You will have to provide specifics to even have a slim chance of getting an answer. Who made the barrel? What weapon is it on? Where is the number on the barrel?
My guess is that the NM stands for National Match, a better quality barrel
Do you mean knockdowns?
Do you mean a hammerless single-shot, double-barrel, slide-action...? sales@countrygunsmith.net
The mean of a single number is the number itself!
A single number cannot have a mean - other than itself. To calculate a mean, you need more than a single number.
MT=Match Target
The mean of a single number, such as 444455566777778899910101010, is the number itself.
The mean of a single number, such as 121317, is the number itself.
The mean of a single number, such as 27528338, is the number itself.
A group of two or more numbers has a 'mean'. A single number doesn't.If you absolutely must have the mean of a single number, it's that number itself.
The mean (average) of any single number is the number itself. Note that normally you would calculate the mean of a larger group of numbers - though it makes perfect sense to talk about the mean of a single number.
The number of electrons do not match the number of protons.