For serveral reasons....
- As I've already stated (and you've avoided because you know you're wrong), guns are smuggled into this country all the time. What are you going to do, force manufacturers in Russia and China to implement your little scanner?

- Technology can be bypassed. Has there ever been a safe that wasn't cracked? Nope. Has there ever been an O/S that wasn't hacked? Nope. History has proven unequivocallly that if it was made by man, it can be bypassed by man.
- Even if your ignorant, naive, utopian dream came to fruition and the U.S. government passed a law requireing all firearms to have a Star Trek biometric scanner on them and they were able to enforce this law over foreign sovereign nations such as Russia and China - what do you do about the existing 6.4 trillion guns in the world that already exist junior?

Isn't it amazing how Dumbocrats never have a plan? Never. Not once. Everything they do is a stumbling, mumbling, bumbling, fumbling act of buffoonery. Their fiscal plans collapse cities (Detroit), states (California), and nations (U.S., Cuba, Greece, U.S.S.R., Cambodia, etc.). Their healthcare policies cause the cost to skyrocket and the availability to plummet (how is Obamacare working out libs?). Their foreign policies cause us to be mocked by our enemies, isolated by our allies, and attacked over and over and over and over (Benghazi, Boston Marathon, WTC I, U.S.S. Cole in Yemen, Embassy bombings in Tanzania and Kenya, etc.).
But, that's what happens when you form your "ideas" based on ignorant, immature, utopian idealism.