Collapsible
stock: makes the weapon easier for two people with different-length
arms, such as a husband and wife, to use–or for the same person to use
while wearing either a heavy coat or just a t-shirt.

Pistol
grip: every rifle has some way for the shooter to steady his trigger
hand–usually, it’s the stock; the AR-15 has a high stock that’s in line
with the bolt, so it needs a pistol grip.

Detachable magazine:
In Ohio, using a magazine that holds more than 30 rounds converts an
AR-15 rifle from a semi-automatic weapon to an “assault weapon”–as if
holding more ammunition changes the way the weapon works. (Hint: it does
not; it’s still a semi-automatic weapon that fires one round each time
the trigger is pulled.)

Bayonet
lug: How many people have been killed by someone who used a bayonet
attached to the front of an AR-15? How many people even carry around an
AR-15 with a bayonet attached to it? I’d wager that the answer to both
questions is “zero”.

Grenade launcher: How many people have
been killed by someone who used a grenade launcher attached to the front
of an AR-15? Again, I’d wager that the answer is “zero”. (Note: the M16
I used in the Army did not have a stepped barrel; nonetheless, it would
accept a grenade launcher.)

Flash suppressor: keeps the flash
of unburnt powder exiting the barrel from affecting the shooter’s vision
in low-light conditions; used on the AR-15 because it has a shorter
barrel than other rifles–on rifles with longer barrels, the powder has a
chance to burn completely before exiting the barrel.