I absolutely see how a grenade launcher would be potentially more lethal than a semi-auto rifle. However, my understanding is that both grenades and launchers are on the ATF NFA list and that to get either you need either an ATF approval and stamp, a special class of dealer license or maybe a munitions license from the state department. If you can get any of those approvals, I think you can also get weapons even more lethal. Also, maybe this is a bad inference on my part but due to language from other AWBs I expect the final version may read "capable of accepting...". This would be something like an M-203. Beyond all of that I'm not aware of any commonly available new rifle in production today that includes a grenade launcher that isn't already unavailable to the public.
Considering all of that, I have no problem with prohibiting grenade launchers.
Making a rifle easier to conceal is something to consider but I will point out that the average AR comes stock with a 20 -30 round mag and that 40 round extended mags for 9mm handguns are commonly available and, AFAIK, all handguns even have a pistol grip. They would be infinitely more concealable than any rifle with any type of adjustable stock. Having said that, I'd also be just fine with a law that restricts the minimum overall length in folded/compacted form of any long gun (shotguns included) to something on the order of 24 -28 inches.
As for grips, shrouds and threads I haven't a clue how any of that affects anything relating to lethality.
Thanks again. 
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