I'm sure glad to see your post. I just got one of the same from Midway this week, too. I haven't taken mine out to the range yet 'cause I'm stuck indoors this weekend with broncitis. Yuck.
I realized they advertised this for an A3, but I put it on my A2. I've got a Bushmaster with A1 rear sights and a carry handle. I played around with it & it attaches around the slip ring just fine.
Any issues using this brass catcher? I sure am looking forward to not having to chase brass!