Early in the story, Rainsford has come back out on deck to have a smoke. While alone on deck, he hears three gunshots.
Somewhere, off in the blackness, someone had fired a gun three times.
Rainsford is immediately curious, because he is on a boat in the middle of nowhere. There is an island off to the side of the boat, but the island isn't supposed to have people living on it.
Even cannibals wouldn't live in such a God-forsaken place.
Rainsford is curious because he wonders who might actually be on that island. He's also curious, because he himself is a hunter, and hearing gunshots always piques his interest about the shooter and the type of firearm being shot. Unfortunately for Rainsford, being super curious about the gunshot didn't work out so well for him. He fell overboard straining to hear or see something else.
While swimming toward the island, Rainsford heard another gunshot.
He heard it again; then it was cut short by another noise, crisp, staccato. "Pistol shot," muttered Rainsford, swimming on.
That tells the reader a little bit about how good Rainsford is as a hunter and weapons expert. He knows what type of gun is being fired just by hearing it (while still swimming to get to the island).
What most confuses Rainsford though is his discovery that the pistol in question is a small caliber pistol. A very small caliber pistol. He knows this because he found an empty cartridge on the ground.
"A twenty-two," he remarked. "That's odd. It must have been a fairly large animal too. The hunter had his nerve with him to tackle it with a light gun. It's clear that the brute put up a fight.
Rainsford is an accomplished hunter, so he knows what it means to put a target down. He knows that when hunting big game, a hunter uses a big round. Rainsford is confused by the fact that the brush damage to the area indicates a big prey, but the empty cartridge and recoil sound is for a small caliber pistol. That tells him the other hunter is very good, very stupid, or very brave.