I'm giving this film one star because that extraordinarily dependable character actor David Warner gives the totally committed performance he always does, even if the material he's forced to work with isn't that great, and another star because at least it means well. At a time when WWII was long over but national service was still a thing that young men had to do, bored soldiers perform guard duties with no apparent purpose in Germany while wishing they could just go home (the titular Bofors gun is purely symbolic and barely appears in the film). Our "hero" is a useless petty officer who will do absolutely anything to get that posting back to Blighty, but ironically, the very night before he gets it, he has one more routine session of guard duty to perform, which happens to bring him into conflict with Nicol Williamson's completely bonkers private, who threatens to screw the whole thing up.
It's not a bad premise, but, this being 1968, we have to have it rammed down our throats that all authority figures are automatically wrong, or at best stupid, cowardly, inept, or all of the above, and the army is intrinsically horrible even when it's not shooting at anybody. The scenario would make far more sense if the characters were trapped in Hell forever with one tiny chance of escape, rather than stuck on a military base in Europe for a few years, because the actions of absolutely everybody are stilted in the extreme. David Warner very nearly makes his character plausible (and full marks to him for that), but every single thing he does is stupid, and often stupid beyond belief. Nicol "point my teeth at the scenery - I'm starving!" Williamson is so one-dimensionally insane, evil, and utterly out of control from the get-go that the script has to provide a completely random reason for him being crazier than usual right now to justify it being plausible that the British Army would have somebody this obviously nuts in it for more than half an hour before noticing they'd made a mistake, and Ian Holm is wasted as the voice of reason who isn't listened to.
I think this must have been adapted from a stage play - it very much has that feel about it. The whole "military service for a fairly short time is worse than death" theme is so overdone that everybody is excessively desperate, unpleasant, or both - nobody in the entire film is genuinely pleasant, well-meaning or sincere, except trivially and momentarily. And the one brief scene that perfunctorily attempts to justify the antagonist's relentless vileness made me laugh because it accidentally channeled Monty Python's "Four Yorkshiremen" sketch.
Overall, clumsily excessive in trying to make its point while being no fun at all on any level. By the way, if somebody is supposed to have gotten hopelessly drunk on Guinness (which is black) and been sick all over his uniform, shouldn't he covered in a substance that isn't pure white?