Sorry-I tried to post this as a reply in an existing thread, but somehow posted it as a new thread. I will post it to the existing thread now.
Here is my experience with an Armalite AR10(T) with 20" heavy barrel. I traded for the rifle and planned to buy a 16" midlength upper one day. I already had a Leupold M1 3.5-10X and planned to mount it on the 20"T model, and then one day buy a TA11E for the 16" upper. I was trying to decide on which mount to get for the Leupold when I had an opportunity to get a TA11E at a good price. So, I was suddenly faced with the decision of which scope to put on the 20" target rifle. I decided to mount the TA11e on it until I had decided which mount to use for the Leupold (and raised the cash to buy a good one).
Now, several months later, I am absolutely in love with the TA11E and intend to leave it there until I eventually get the 16" upper. I have used a TA31 for years on my M4gery and am a big fan of the BAC feature. The TA11E is more of a great thing. The eye relief, field of view, close-in speed with the BAC and the bright image are just outstanding.
Most interesting is how capable the scope is at middle ranges. I have been shooting it a good bit lately at 300 yards and can hit an 10x12" (approx) steel plate from prone and sitting with boring regularity. From the bench I can hit clay pigeons on the berm. I am by no means an expert marksman, but I can make shots with this rifle/scope setup that I have never been able to make before.
Two things I have learned about using ACOGs at really close range is that it is a learned skill and a perishable skill. Inside seven yards you have to learn not to look through the scope but to look at the target and let the BAC do it's magic. If you mount the rifle properly the doughnut reticle will appear on the target and you will never notice the scope or it's magnification. If you peer through the scope at close range it won't work nearly as well. I thought I had that skill down, but I found that if you don't train frequently at close range you can lose the knack of employing the BAC properly. You don't have to fire live ammo to train; dry practice indoors works great.
This is the third ACOG for me. Each time, I agonized over the price but finally justified it by telling myself that I could always get my money out of it if it didn't work out for me. None of mine are for sale!