Nobody in their right mind would rent you a weapon to kill yourself with and then take it back! They would stand a high chance of being charged with murder if not manslaughter. There arw however ways people make their own firearms with schematics freely available on the clearnet and dark net. They're not particularly safe and often involve the use of a 3D printer and buying unprintable parts legally. Ironically you can buy certain parts of fire arms online. Some of them need minor modifications to be used but if you're technically minded it's an option. Then there are other loopholes like in certain countries you can own certain weapons for killing local 'vermin'. These usually include shotguns and low calibre rifles IIRC. You can also own them for sport. I think you have to have a background check for any previous offenses before obtaining a licence. I don't know how long it takes to obtain your licence but I would guess at least a couple of weeks. Once you have your liscence you visit a sporting goods store and present your liscence and buy what you like. It's actually quite crazy how easy that option is in some countries. There was a time I was interested by this stuff as I enjoyed anything involving precision and intricacy but I never looked far enough into it to even think about applying for licences or anything. It was a brief bit of research mixed with a few chance video views online so unfortunately I can't advise you in any useful detail.
I have no idea how career criminals in places in europe get seazures of american guns and rifles. Obviously they come from somewhere. It must be quite sparae though because there's often repurposed old handguns and revolvers that look like they belong on a cowboys hip. I wouldn't want to go searching down those avenues anyway. Anybody offering to sell you a weapon is somebody I would be vary wary of meeting up with.
Our easiest way here is a shotgun with a license - just what is ideally needed. But many restrictions apply.
The making it yourself way is not easy without a workstation and power tools at least. No such things as 80% lowers here. You will have problems buying parts or ammo, because a license is always required and parts aren't in abundance like in America.
Sure guns can be modified, but by the same career criminals that do it for their own purpose, notable example are the chechen fighters in the 90s. Career criminals get their guns from ammo stockpiles off Albanian warehouses, or smuggle them through Turkey or Bulgaria - again not an option for ordinary people.
Sure you can go to some flea markets here in the east, or the balkans for cheap and get some very good albeit old stock, but you have to know who and where, and this information is nowhere to be found.
