You have to keep trying firing. If I'm not mistaken a merc's stats (which ones?) and gun's condition (?) determine how quickly this will get done.

Guns get damaged from repeated use but also whenever a mercenary carrying a gun is ordered to swim or is caught in the blast radius of an explosion. Guns with a condition less than 95% have a chance to jam while you're firing them, the lower the condition, the higher the chance. To unjam a gun on the field you need to keep trying to fire it. Anyone can try to unjam a gun but mercenaries with high mechanics stat have an easier time unjamming them.

The best way to prevent guns from jamming is to hire a cheap home mechanic, like Sparky or Speck and have her or him stay at the base, fixing your broken equipment all day long. Before long you'll have acquired and repaired enough good guns that you will be able to do a rotation, bringing only guns at 95% and above and fixing those that fall below 95%.

They jam on a 99% too. With a bad ammo. At least in JA1.

And just in case somebody doesn´t know, to repair, your mechanic needs to have a repair kit ready and will repair (so long the repair kit lasts, as it will be used up during repair) all equipment in his inventory.

AFAIR the mechanic needs to have the tool kit in his right hand, a spare one in his inventory will be used automatically.

No need to put a tool kit in the left hand, as repairing never deplete the tool kit entirely and you can combine tool kits to refill them in between missions. And if I recall correctly, a mercenary will only repair the items present in his left hand and his pocket vest. He'll first repair the item in his left hand, and then proceed to repair the items in the pocket vest, starting from the top pocket to the bottom pocket, meaning if there are equipment you want repaired as soon as possible, put them in the left hand or top pocket. You can stack items you want repaired too, for instance if you want to repair five grenades, you can stack them in the same pocket and once a grenade has been repaired, the merc will repair the second grenade in the pocket, and then the third, etc, until they have all been repaired, before moving to the next pocket.