It seems that 8.8 mm is about .348 as in .348 Winchester. It would appear that you may have a .348 Winchester. Do not fire a high pressure .348 Winchester cartridge in the rifle unless the chamber has been cast and measured by a gunsmith!
"I have a rimmed, straight walled 9.xx"
The 348 is hardly straight walled. Base diameter of a 348 is .553, the 38-55 is .422, more than a tenth of an inch difference. A 38-55 would rattle in a 348 chamber. Besides a bullet fired in a .348 bore wouldn't come out with a diameter of .365
My
guess would be 9.3x74R, but it's time for a chamber cast before any more firing. There are several 9.3 cartridges that are possibilities.
