On the contrary, there is nothing "simple," or even legal in the US about what you are suggesting.
Here's what is simple, if you are caught with illegal possession of a firearm, you will be arrested, and the gun you have on you taken from you. Additionally, if you are convicted of a felony, you cannot legally own a gun in the U.S. The percentage of felons that give a **** about that law is probably, however, pretty low. And felons know where to get guns illegally. Thus taking guns away from felons in the US has very little effect on the prevalence of guns, especially among the people we least want to have them.
Having covered the felony case, let's consider misdemeanors. What if you are arrested for a non-violent crime and you are legally in possession of gun? Are you suggesting that that gun should be confiscated? That would almost certainly be illegal in the US. For better or worse, we have a Second Amendment that our current Supreme Court has interpreted very permissively.
Except that under US law, the police can't confiscate a gun you legally own (unless, I presume, you have just used it in the commission of a crime), and even if they could and did, you probably have more guns at home anyway; and even if you don't, you could easily obtain another one illegally.
So, no, no exception. You "simply" don't understand the situation here.
I don't really understand US law, so forgive me if I misunderstand.
1) I assume that this means state law not US federal law?
2) I thought police in at least some states could seize anything thought to be associated with a crime?
Civil forfeiture allows police to seize — and then keep or sell — any property they allege is involved in a crime. Owners need not ever be arrested or convicted of a crime for their cash, cars, or even real estate to be taken away permanently by the government.
Could not civil forfeiture be used to seize guns in this situation? I realise this doesn't stop them just buying another gun.