If it is in 45-70 I would expect the bore to be .45 caliber that is distinctly larger than 8mm. Distinguishing between 8x50R vs 8x52R. Might be possible with calipersAlternatively, you can buy a Kit to make a Chamber casting, then measure the casting and compare it to cartridge specs.
CERROSAFE® CHAMBER CASTING ALLOY | Brownells
There are various products on the market which can be used to make a cast of a gun chamber. To be certain that the product we are offering you is the best and most practical, we contacted the basic manufacturer. They recommended Cerrosafe because ...www.brownells.com
If you click the "Learn" icon next to the Description, you will find "How to" info