“OMG, how dumb. It only takes a quick look at the receiver to see if a weapon is capable of full auto fire. For barrel length, it just takes a tape measure.”
Hmmmn... I’m not sure if I should get into this because my mind is still being made up about the organization that I am about to reference. However, Jews for the Preservation of Firearms Ownership (JPFO), have asserted in many of their podcasts that the ATF’s protocols for inspecting firearms are undocumented and politically motivated. As one example, the ATF has repeatedly “proven” that common semi-automatic AR-15s are “assault weapons” capable of fully automatic fire. They do this by “inspecting/test firing” the weapon using ammunition with soft primers. This is known to make the AR-15 fire automatically as the firing pin coincidentally hits the primer on each newly chambered round. As with many semi-automatic firearms, this minor primer hit by the firearms is normal and should not ignite the primer. Using soft primers is also dangerous to the operator, dangerous to bystanders, illegal (only in this case because it causes full-auto fire) and tactically stupid so don’t even try it.
I believe there has been some attempt in congress to force the ATF to publish and follow their protocols during testing but I have no idea where that is. If you own an AR-15 and the ATF decides to send you to prison, make sure you don’t drop the soap.
Now... If you have a whole bunch of other factors as well, you can end up with a condition called "hammer follow". It's a malfunction that will allow more than one shot, but it will also destroy your trigger group in the process as it puts a lot of stress on the sear catch where the selector cam rides. For the record, I've never tried to induce this condition in any of my rifles as they are worth more to me in a fully functional and safe condition.
This is what got that po'bucker in Wisconsin sent up the river last year. He loaned it to a buddy who took it to a range that just happened to have a Feebie at it. Buddy malf'ed the rifle, and the rest is history.
The Feebies have a long and notorious history of fudging their "test" results. Even Randy Weaver's debacle may have started because of Feebie tampering. Randy insists to this day that the shotgun he cut down was cut to 18" legal minimum. What got him was the Length over all by a half-inch. Someone had sawn off part of the stock. Before or after he cut the barrel is unknown.
If you bet on the F-troop lying to make a case, you won't go broke any time soon.