Interesting issue to be wary of, I got my FAC issued for .22lr &.17hmr, decided to Go for a Sako quad with both barrels, having discussed it with my FEO that it would be acceptable. After a few weeks got a call and told that the .17hmr barrel only, was not permitted according to my FAC, and instructed me to place the barrel into storage at my nearest Dealer and apply for a variation to cover the barrel only! Had to send a receipt of the storage too.
variation arrived within a week and I picked up my barrel, that was a few months ago, BUT, just recently checked my FAC and realised that it stated ".17hmr barrel to purchase" not in possession!
I should have checked at the time, but effectively had a barrel "firearm" I'm not supposed to! In went the call and a variation straightway! Turns out when the dealer returned the barrel, he and I should have filled in the FAC as if I bought it, and sent the FAC in again??!
I'm told the variation is done and in the post, covering the barrel in my possession now, and I asked for permission to acquire an additional action, so will have two dedicated rifles! Looking for a quad action and stock.
it seems not all forces apply the same rules with interchangeable barrels, all is not clear as it may seem, might be useful for somebody in the future!