As of today, guns barely deal any damage, and very rarely contribute to any kills during sieges.
Shooting at opponents with a gun feels useless, they will just take a hit or two, move aside, and replenish their health by eating.
In rare occasions they may spend a splash potion but I've never seen any half decent player ever being killed with guns during the skirmishes.

Example: I was recently shooting at multiple people with a gun during a siege, they shot back at me, in case I take two or three hits in a short amount of time, I just move aside, eat, and go back to shooting. I believe it's same from their perspective too, if i land two hits in a row or a head shot, they just move aside, eat, and come back.

Guns are nothing but annoyance in this current state. They are not really capable of killing a player, reason being that walking into someone's direct line of sight while they shoot at you with a gun is nothing but a slap on a wrist.

So what's a solution?

In my opinion, there are two different solutions:
Obvious one being to buff the damage, this is necessary in both solutions:

1st Solution: Make guns kill people.
Double their damage, give them a chance to crit for the double of the doubled damage.
Normal body shots should have around 10% chance to crit, head shots 50%.
A headshot from a sniper currently deals up to 4 hearts of damage (on ideal range, and it is extremely hard to land it on a moving target from that far)
If the damage was doubled, it would deal 8 hearts, and kill a player instantly if it crits. (theoretically 16 hearts). Which would make head shots lethal in 50% of the cases from ideal range.

Of course this would change some things, I understand that minecraft is a game in which you make your own armor and it takes time to grind materials and XP and everything necessary to have it, but this argument against guns being able to kill is outdated, since we no longer drop armors if killed during a siege.

It would also be necessary to reduce the amount of points that kills drop if we were to actually make guns lethal, and I would reduce them by half.

Another necessity would be to make a plot type that allows nation players to respawn directly in the besieged town.

Implementing this would not only make guns useful but it would also add a lot of depth into the game. Sieges would be more than just rushing at a banner and who wins, wins. It would make kills more common and arguably PvP would be more enjoyable to play.

So it would not only reduce time required to kill an opponent , but also add depth to the game and make already existing features more functional.

2nd Solution is a less radical one.
We can just embrace the fact that guns don't really kill in current state of the game. They can still be used to assist allies as a ranged support during the fights, which was nerfed heavily since friendly fire was implemented, but still remains the only viable strategy to utilize guns effectively.

So embracing the current role of the guns without breaking a game could be done by simply giving them a short slow effect.
1-2 seconds slow 2-3 (if shot from further away than lets say 20 blocks to prevent cheesy close range kiting).

That way they would stop being almost completely obsolete without breaking the current way the game is played.

Both of those solutions are in my opinion better than just leaving them in the current state.