This is likely to be something in the realm of late 19th Century black powder big game double rifle firearms such as the Holland & Holland. The cartridge is rimmed and centerfire and looks tubular. If the "end" is the rim width and is about 20mm, the cartridge tube is a bit less than that diameter and may be a "10 bore". These rifles were cited as "bore" much as period shotguns would, such as 10 gauge, 12 gauge, etc. on the USA side of the pond. These were used for hunting large African and Indian game. The "K" on the headstamp likely refers to Kynoch, a well regarded English ammunition firm of that era, even into the 20th Century. As Kynoch headstamps go, this is an unusual Kynoch headstamp, if it is a Kynoch cartridge. It is quite plain.
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Hope this helps.