On pistol grips:
Some people have gotten the idea that pistol grips enable a person shoot rapidly and accurately from the hip. This is stupid, and a moment of thinking about the shape of your body should reveal why.
Here's a man hip-shooting with a pistol grip. Note the awkward posture and the rearward lean adopted to keep the wrist out of a ridiculously unconfortable angle.
It would be very difficult to advance, or even retreat, while firing in this position.
http://www.dortignac...dtrip/ORT14.jpg [Image display blocked; image is linked instead.]
Compare that to a classic straight stock. Note the far more upright posture and the far more natural angle of the wrist on the trigger hand.
Pistol grips are an ergonomic feature that makes shooting from the shoulder more comfortable by allowing a more natural grip on the firearm and allowing easy access to the firearm's controls. (safety/selector, trigger, magazine release, bolt release, etc.) They also allow the action of the firearm to be moved closer to the shoulder, making the gun easier to manage; particularly for smaller shooters with shorter reach.
On bayonet lugs:
Bayonet lugs are a means to mount a bayonet on a rifle, and they're pretty common on surplus rifles. If you're not sure what a bayonet is, picture a stabbing instrument on the end of a rifle, allowing riflemen to double as pikemen between reloads.
I've never really understood the argument for banning these, beyond "you have no legitimate need." I have no legitimate need for half the crap in my house, but it doesn't follow that it should be banned.
My rifles with bayonet lugs are functional firearms I've used for target shooting, hunting, and pest control. While I could take a dremel to the bayonet lug without hampering that functionality, I see no compelling reason to permanently and significantly alter a rifle from 1937. My gun collection is as much a personal museum as safes full of things that go boom.
On magazine capacity:
The idea that anything with a capacity greater than 10 rounds is high capacity, while anything smaller is standard, is ridiculous. If a Glock 17 was designed and sold with a 15-round magazine, that's a standard magazine. You have the option of buying extended magazines, even drum magazines, but these will extend further than the standard magazines and alter the balance of the gun.
These are Marlin M60s, . The top one has an 18-round fixed magazine and the second has a 15-round fixed magazine. These are classic rimfire rifles, and great starter guns for kids.
One significant problem with banning firearms that are able to take these extended magazines, never mind those able to be used with >10-round magazines, is that pretty much any firearm with a detachable, or sometimes even fixed, magazine is included. I can remove the fixed magazine tube on my pump shotguns and add an extension or drop in a longer tube; depending on the model, it might be a little more involved, but it's something that could generally be done easily in a garage or basement workshop.
Because a box magazine is basically a box with a spring and a follower, making an extended magazine is a matter of extending the box and spring. As long as the box is the right shape in the right places to pair up with the firearm, it fits. Getting the spring tension just right becomes more difficult with greater capacity, as it needs to be sufficient to push the last round up without binding or double-feeding early on. Because of this, extended magazines are not generally associated with reliability. They can be fun at the range, where a jam or a misfeed means little more than taking a few seconds to clear the gun, but I would think twice before planning to rely on them offensively or defensively.
On barrel shrouds:
If you're not sure what this is, that's okay; you're not alone. A barrel shroud is a shroud... around the barrel.
As you shoot a gun, whether dumping magazines as quickly as possible, or even just shooting after reaquiring the target, the barrel can get pretty warm. To keep you from burning yourself, some guns come with a ventilated sheet of metal wrapped around, and standing slightly off from, the barrel.
My understanding of the reason behind banning these is pretty close to bayonets, except that barrel shrouds do serve a utilitarian purpose for recreational shooters.
On grenade launchers:
This one is insane. Grenades have been restricted under the NFA since 1937, as have firearms with a bore diameter greater than half an inch. This feature is basically a cup on the end of the rifle that you can put a grenade into, then fire a blank round (no projectile) to create pressure and launch the grenade. Since grenades are already so heavily regulated and taxed at a rate of $200 per unit, prohibiting this feature does basically nothing except negatively affect collectors of war relics. There are also rod-type rifle grenades, which do not require any special launcher and would not be regulated under proposed legislation addressing grenade launchers.
I get the idea that this is added so the authors and supporters can point at anyone who opposes the collective package and say "Look, this person thinks criminals should have grenade launchers. See how silly and stubborn they are."
Legal fully-automatic firearms in the USA are taxed and federally registered. This registry was closed in 1986, and no new machine guns since then are legally available except to law enforcement or military. Some states prohibit private ownership of pre-86 machine guns which are legal under federal law if the guns are registered and the tax is paid.
New legislation at the federal level does not deal with this sort of firearm, regardless of assault weapon features.