2.1.8.2 Large caliber weapons:
Certain destructive devices may also meet the definition of machinegun because in addition to having a bore diameter of more than one-half inch the weapons are capable of fully automatic fire. ATF treats NFA firearms of this type as both machineguns and destructive devices. The weapons are coded as machineguns in the NFRTR with an annotation that they are also destructive devices. Any such weapons manufactured on or after May 19, 1986 are subject to 18U.S.C. 922(o). In instances where a weapon of this type is being transferred, it is imperative that State and local laws where the weapon is being transferred do not prohibit possession of destructive devices or machineguns.
Right. I have never seen one papered as a two stamp gun, and nothing in that section states that there is another stamp due; it is just annotated in the NFRTR. It is a machinegun, and also a DD by virtue of the bore diameter. Much in the way that MG status trumps SBR status when considering a single device. A MAC is still a one stamp gun, not one for being full auto and one for being short barreled. In fact, the only two stamp guns I have seen are either an unmarried SBR & mg conversion device combination, or something along the lines of the MP5SD mg/suppressor combination.
I witnessed this same discussion about that same MK19 about 10 years ago, and if I recall correctly, it was a one stamp gun. I think it's the only MK19 in the registry.