#GameMaker tips.
If you're making a sequence with several shots, make one separate sequence for each shot, and then make another sequence containing all of them. If a single sequence contains too many elements it can make the game outright stop working, but a sequence can contain other sequences.
Thread of #GameMaker tips from my experience.
First of all, never use decimals in object coordinates (x and y properties), even if the engine allows you, it breaks the collision detection. If you need them, round the numbers and save the remnants for next frames, but always move in whole numbers.