Most modern knappers using abo tools use Moose antler billets. They have loads of mass and can be used for early stage biface reduction. And they work well, no doubt about it! The early stage reduction with these billets causes deep pitting and wear on the ends. As a result, most modern knappers constantly redress their billets using a heavy rasp. Most will shape the end to a hemi spherical shape.
A few years ago, after seeing what Marty Rueter coud do with hammerstones, I started using them and found them to be more efficient for early stage reduction. The past couple years I have been using hammerstones for early stage reduction to a W/T ratio of around 4/1 or so. At that point I usually transition over to an antler billet because it gives me more control. My antler billets no longer become heavily pitted but instead become faceted on the end, very similar to the wear patterns on my hammerstones. I stopped redressing my billets with the rasp and found the faceting actually improved the the billets versatility. I now use the flat faceted faces for most major thinning flake removals. With slightly isolated platforms the billets flat face makes full contact with the platform and allows good control and predictability of the flake path. I then use the ridge of the billet to remove a few small guide or isolation flakes to set up the next platform. The faceted billet gives me more control and allows me to do things that the dressed hemi-shperical shaped billets could not. It also greatly reduces billet wear.
I use to wonder how the old boys re-dressed their billets? Modern rasps work fast, but reshaping an antler billet with a chert flake or rough stone would have taken considerable time. The answer may be that they let natural wear do the job for them!
The billet in the pics below is my favorite billet. It is a good solid whitetail deer shed. I have not taken a rasp to it in months and it has an optimal shape created by natural wear. I use it for points ranging from 2" to around 6". ....jim