Is my BSA .303 rifle an ex-military conversion?

Is my BSA .303 rifle an ex-military conversion? I am writing from the US, and I have a sporting rifle in.303 British calibre. It appears to be a military rifle which has been altered to a conventional sporting rifle.

If I shorten a rifle barrel, will it affect accuracy?

Shorten a rifle barrel: I have a CZ BRN .17 HMR rifle and would like to change the moderator for one that does not shroud the barrel but would increase the overall length by 3in. If I had the existing barrel shortened by that amount, would it affect accuracy?