If owning an object was very likely to result in killing someone, it could prima facie be reasonable (though not exactly in line with libertarian principles) to prohibit that item.
No one really knows how many AR-15s there are in circulation in the US, but Wikipedia claims 10-12 million. Let's use the lower number of 10 million.
According to the FBI there are around 300 homicides with a rifle per year. Keep in mind that is ALL rifles.
If we assume there are 10 million privately owned AR-15s, and all 300 of those homicides are with an AR-15, we can conlcude about .003% of AR-15s are used to kill a person each year.
So 99.997 of AR-15s are used in a peaceful manner. That's not exactly cause for alarm.