Even if I do see some merit in the idea that a gun can't be S5 if it isn't in the UK.
But to turn to "conversions". To me a "conversion" is an after-market affair. If a gunsmith were to take a revolver with a 12" barrel but a normal grip, and attach a bit of wire to the grip to bring it to an overall length of 24", I can see that that would be a "conversion", and that there would be good reason to disallow that, as it could easily be unconverted.
But then, of course, it would be the work of only moment with a hacksaw to "unconvert" these:

Just as with the same hacksaw one could shorten the barrel(s) of a shotgun.
So I struggle to understand what the aim of the lawmakers was when they made the "once a S5, always a S5" rule. (Yes, I know that only madness lies in looking for logic and reason in firearms legislation).
If one of those LBRs was deliberately or accidentally modified, my understanding is that it would become illegal because it would fall into S5 due to the overall length being < 60cm.
But if the law says that once S5 it's always S5, doesn't that mean that it could never be repaired and regain its legal status?
In the now-locked thread there was discussion of by whom, and at what stage in the manufacturing process, and where, a handgun which would fall under S5 could be modified and then be regarded as an S1?
At first glance the Alpha 357 Magnum LBR bears more than a passing resemblance to the carbine they make. I don't know if they also make it with a normal pistol grip, but if they do where/when/how does the one with the extension piece get made to make it legal in the UK?
Could S&W add an extension to this before it leaves the factory and have it be legal in the UK?

(I bet there would be some takers
