It is legal to have a firearm with no serial number under two circumstances:
1) It was produced before 1968 without a serial number (you cannot deface/remove the SN on one that had a serial number)
2) You build your own gun and you are not a manufacturer.
So if you build your AK or 1911 from a parts kit, there's no legal obligation for you to stamp it with anything except the caliber.
If the Made In Montana laws passed in a couple states (and being debated in others) manage to tell the ATF to go fuck themselves, then federal laws don't apply to guns made in Montana, stamped Made In Montana, and not transferred outside of Montana (or whichever state they were made in.). Meaning:
1) No barrel length limit, no overall length limit. SBR's and SBS' are good to go, so long as you made it yourself or bought it from a manufacturer who makes "state" guns.
2) Silencers, w00t!
3) Destructive Devices are go! Build a 20mm rifle, a recoilless rifle, or make a copy of an RPG-7 and your own HEAT rounds for it.
4) In states that don't specifically forbid machine guns (like Montana's law does), you can build unregistered machine guns. Fuck the ATF! Texas' version of the law doesn't forbid machine guns like Montana's does, and even if it did there's a loophole: in Texas, it has to fire more than TWO times per trigger pull to be a machine gun, not more than once. So a gun that fires 2-rnd bursts or a double-barrel shotgun that discharges both barrels simultaneously with one trigger pull would be kosher.
5) No serial numbers.
6) AOW's are even cheaper! Stick a vertical foregrip on your pistol and a detachable stock, it's good to go.
Note that guns with receivers made outside of the state with the state gun law and guns made in other states count as interstate commerce and therefore all federal regulations apply.