Firearms are very reliable these days. Jams are rare, although they can occur if the weapon is poorly maintained (not cleaned, or sand is allowed to get inside the gun etc), or if the user handles the weapon poorly (like I have done

). We can assume the Chosen One takes care of his/her weapons...
Most jams are not really "repaired", you simply cock the weapon again, that is, do the reloading motion - takes a second or so. The offending cartridge or an empty casing drops to the ground, and the gun is then ready to go again (new cartridge is in the chamber), assuming of course, that there were still rounds left in the magazine.
Stoppages can occur if a firearm is handled too "gently", for example when reloading, doing the reloading movement in a wrong way, the bullet does not enter the bullet chamber properly. When fired, I was able to get many stoppages on a real Glock 17 handgun that way in one training session, and that weapon can usually be fired thousands of times without stoppages. In some tests they even covered the weapon with sand, and it still worked like a charm.
I had a cut in my finger, and thus had trouble cocking the weapon, and I was a newbie, so that explains at least a little bit

.
If something breaks inside the gun (extremely rare), then it certainly cannot be repaired during combat. You need to disassemble the weapon, and field stripping can require a screwdriver or other tools, and perhaps a manual if you don't know the weapon. And it also requires concentration, possibly a spare part, something you can't deal with when someone is shooting at you

. Military weapons are usually simpler to open and maintain, as they (usually) don't need screwdrivers or other tools to open.
But no, while I have some experience with firearms, I am not an expert in these matter either. Anyways, if this was implemented, the chance of a stoppage would have to be very low if realism is desired.