First off, the SBR and the pistol are not exactly the same gun. They are nearly the same gun, but there is both a slight physical, and a huge legal difference.
The SBR is the standard semi auto carbine, just with the 10.5" original Tommygun length barrel. The frame where the stock attaches is flat, and tapped for the stock attachment screws, the same as the regular carbine.
The pistol is virtually identical in appearance, except that the frame where the stock would have been attached is not flat, it is rounded, and there are no mounting holes for the stock screws.
The real Tommygun (full auto) had a 10.5" barrel, and some models had a detatchable buttstock (button catch) and others could be unscrewed.
Never heard of any gansters shortening the barrel of a Tommygun. They would sometimes take off the buttstock.
Bonnie & Clyde were noted for shortening the barrels and butts of BARs they stole from govt armories.
The NFA 34 was our first national gun control law, and as near as I can tell was passed to give work to those Feds put out of work by the repeal of Prohibition.
It restricted ownership of full auto firearms, short barrel rifles and shotguns, and devices intended to muffle the sound of a shot, commonly called Silencers.
Tommyguns, fell under the full auto section, even though they were also short barreled. In general, any stocked firearm under 26" overall length, or with a barrel under 16" (rifled) or 18" (smoothbore) is covered under the NFA. Also, a handgun with a barrel over 16" (and no buttstock) is also covered.
There have been, in recent years, some exceptions allowed, under curio & relic rules, but for many years, there were virtually no exceptions.
The semi auto Tommygun, is outwardly nearly the same as the full auto, but internally, it is quite different.
Basically, the semi auto carbine is a rifle, or SBR, leaglly, because the manufacturer registers the receiver as such. And the pistol is a pistol for the same reason.
All else being equal (and it almost never is) bigger bullets tend to work better.