Read Notice 1
https://www.gov.uk/government/uploads/s ... update.pdf
So £390 of 'Other Goods' 1 litre of booze 200 cigarettes. As to firearms related gear it used to be quite simple. If you imported a firearm you needed your FAC showing you had a slot for it. If it was a part that required proofing such as a slide, barrel, receiver, or frame then you needed a slot for it on your FAC. In both cases the FAC was being used as a form of import licence.
If it did not require proofing as a pressure bearing part you could bring it in so, magazines, scopes, triggers, scope mounts etc were only covered by the PERSONAL USE £390 limit. If you had multiple items the same such as 20 AK47 mags then it might well be considered a Commercial Importation with Duty and VAT due and LOTS of lovely paperwork. Believe me you do not want to go there unless you know what you are doing or have a good agent.
However since I left H.M. Customs, now Border Force, they are with Department for Business, Energy and Industrial Strategy (BEIS) (What used to be the Dept of Trade and Industry) tightening up on what requires an import license, so you may have to contact them (Please let us know how you get on) Google is your friend Type in 'what firearms parts need an import licence in to the U.K.' and you will get lots of useful information such as
https://www.gov.uk/government/uploads/s ... n-2016.pdf