Question about my SPENCER-HELP

[99savage]

I have an old Spencer carbine. I cannot figure out what caliber it is though. I measured the bore with my calipers and think I keep getting .500". An old family member of mine who remembers the rifle said he was pretty sure it was like a 56-50 rimfire. Would this be like a 50 cal. with 56 grains of black powder? Seems bacwards of how the old Win. cartridges are designated. They have the caliber first then the powder weight. I just do not think this is a 56 cal. carbine. Any help would be great. 99

NOT sure what calibers these rifles were even built in.