That power's mechanical description is not totally clear.
Effect: You may fire at as many enemies in burst as you have bullets left in your guns. If you reload, this attack ends.
This effect line is fairly clear: if you have five bullets left in your gun, you can fire on five enemies. However, the Attack line says this:
Attack: Dexterity - 1 vs. AC, eight attacks
The "eight attacks" part suggests you get to attack eight times - regardless of how many bullets are left in your gun. Those attacks have to involve firing your guns because the effect line - which has mechanical weight - states this attack is performed by firing at the enemies.
Those two lines in combination suggest that if you have three bullets in your gun, you can target up to three enemies and distribute your eight attacks between them. In fact, it could even suggest you can make eight attacks per target (!!!!) for a total of 8 × [W] damage!
But wait! If you reload, the power ends! You can only take three shots, right?
Sure, that's true. You might imagine this means you have to stop after three attacks, but in 4e, simulation and realism don't matter. The mechanics work the way they say, sense and realism be damned, and it's up to you to work out how stuff makes sense. The mechanics never say you use one bullet per attack. They do however say you could potentially be making eight attacks against one creature with one bullet. Maybe the bullet ricochets a few times, or it's one super-effective shot.
This power is poorly expressed, in part because the author appears to have misunderstood the purpose and full utility of the effect, target and hit lines.
So what do I do?
Talk with your group about it and rewrite the power so it works the way your group (and the player of the Gunslinger!) agrees it should work. The minimum change to make it work might be replacing the last four lines with these:
Target: Any number of creatures in the burst up to the number of bullets you have left in your guns.
Attack: Dexterity - 1 vs. AC
Hit: 1[W] damage.
The effect line gets dropped altogether, since only the second sentence in it matters now and it isn't clear how the character could reload in the middle of using this power anyway.
You could add "up to a maximum of eight" to the end of the target line sentence, but if they have more than eight creatures in their adjacent squares, they probably need all the help they can get.
Strongly consider not using this class at all.
Pick an official ranged class to refluff instead. Don't take this for an expression of dislike for homebrew in general: some of it can be very well done. This homebrew in particular, however, seems to have ineffectively written powers, so this power is probably the least of your problems, and you'll have to tread with caution. The official classes are, by comparison, fairly mechanically solid for the most part, so refluffing one may give you the aesthetics you desire on top of good mechanics. You and your fellow players and GM might even agree to treat your bows or crossbows as guns, aesthetically.