1st clarification: No beat cop has or has access to weapons of war with the lone exception of snipers. If you're talking about AR15's, they've never been used in a single war by any country on earth, full stop. They're not "weapons of war". The exception is going to be SWAT, who likely DOES have actual "weapons of war" like MP5's, UMP's, P90's, M4/M4A1's, etc. Cops in many agencies DO keep an AR15 in their patrol vehicle in the same way that they used to keep a shottie for those "just in case" moments but those are semi-auto and, thus, are not weapons of war. Also, the majority of gun owners in this country are not "gun enthusiasts" by any stretch of the imagination and there's extensive, non-partisan polling to prove that.
Now...
When you get robbed, is there a cop on hand to stop it or protect you? No.
If someone breaks into your crib, is there a cop on hand to stop it or deal with the intruders? No.
Police in this day and age are purely reactionary. The only time they can somewhat prevent crime is by their constant presence i.e. frequent neighborhood patrols, which they largely don't do anymore. This means you're gonna get robbed, your house is gonna get broken into, someone might get raped, etc unless you have a firearm or some other means of self defense and if you do have a gun on you, why shouldn't it be the same type and have the same capacity that a cop uses to stop those very same criminals?
The average cop in this country carries a Glock 17 or a 19 with a 17 round mag or 15 in the case of a G19 and the AR's they use have standard 30rd mags. Why limit the citizens from having that for their defense as well?
Also, you're limiting this to "home defense", ignoring that the majority of incidents where someone would use a firearm occur outside of the home. Likewise, most rifles that shoot "high power" calibers are hunting rifles. The .223/5.56 cart that the average AR shoots is a medium power, intermediate round, not a high powered round like the .308, .350 Legend, .30-06, .300 winmag, etc and is unsuitable for anything other than dispatching coyote, wolves, or similar sized animals.
