Weco's right - with most rifles, it's not too difficult to rest the end on the barrel on some part of your body (head, chest, etc) and still be able to reach the trigger.
Most AR-15's I've seen have anywhere from an 18.5" to 22" barrel (though it could have been any length - these are highly customizable guns). The trigger is only slightly behind the breech of the barrel, so you can see how it would be easily reachable.

What is the point of rules that are not enforced?