What is a "weapon" ? A Class 3 Destructive Device? Yes. * Cannot legally own in most states, 6 months background check.
If you mean a machete or baseball bat, no.
A firearm? Depends , not all firearms, or guns, are "weapons". To buy a firearm, legally, If over 18 (21 for pistols) in most states, takes a background check, going thru a dealer; $28, and 3-10 day wait. Still, not "hard" if clean background. You must not have a felony or domestic violence conviction, or adjudicated incompetent ; w/o rights restored. A simple "criminal " background does not necessarily bar firearm possession or for life. Only felony convictions. or , recent indictments for a felony ( temporarily) or DVs; count.
In a very Few states, you can buy privately without a BG check or dealer. Even then, he doesn't Have to sell to you. He must legally use reasonable discretion; that is, not sell a 50 round mag to you if CA plates; not sell 50 revolvers to you in one day; ask for ID and refuse sale if you don't show it. This "gun show loophole" is a leftist liberal Myth. If in a state, like CA, that requires gong thru dealer and BG check, it Must be done at a gunshow, too. I Know; I Lived in CA 44 years; gunshows went from fun to really Orwellian , few I know bothered going to one in 2003.
Edit: I remember buying 3 guns from private sellers in my state 6 years ago, none knew me. previously, nor asked for ID. But, I bought a 1934 .22 rifle, a broken 1890's lady's .32, a relic of an 1890 Winchester that had been thru house fire, then dipped in a chrome tank. No "AK-47s" or such.
A Gun, a bb Gun, CO2 gun or antique (before 1898 non automatic) or any muzzle loader generally OK if over 18, no BG check, But very few muzzle loaders ** , Red Ryders, or $1500 1878 black powder Merwin & Hulberts ever used to hold up a bank in the 20th/21st century.
To buy black market illegally. may be hard. It will cost more, you have to make connections. Dealers are suspicious, if he thinks its a sting. might stab you. So, Very Hard.
* C3 DD is automatics, suppressors. short firearms, over .50 caliber firearms. Only Ok in a few states, requires lot of paperwork, restrictions. Even then, sheriff can deny as a Class 3 is a privilege, not 2nd Amendment right. I know a lot of Class 3 possessors. Ma Deuce, STEN, etc.
** legal in any state to own a sawed off, Muzzle Loading shotgun; a coach gun.