So, say I have ten friends with +1000 and you have 10 friends with +1000. Now we just killed all of you. Now me and my friends have 20 guns with +1000 and we can just give them to our 10 other friends?

Additionally, what does assembling your gun mean? How does one assemble it wrong? What is cleaning my gun? 

If people had to assemble and clean their guns, what does this add to the game and the user experience?

Is it difficult to clean and assemble? Or is it just additional button clicks for the sake of clicking more buttons?

Why would I ever not put the fire pin back in?

Now, if I were in the 1950s and carrying my gun around. How often am I cleaning it? How often am I taking it apart and reassembling it?

If Giorgio-Esposito can sell his gun, what's to stop him from selling it to a completely new user right before he dies, now that gun just continues on living with a completely new player? Why wouldn't he just keep his gun with a random crew member to keep it safe if he gets shot. Then he just takes it back when he needs to shoot someone?

I can't be arsed to fill out a shot report, do you really think I'm going to maintain a pretend gun? 

It was an example a gun has many parts, and if u dont pay attention and reassemble it correctly it will not fire.

Yes, it was an example. Now, given the many parts. If I am reassembling my gun, why would I ever not put ANY piece in? Is it time consuming and I want to shoot my gun immediately? Or is it super simple and just extra clicks and there is no excuse for anyone to ever not fully reassemble their gun? (In which case, what purpose does it provide beyond being extra clicks?) 

In real life you can let people hold ur gun and it is a daily thing where i live. 

So you do support, with this idea, me shooting you with my +9999 gun and then passing it immediately to Giorgio-Esposito to keep it safe incase I die? And then my +9999 gun just continues existing in the game forever?

Just suggesting that we make this game more realistic

How often does one clean their gun? How often does one disassemble and reassemble their gun? If they don't do it every time they use it, how much does that impact the next time they use it?

 and more things to do.

Should we tie our shoes or we trip doing crimes? Should we put on clothes or we get sick from being naked outside? What value does this add to the game beyond being additional (mandatory) clicks for clickssake?

And say u can buy Silencers wich can make u stealthy and raise % chance that no one will witness the kill. And maybe making the gun more powerfull.

I think you idea could be more impactful if you took more time to think about it before posting. Why not come up with a list of different modifications one could buy for their gun, a price, and what the mod would do to benefit them? Right now, it doesn't seem very thought through, or really necessary. 

If you really like the idea of people being able to buy mods for their guns to add new stats or whatnot, then take some time to really think about it and then come back.