Gun barrel length to string

[snipercup]

Summary

SUMMARY: none

Purpose of change

Contributes to #36110. I say that because #14825 introduced the barrel_length and it shows the properties as integers. Also, the documentation (JSON_INFO.md) suggests using a string.

Describe the solution

Change barrel_length property in guns to strings. The documentation tells me '9ml per inch is a good approximation'. I assume the integers represent the amount of inches of the barrel, therefor I multiplied it by 9 ml.

Documentation suggest 9ml per inch but it should be 250 ml per inch.

Describe alternatives you've considered

Not changing

Testing

The original guns:

Sawn off before the change:

Sawn of after the change:

Note that the results are the same.