I'm on my third AR right now and enjoy them very much.
There are MANY model configurations out there so I won't bother to address specific models but rather lend my opinions on features.
As an LEO you can get a short carbine barrel - 11", 14" or shorter - but why would you? You'd have a monster muzzle blast, a reduction in long range accuracy and a slight drop in velocity. Velocity is important for the .223 caliber. IMO, stay with at least the 16". Whether you choose a 4-position CAR stock or the standard A2 stock is strictly preference.
Twist rate - The 1:7 is typically used for Competition at long ranges because much heavier bullets are used, 69-80 grains. These heavier bullets require a highter twist rate to stabelize the round. A good general twict rate is the 1:9. You can easily shoot bullets in the 50-69 grain range with very good accuracy potential (1-2 MOA easily) if you do your part
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I suggest sticking with a brand that has forged lowers, not cast. If you get standard A2 iron sights, have a National Match front and rear sight installed. This will reduce the diameter of the front post and give smaller increments of adjustment for elevation and windage in the rear.
One other thing, as an LEO you can have "evil features" that a civilian cannot. Choose wisely as it is possible that you may not always be an LEO (for whatever reason) and would not be able to keep a model that is considered "LEO only".
My .02 Good luck. If you have any other questions I'd be glad to help if I can.
CMOS