In pack a punch you are enable to upgrade your weapon one time . So if you have all your weapons upgraded you won't be able to upgrade them again so you could kill easier zombies which will be realy useful at the high rounds. There could have been another machine near the PaP (Pack-a-Punch) which will do this for you but you will have to pay double the points of the pack a punch that means 10.000 points. Now your weapon will be stronger. Some weapons may not have a third upgade . There it could have been another machine that will link your two weapons (Upgraded or not) so they will only use one weapon slot. For example if someone links Crossbow with RPK His new weapon will be (those two choices are not for the player but for the game designer)
- An RPK with a crossbow like an attachment
- An RPK which will have for ammuntion crossbow arrows
( I think the first is more possible to happen)
How could you enter two weapons in this machine ?
-Well, you will put in it one of them and then it will give you the new weapon. Your second weapon slot will be empty so you could have another weapon. you want be able to do it again unless you have two not linked weapon
Judging by this and the other post you just made on buying and selling guns its likely you just read this http://tinyurl.com/diezombies then reposted the ideas in not so grand'a fashion. Sorry the original was better.