Depends what you mean by badass. The requirements for obtaining a UK firearms certificate (FAC) amongst others are that you;have a good reason to own the firearm in question (that may mean being a member of a relevent club)are able to use it in an authorised place e.g. a MoD approved rangeHave an approved gun cabinethave no criminal recordare of sound mind (in the opinion of your doctor)and are vouched for by two upstanding members of the communityI may have forgotten something and I’ve not counted actually paying the fee! Obviously some of the requirements change depending on thegood reason. For example bone fide collectors wouldn't necessarily use their weapons so wouldn't need to access a range or be a member of a club.There are many good reasons which are recognised including collecting, target shooting and so forth. There is no particular reason why you should not get a FAC for a 0.5″ Barrett sniper rifle. In fact my local Gun Shop has one for sale.*NEW* BARRETT MODEL M99 BOLT ACTION RIFLE .50BMG F4218You would need to find a club that shoots such weapons and a range authorised for this calibre. We are a small island so the later isn’t so easy given the distance that stray rounds might travel.There is no absolute limit on calibre for a Class 1 FAC so it is possible (as part of a collection or as the centre piece of a militaria collection for example) to get a cannon on certificate or even a fully functional modern artillery piece or tank. Bone fide collectors are not normally expected to fire their weapons so actual use of such weapons is excluded.If you want a really long read on what is permitted in the UK read the following document;https://www.gov.uk/government/up...