Could be the way guns are defined in UK are different. There is a fundamental problem in US law specifically, that you can purchase legally nearly any part of a gun separately, but only need to register the lower receiver. These are parts that take very little stress and can be relatively easily printed and used to hold together all the other parts that actually hold the stress of firing the bullet.

This is at least true for some specific rifles, where there’s a whole industry around selling unfinished receivers that are relatively easy to mill down with common machining tools to be able to assemble unregistered rifles.

My guess, is that these bills are a knee jerk reaction to constituents who’ve seen some tik toks talking about this. Though the conspiracist in me thinks that it’s mostly an excuse for control. This means, this bill is also coming for the UK too…

> These are parts that take very little stress and can be relatively easily printed and used to hold together all the other parts that actually hold the stress of firing the bullet.

A lot of the polymer guns (1911, AR15) need to be reinforced with metal at certain places for any kind of reliablity. A Glock doesn't need to be, because the material was invented by the designer of the gun and the gun was intended to be a polymer frame from the start.

Lower receiver being the serialized part isn’t universal. Many firearms have only a single receiver or only the upper receiver is serialized.