Actually, I don't think this has anything to do with his knowledge of weapon training or weapon discipline.
If questioned, I expect him to be able to quote chapter and verse on weapon safety, demonstrate correct handling drills, everything he should know all about. I expect he would be able to state correctly the do's and don'ts of weapon handling (e.g. never point at anything unless you intend to destroy it, unload before entering buildings etc). He was, after all, trained to do just that in his primary role of airfield defence, and would have spent months and years doing that (and not much else!)
What I think happened here is that the guy was larking about and didn't have the wit to apply the rules he was all too well aware of to the situation at hand. If he'd been larking about with a broom handle or a bucket of water, no big deal. But a weapon, loaded or not, is a different kettle of fish. It's the "familiarity breeds contempt" situation. That, and a massive overdose of stupidity.
That he only got three years for this is surprising to me. If ever there was an example to be made, this was it. If he'd been a civvy, just having possession of the weapon would have got him 5 years - without it even being pointed at anyone (an offence), fired (an offence), shooting someone (an offence) causing GBH (an offence). There was no military purpose to what he did, so why didn't he get the 5 years as a starting point, add on the aggravating factors, and then a multiplier for the fact that he damned well DID know better. And a bill for the round, the medical treatment of the victim, and the cost of his prosecution and imprisonment.
****, indeed.