Rare military term for a rapid-fire anti-aircraft weapon; more commonly referred to as 対空機関砲 or simply 高射砲.
The notation '高射機関砲' can be found in documents of the former Japanese military.
More common modern term for an anti-aircraft autocannon.
General term for anti-aircraft artillery; 高射機関砲 specifically implies a rapid-fire automatic weapon.
Compound of 高射 (anti-aircraft) + 機関砲 (automatic cannon). The exact historical derivation is uncertain; the term appears in older military contexts.