How can a gun be designated an assault rifle without knowing the intent of a purchaser or owner?
A "defensive" rifle can be defined in several ways or terms, most of which are interchangeable with "offensive" rifle. In the Army we have what are called DDMs or Designated Defensive Marksmen. They are soldiers—infantrymen or sometimes special forces—who are designated for their mid to long range shooting skill to carry accurized M16/M4 rifle systems (sometimes M14 variants) equipped with higher than standard issue power rifle optics and bipods. Their primary function within a combat arms unit is to provide mid to long range highly accurate
defensive covering fire as their fellow troops advance on the battlefield. Of course, their role can be offensive as well. I myself worked a year for a private military contractor in Afghanistan as a DDM riding "shotgun" on civilian contractor convoys. Thus is it could be safe and factual to define a
defensive rifle in the preceding manner, same as
defensive rifle could be any civilian carbine or rifle owned and set up by an American for the purpose of defending his home and family against intruders. Such defensive equipment as flashlights, laser sights, red dot sights, folding iron sights, etc. could be added to the rifle for this purpose.
An "offensive" rifle definition does not really translate well into civilian terms. In military terms an offensive rifle could be defined as a Main Battle Rifle or any modern standard issue service rifle. Or it could be defined as a Long Range Personnel Target Interdiction Rifle or, in laymen's terms, sniper rifle. This type of weapon would be any rifle of higher than standard caliber tailored to neutralize enemy soldiers at extreme ranges, usually 800 meters and beyond. Such rifles could be bolt action or semi-automatic, and could be chambered in a host of calibers from 7.62mm and .338 variants, all the way up to special purpose cartridges such as .510 whisper and .50 BMG.
But here's the "rub", right? A civilian could easily justify desire for ownership of an "offensive" rifle for long range hunting, defensive purposes, target shooting or competing in shooting competitions. Thus we have arrived back the interchangeability between the terms "offensive" and "defensive" rifle.
Suffice it to say any American who sets out to purchase or build an AR15 cannot automatically be accused of wanting to own an "offensive" rifle for some nefarious purpose. And even if they did intend to acquire an "offensive" rifle there's absolutely nothing wrong (or illegal) with that intent or desire.