[Joe_Snuffy]

While not an 18B [though I do aspire to be one], I decided to toss my hat into the ring and bring up some small M4 related points not mentioned:

The M4A1's selector switch can be set to Safe, Semi or Auto , while the M4 [note the lack of additional annotations to the model number] comes only with Safe, Semi, or Burst [3 rounds].

Anyone who is particularly tall, or just has long arms in general will have some trouble adjusted to the weapon's shortened size in comparison to the M16. Being 6' myself I've found that in the end just holding the magazine well paid off more than finding a comfortable place to grip the rails.

For those who have never tried it, installing the upper rail is a pain if you're not paying attention due to a small forked bracket who's forks sit on either side of the gas tube and hooks up into the star shaped ring in front of the hand guard retention springs. The easiest method to get it to slip into place uses gravity or a small object [like a thin pen, or small screwdriver] to nudge it into place. Because of the difficulty to install the upper rail system and the likelihood of damage from incorrect installations by users has led to the directive that user-level maintenance does not permit the removal of the upper rail system.

Because of the carry-over of the Picatinny Rail System to the M4 nearly all accessories designed for use the M16 are compatible with the M4.

Personally I've found both the M16 and M4 to be fairly decent weapons on the range when using standard-issue ball ammo [and with the proper amount of elbow grease applied to keep it clean], as for combat I can't attest for either one's attributes due to never managing to be in direct contact with the enemy [I did receive a regular showering of mortars/rockets my last deployment. Honestly rain is better, even if it leaves that syrupy mud in it's wake] so I'll leave that to those who have.