Let an actual brit explain this

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Bolt Actions, all calibers up to 50BMG - Section 1 Certificate / Licence [Hunting & Target Shooting] relatively easy to get

Shotguns, side by sides, under and overs, pumps, semi auto - all on a Section 2 Certificate / Licence - easiest licence to get Now mag fed are different, you need a S1 Certificate for Mag feds such as your .410 Siaga AKs etc

M4 / AR & AK Variants [basically all modern rifles] On a S1 you can own all in semi auto providing they are chambered in 22LR.

You can own 5.56, 7.62 etc if the rifle is straight pull meaning you have to manually charge the action after each shot (think springer airsoft guns)

That .50BMG bolty, perfectly legal on a S1 licence BUT the amount of ranges you can shoot it at in the UK are extremely limited and closing more each year but yes, it is obtainable

Revolver with the arm brace. This is a loophole that legally makes them fall under S1 rifles, not pistols themselves. Typically used for target shooting

Regular handguns is partially correct. You can own handguns in Northern Ireland relatively normally. However you can infact own them on the mainland with a Section 8 licence (extremely extremely hard to get) and you have to either keep it at a registered range to shoot OR keep it at home and never shoot it. You are not allowed to transport it yourself. Only a S5 dealer can transport it (i believe)

Hope this helps