I have to disagree actually. I don't think this is the only way to fix it. I also might not have been clear earlier, but in 1.20 you were able to take loose ammo from the level even when you had 50 pieces already. Much like carrots in jj1 where you can always pick them up even if you are at full health.

Removing the check on ammo limits would work; but an alternate solution would be to find where jj2 handles the packet for networked player picked up ammo and see if that code is separate from when the current player picks up a piece of ammo. Given the history of the jj2 code (1.20-1.23 this may be true but it also may not). I have not been able to locate where the incrementing of ammo takes place, I am also not sure how to go about finding it.

In the case that this is not a suitable fix it might be possible to add in a jump to wherever ammo amounts are decremented from firing when another player fires a weapon. However, the process of setting up the stack for what would likely be calling a method of an object could take a fair amount of code injection and a lot of very difficult problems to figure out first.