Ok, the question is tricky. Maybe you have yet to figure out how to use Autofire. (If I'm assuming incorrectly, please take no offense)

In JA2 1.13 and its derivative mods, BURST and AUTOFIRE are two separate things. Some guns can do both, but most guns can do one or the other.

BURST mode is a set number of bullets fired at the target. There's a mechanism in the gun that makes sure you don't fire more (or less), so it'll be the same number of bullets every time. Very few guns have burst mode, although you can get a Trigger Group item to add burst capability to many assault rifles and SMGs (and even some semi-automatic rifles!).

AUTOFIRE mode is different - it will shoot as many bullets as you TELL it to shoot. Of course, it is hard to be accurate, and a merc with low experience level will usually fire more bullets than intended. Again, in autofire mode you'll need to specify HOW MANY BULLETS you want to fire. If you fail to specify a number, you'll only shoot one.

You can toggle to autofire just the same as you toggle to burst mode, but the cursor and the interaction will be different. Autofire mode will show you three numbers on your targetting cursor (when it is pointed at an enemy). The first (top) number is the number of bullets you're going to shoot (approximately). The middle number shows how many APs it's going to cost you. The last number shows how many bullets are left in the gun's magazine.

To increase the number of bullets you want to fire, simply right-click. This works the same way as adding extra aiming APs for a more accurate shot. Right click a few times, and you'll add more and more bullets.

Any more questions?