I figured this selection from "Balance of Power" explained it best.
What follows contains minor spoilers for the fic:
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Chapter 10 (about halfway through)

There were supposed to be 1000 balls of paint for each of them, and the hoppers held approximately 200 rounds. Ed had reloaded twice already, which meant 600 rounds. He estimated Hughes had only reloaded once.

Hughes should have been nearly covered in bright red paint. Ed looked down at himself and sighed. At least as covered as I am, he thought. But he wasn’t. In fact, Hughes barely had a mark on him, which only served to slam home the fact that Ed couldn’t shoot his way out of a wet paper bag. It didn’t help matters that Ed couldn’t sneak up on a deaf man, either.

He cautiously peeked around the corner of the shed, peering with one eye in the direction of Hughes’s last known position. He thought he caught movement behind the bush and grinned. Maybe I’ll finally catch him off-guard.

Ever since the game had started, Ed had been more and more in tune to the subtler sounds around him. It was self-preservation, after all. Hughes was one sneaky bastard, and the man seemed to take great joy in ambushing Ed. So when he started to creep out of hiding to take what he thought was Hughes hiding behind the bush and heard a barely-there sound, he knew he was done for. But he wasn’t going out without a fight, damnit.

Ed spun, gun at the ready, finger on the trigger; and was hit square in the chest. Again. He’d overbalanced when he reacted, firing wild, and fell back on his ass. Not a single shot hitting his opponent.

“Damnit! Why can’t I hit you?” Ed yelled as the other man disappeared around the shed.

“Because your aim sucks, noob!”

Ed blinked. He really has been here too long, he thought with a laugh.