This is not a bad thought. I think combine it with you actually need to be online to serve the sentence and a bunch of things you can do in prison so it isn't boring and that might have some positive affect.
To be honest this is something we already discussed many moons ago and worked on but never got around to finishing it. Mainly the adding fun things to prison part that can take a lot of effort.
Only thing against it would be:
A lot of people complain that guns take hours to get and seconds to lose which in some sense is true, if you make them lose it more often I'm not sure that is going to make it more enjoyable
So I did some maths on this last night. If you brought back gun crafting to gangs then it would work like this
4 People go out mining and smelting and come back after an hour with 1000 steel (Each person gets 500 ore and 250 coal and smelts it)
With that steel you could get on average 10 guns
Then you split the guns with the 4 people so each get 2 and 2 spare (Maybe the group sells them maybe the group stockpiles them and next time they do it they spilt the guns up as they will have 4)
So after getting on at 6pm restart. Getting people around and doing what needs to be done they then head to the mine at 7pm and mine. That takes us to 8pm
At 8pm they come back and speak with their group and decide to do crime each with a gun
Half an hour later they get caught and all sent to jail for an hour for attempted murder (rough number up for discussion)
They each lose a gun and ammo so 4 guns 400 bullets lost plus a 25k fine (Again up for discussion)
They get out 30 mins later after doing activities in the prison
They then have to fight a gang war so get geared up and go fight that
Now its 9pm
An hour later (10pm) they find and fight gang and lose (Police intervene and pick up said 4 people)
Each get the same charge again gun lost plus ammo (which takes time to get too) so again 4 guns lost and 400 bullets plus a 25k fine (Again up for discussion)
So they spend half an hour in jail again after doing prison jobs they are out
Its now 10:30pm Police have taken 8x guns 1600x Bullets and 400k from 4 people.
It then forces people to constantly be active on the server to want to do crime and also police get their wish of keeping guns off the street
Now imagine this on a gang level scale like nowadays
It would be 12 people. So you 3x everything
A gang then loses 24x Guns, 4800 Bullets and 1,200,000
Dont see how anyone loses here but maybe the crim. Police literally win this way instead of having put someone in for 12 hours jail and barely a fine and lose of 1 gun over a whole night. The crim can go out and do whatever he wants and go roleplay with people instead of stuck in jail for 12 hours. The cops are getting money and guns off the street. Literal win win for everyone but this only works if gangs get pistol crafting back
Looking back on this maybe the fine needs to be a little lower but thats what discussion is for.