I am in the process of building AR15 in 204 Ruger, Its close to the 5.56 but is a speed demon at close to or even over 4,000 fps. a great Vermont round. One of the positives is that its not very popular. good because you can probably get it at a great price. if you can, it would be a great investment. Plenty of brass available, lot of bullits. Go for it.There is one available at local pawn shop and it is real tempting.....