I've got an old Hi-Wall that has been bored out and rifled to 38-55.I wanted to start reloading for it but goofed at the store and bought 270 GR RN interlock ( Hornady .375 ). I relise that this is a 375 H@H bullet, But is there any reason I couldn't use them?

You have at hand the source for your answers. Since it was rebored, you will need to determine which standard was used for the bore, and which twist was given it.
Probably you will find that what you have is quite satisfactory.
Cheers from Darkest California,
Ross

using a wax slug looks like the diameter is .374. I'm going to try 29 Gr of 3031 and 4064 each. Any guesses as to bullet performance/expansion? I'd like to find a load to use on deer and elk this fall. I'd really like to take an animal with this old rifle.

Scout,
The roundnose might work in the tube magazine, but most folks would discourage the practice.
I'm not familiar with the construction of that bullet, but most .375's are designed tough enough to hold together at magnum velocities. That enhances penetration at the cost of expansion, though a 3/8" hole is not too shabby.
The diameter should be no problem if you work up your loads judiciously. Old Winchesters had groove diameters that were somewhat large, but a current barrel will likely accommodate the standard .375.
You will grow to enjoy that cartridge if recoil (because of the stock design) does not become a problem for you.
The rate of twist will dictate the length of bullet that can be stabilized, the quicker the twist, the longer the bullet you can use.
I do not have a .38-55, but do have a similar .375 Big Bore, which I like to use for cast bullets.
Regards from Darkest California,
Ross

I got a few combinations put together and had time for a very informal test shoot. I was pleasantly surprised. Shooting at 100 yards from a sitting position on the ground 18 Gr of 2400 shot just uner the paper,7-8 inches low. 29 Gr of 3031 gave me a 5 inch spread 5 inches low. 31 Gr of 4895 gave me a 1 1/2 inch spread 4 inches low. (three shot groups) So Ive got possibilities for a very accurate load. What really amazed me though was the expansion. Fired into the dirt the 4895 load had a mushroom that came down almost halfway and maesured over half an inch in diameter. No problems and I love the balance and feel this old gun has when shooting it.

Scout,
It looks like you have a winner there.
In olden days we sometimes would flatten a roundnose bullet a little with a hammer or press to reassure ourselves somewhat that, in recoil, the bullet nose would not pound the primer ahead of it in the magazine. We could not detect any difference in accuracy at the short ranges we were using. That was so that we could use roundnose bullets in the .30-30.
Regards from Darkest California,
Ross

Ewin, doubt my load would work in a lever gun as the over all length is slightly longer than the max called for. Since I'm using it in an old falling block Hi-Wall (model 85) I have some lee-way. I will get ahold of some standard 255 Gr cast and jacketed bullets and give them a try as well, but may have stumbled onto a unique bullet combo for an old round.

quote:

Hi-Wall (model 85)

Oops, I forgot that.
Ross