-assuming you had an 1886 solid frame in .33WCF that was destined to get a new barrel (please don’t belabor the reason why...just roll with it)

-would you go back with a barrel for .33WCF or would you go with a .45-70 barrel and convert the rifle from .33 WCF to ..45-70.

If I was already shooting it as a 33 and had dies, brass, and a cash of bullets, I would keep it a 33. If I just had the gun and was starting fresh I would go 45-70 to keep it simple.

I had both. I found the 33 WCF easy to reload once you found components. I tried reforming my own cases and quickly found that I just needed to buy them.
Recoil was less and still thumped deer. The 200 grain JFP is a fine bullet.
A bit flatter shooting, though the longest shot I ever took was 130 yards. That was with an open sighted Marlin 1895 and top end 300 JHP loads.
I was able to find components at Buffalo Arms.

Converting to 45-70 is just as easy as they take the same cartridge base. Both headspace off of the rim.
I remember a local gunsmith that had a take-down in 33 WCF and he built a 45-70 barrel section for it that allowed him to switch from one to the other.

Be different and go .475 Turnbull

Brass, bullets and molds for 33 Win, stick with it otherwise 45-70.
Keep the 33 barrel.

I would stay with 33WCF

45-70 rifles are being made every day.

If you don't care about a 33WCF, sell it and buy a new 45-70

probably have money left over, and make 2 people happy

I would do a 50-110!

I have a couple of 45-70's. I wish one were a 33.

My 1886 is heavy and I can shoot full power loads in it, no problem. My Marlin is very light and full power loads rap my shooting hand pretty good. My brother had a very light 1886 and he didn't care for full power loads in it.

What are your plans for this rifle? How heavy is it? Are you going to mostly cast bullets?

If you are going after deer, both cartridges will work fine. If you are shooting cast loads the 45 would be a step above the 33. If you are going after buffalo or big bear - definitely 45-70.

I'm doing a .50 Alaskan conversion on my old Win 71 .348. After researching .50 Alaskan vs. .50-110, I've concluded that the .50 Alaskan will do it all, whereas a heavy bullet in a .50-110 case may be too long for the action; whereas, that is not so with a .50 Alaskan.

Don't rule out my favorite 1886 round the 40-65 WCF. They make sweet shooters.

Go in between to 38-55. You can also shoot.375 Winchester if it is a newer action and strong enough..It will do everything the 45-70 will with less recoil..

What do you envision you will hunt with it or is it primarily shooting sports/range time you’ll be doing? Can’t ever go wrong with an 86 chambered for 45-70. I would be tempted to make it very portable with 20” barrel and full barrel length magazine.

.45-70 more supplies, more info available, will do all you need.

LOL. You've been running some weak 45-70 loads it appears unless you really believe the 375 Winchester is a good candidate for Brown Bear and Africa's big 6. Guys do that stuff with 45-70's. As far as the 375 Winchester goes I used to own one. It is a glorified 35 Remington

I think you mean 38-56, unless you are planning a total rework of the internals.

For the OP - if you already have enough rifles in 45/70, then a rebarrel in 33 Win or go big and build a 50/110.

A fast twist 45/90 is another option, but in reality in an 86, using bore riding cast designs, you can load a 45/70 to 45/90 length and performance. The 45/70 in an 1886 is pretty hard to beat for versatility, component and load data availability.

A 33 WCF in the 1886 will handle most game. A Nosler 210 gr bullet in the chamber and last round
will work even in Alaska. Hawk 200 gr bullets are amazing if you run short of 200 gr Hornady
bullets.

Ben Lilly killed grizzlies and mtn lion with the 33 WCF-factory loaded 200 gr bullets. Unless you live
in Alaska, the 50 calibers are not necessary and components are drying up. I have had both
in the Model 71-the 450 Alaskan is more accurate.