Just wanted to throw in here, just for clarification, that there is no difference between the line of sight between the small a large app on the rear sight. Both are the same, the small app just give you a crisper, more clear sight picture, thus finer accuracy at longer ranges.
The 50 yard zero works great for carbines. Have both of mine zeroed at 50 and love it.
For fullsize 20 inch rifles I still prefer the 25 meter zero. Just remember, that if you use anything other than the 25m zero on a fullsize A2 fixed handle rifle, you cant accurately use the rear elevations after zero, because they are set to work off of 25m. I've always had great results from 0 - 300 meters on a fullsize with a 25m zero.