt, .45- 2 7/8" (AKA .45- 110) w/ a shotgun butt. It does get tiring after 'bout 30+ rds or so. Lets me know real quick that I have touched off 95-100 grns of BP w/ a 535-540 grn bullet. & the larger chambers seem to shoot much better w/ heavier bullets. The heavier calibres in a light rifle or one w/ too much drop &/or a crescent type butt stock can be much more uncomfortable.
Short Stake, I think a .45-2.4" is a better cartridge. Prior history & today's match results seem to bear this out. The Sharps .45-2.6" was a very short-lived round. I follow the match results, & I don't recall seeing too many .45-2.6" rifles mentioned or in use. Of course, there are ,matches that I could have overlooked. I've talked to to a few .45-2.6" Shooters & seems it can be a rather persnickety round to shoot well. A few have gone the .45-2.4". Or throw all caution to the wind & get a .45-One-ten, which has more than enough steam for 1000+ yds & lets you shoot thru the wind.
John King--King Machine Service, Kila MT-- is a good gunsmith, but he is usually booked up. Same w/ Lee Shaver, the Pedersoli factory representative. Do a search for "John King" & "Lee Shaver" in this chatroom. Their addresses, etc., have been posted here in the past.
The problem w/ the Remington pattern Rolling Blocks & the longer cartridges is that often times the hammer needs to be "bobbed" to allow the longer cartridge case to clear the hammer nose.
