That could be done rather easily for an M-16. Now, how are you going to do that for a 155MM howitzer? And, there is the small thing about having a suitable range for demonstrating proficiency. Shifting the burden of certification from the government to the seller doesn't really address any of these things. There still must be a certification standard that ensures safe storage and operation. Considering the damage one can do with your own howitzer, I would suggest it should be rather high. Also, a howitzer requires a crew and that means crew drill. It's an awful lot more than just cramming a round in the breech and yanking the landyard. Things like setting powder charges, what to do if there's a hang-fire, laying the piece, etc.
I will admit that machine guns, M-60 size or less, seem to fall into a debatable grey area. M-16s and other full auto individual weapons fall on one side of that area and I see no restriction for individual ownership. You get much above a M-60 and I begin to believe that one should proceed with utmost caution in advocating individual ownership without restriction.