Almost as Billy finished pounding on it, the door opened with a jerk. (sentence structure) What Billy saw inside the house made his heart sink. (Don't tell me. Show me what is happening, what he is seeing, how his body is reacting) In an instant (cliche) he saw a man crouched against an opposite wall, the man (a man/the man--repetitive) was pointing a weapon (what? how did he know it was a weapon?) at the open doorway. Billy shouted, “he's got a gun” and lunged away from the doorway. (usually you lunge toward something, not away from it; also, who's he shouting to? are there people with him?) When he (who?) dove for cover, (awkward) He crashed through the wooden porch handrail and he was unconscious before he landed in the bushes.
Critique: Need to tighten up your writing. You have some basic grammatical and punctuation errors here. You've told us everything. Show us.
Would I ask for more? No.