The UK's Finest Independent Forum for Shooters and Gun Owners. Est 2010.

I've read that going from .50 cal to .416 Barratt is fairly trivial (shared bolt, magazine etc.).

How feasible is this going to be (if required...). At what point does a rifle cease to be one calibre, and become another (legally)?

Receivers and chambered barrels are FAC parts.

When you remove the 50 cal barrel you will have an FAC receiver and an FAC barrel, two slots needed on the FAC and you no longer have an 50 cal rifle, the new 416 chambered barrel is another FAC part so another slot on the FAC.

When the chambered 416 barrel is fitted to the receiver then you have a 416 rifle for which you will need another slot on the FAC.

RFD gunsmith would be the way to go.

Are you going to do it yourself on your fac ? Then you'll need slots for all the parts and rifles and just let them know when you have a new rifle in a new calibre and what spare parts you need to dispose of.

If rfd doing it then you give them one calibre and pick up another, the rfd does the change on his books as he needs to.

Beat me to it, Dromia.
Just one thing to add, if you want to keep both barrels then you will only need one extra slot for a 416 barrel and the existing one for the 50 rifle. Even if you separate the parts and make a 416 rifle and have a spare 50 barrel you will still have only one 50 rifle and one 416 barrel, just not all necessarily in the same place.

I had a 308 rifle with a spare 6.5/260 barrel, on my fac and it didn't matter if I went hunting with the 260 and left the 308 barrel at home, I still had what I was allowed on my fac. I also have a plethora of straight pull AR style rifles in various calibres and I can swap uppers and lowers without any problems and effectively change calibre and serial numbers but all are on my fac so there is no licensing problem.