That's precisely how I started collecting military rifles; on the old UKshotgun certificate, starting when I was 17. There was no such complication as mag restriction then; if it had a smooth barrel of 24" or more & was not capable of automatic fire it was a section 2 shotgun. My first acquisition was an Enfield 1918 SMLE, & it wasn't long before I had a couple of dozen, including a Brandt 60mm mortar, a GrB 39, & other weirdies, although as things developed they all slowly went (to be replaced by others still with rifled barrels of course). Sadly more & more types of firearm have been taken from the general public & elevated to section 5 status ('banned' in simple terms) which could be relatively freely owned quite legally in the UK when I first started. Society isn't any safer but they're hardly likely to give us them back..............
Just having a nostalge!