I have a Bushmaster 16" M4 type upper with the A3 flattop receiver. Ihave an ARMS #40 BUIS under a TA01NSN. The TA01NSN has the flattop adapter (as supplied with the scope.) When I mounted the optic I found it hung up on the BUIS and could not fully seat on the receiver. I ended up having to shim the TA01NSN so everything would fit just right. Is this normal? Anyone else run into this problem? Thanks for the info.

I had the problem too. It touched the NSN ever so slightly. I solved it by getting an ARMS 19S for the ACOG. It cleared fine after that.

I knid of have the same issue, I have a TA31F(mounted on the TA51)over a ARMS #40.  I just put the ACOG foward one slot,  I still get plenty of eye releif, and the #40 is mounted where I zeroed it.  I called ARMS and asked if a #19 or 19S would allow the ACOG to sit a bit higher and further to the rear, they thouht not.  So I'm staying with the TA51 mount for now.  The rear of the TA31F and the back of the #40 ARMS BUIS are about the same.  Hope this helps.

http://home.comcast.net/~cws357/TA31F_002.jpg

Thanks very much for the input, guys. I appreciate it.