SecOf--
The M-16 series of rifles is somewhat inadequate for the job you are specifying. A .223 or even a .308 is more than adequate to keep someones head down in a serious firefight, but is limited to line of sight. What if your suspect actually gets into a house? The .223 and .308 rounds are normally too small to penetrate the walls and retain sufficient accuracy and power to take out the suspect.
My suggestion would be for a small group of elite security personnel. Divide the group as such-For the commander, who will also work the radio if backup is needed, he should have an MP-5 (preferably in the Supressed SD version-in case you have to take out a sentry):
[img]http://www.hkpro.com/image/actionksksd.jpg[/img]
Two of the members should have Barrett M-82's--they are more than adequate to shoot through walls if necessary:
[img]http://www.barrettrifles.com/images/model82A1.jpg[/img]
The final member should have one of these mounted on a rapid response vehicle. It is ideal for taking out the suspect if they get behind any significant cover that even the Barrett cannot get through:
[img]http://www.hkpro.com/image/gmgcover.jpg[/img]
I suggested each of these specific firearms for a reason. If you or your chief will write a letter--preferably on letterhead--expressing the need for neighborhood security (occasionaly, a little name dropping if there are any celebrities in your neighborhood doesn't hurt). Send a copy to the US State Department, Heckler and Koch, GMBH in Germany, Barrett in Tennessee, and the Treasury department of ATF, you can frequently get a variance letter from the government offices, and a "lend-lease" on the equipment (Colt doesn't do a lend-lease, which is why I would tend to skip the M-4) for testing purposes. Net cost to you--the stamps for the letters, and the ammo you would need (the HK GMG comes with 500 grenades--HE, AP, WP mixed unless you specify a different configuration).
One caveat--if you are having a problem with Girl Scouts in the neighborhood, you may wish to consider switching or adding a Combat Shotgun loaded with #11 shot for increased spread. Those green suited (perfect camoflage against the landscaping--did you really think it was accidental?), cookie selling (attempting to poison America via slow occlusion of the arteries) icons of Evil are too small and quick for the Barrett to hit them with anything but a lucky shot. A #11 shot will work every time (make sure the Brownies don't get behind you first).