Having a quick peek at the EE, the average AR up for grabs today is an M-4 based carbine, rather than a 20", and the most predominant bullets on the market are the 55 grain, rather than the 62 grain or heavier.
I would suppose that of the thousands and thousands of civilian owned AR's in Canada today, very few (probably in the hundreds) have been fired at targets in excess of 300 meters. (The last number I saw on Civilian AR ownership was approximately 50,000, a number dating from the early 90's which is now probably doubled.)
I'm not saying the rifles aren't capable of more, I'm saying they aren't used for more.
(Or, equally likely, a longer range is not available for use at the local shooting range.)
The 55 grain rounds typically don't perform as well at longer ranges as compared to heavier rounds, and given the comparative rarity of heavier rounds, I would say that most folks seem to be happy with the adequate performance of the 55's at shorter ranges.
So, it would be my suggestion that MOST shooters using the AR platform will rarely fire past 300 meters, and the average distance MOST shooters will shoot is probably closer to 200 meters, and many small/local ranges don't even have the ability to shoot past 100 meters.
The system is capable of much more, but with the swap and play capability of the AR System, there are so many variables that come into play that it's really hard to give a solid number.
Example: a 7.5" Kitty-Kat upper will be effective at shorter ranges (50-100m) but with two simple push-pins, you can swap on a 14.5" carbine upper that can easily reach out several hundred meters, and with another couple of push-pin, you can swap on a tuned match upper with a fancy scope that could drop 77 SMK's into a little bullseye at 1/3 of a mile or more.
Thinking of the "average" AR with the "average" shooter, using "average" ammo, and I will stand by my 200 meter guestimate.
Honestly, I feel that if you're consistently hitting an 8"x16" bull at 300 meters with an AR Platform, you're above average.
If you're consistently able to hit an 15" bull at 500 meters, then you're probably in the top 10% of civilian AR shooters in Canada.
Anyone shooting beyond that and getting regular hits/good scores is an exceptional shot.
YMMV, but those are my thoughts.
NavyShooter
(Of interest, I had a peek at the first few pages of the Black Rifle EE, and found 37 AR type rifles currently listed for sale, with an average barrel length of 14.9 inches between the 37 rifles. There were only 5 full 20" barrel rifles for sale.)