I have a 6601 with a large hole receiver, and am considering replacing the A2 upper with a flat top... I was wondering if Colt ever offered the 6700 upper for sale. The 6700 being the 20" HBAR with flattop receiver... If they did, did they use a M-4 upper receiver with the m-4 cuts...? Are M-4 uppers with a large hole fairly rare? Any input would be greatly appreciated, thanks!

Dave

Yes, they offered a "conversion kit" w/ 1/9 20" HBAR, small hole flat top receiver and carry handle.

The one I have is not an M4 receiver, no feed cuts.

Large hole flat tops are pretty rare, only offered on the complete 6700 rifle for one or two years.