The term "Dry-Fire" speed is a bit confusing but in flight shooting, high "Dry-Fire" speed simply means that the bow remains very efficient even with very light arrows. I believe that Dan Perry coined the term.
I have shot bows that could not get out of their own way even with if there were no arrow mass on the string at all. The drop off in efficiency with lighter arrows is very abrupt and the hand shock unbearable. A 3-4 ggp arrow in this bow would be little different than a dry fire.
On the other end of the spectrum, I have shot an unconventional bow design (modern materials), where I accidentally dry fired it. The string sounded like it cracked the sound barrier and sheared through a 1/8" steel protective plate before destroying itself. This is insanely high dry-fire speed. A 3-4 ggp arrow in this bow feels like the equivalent of lobbing a heavy brick compared to a true dry fire. Although this would be a very FAST brick! Even a 2ggp arrow slows down the string considerably with this kind of bow.
It is a lot of fun to see how changes in design an construction affects performance with very light arrows. It certainly helps keep bow building from becoming dull for me. There may only be a few fps difference between bows with heavy hunting arrows (10 ggp +/-), but the performance difference is magnified with light arrows. A great side benefit is that you will find that your target and hunting bows will improve based on what you learn from your flight shooting bows.
With regards to durability, a well made primitive flight bow can last a long time and I have seen many examples that continue to set records years after they were built. Dan Perry is a good one to ask about that.
There are a couple of design approaches to take to get a high "dry fire" speed. The first is to design a bow that focuses the bending area over as small a portion of the limb as possible. The inner/handle and outer portions of the limb should be stiff as possible. This forces all of the limb kinetic energy back through the string and into the arrow before it leaves the bow.
The other approach is to play around with "gear ratios". What is meant by this is that the design allows a long draw but the heavier limb moves as little as possible so there is little energy lost to accelerating the limbs in the first place. A compound wheel bow does this and there are some other unconventional designs that do this.
Regardless of the design approach, it is always good to keep the limb mass to a minimum and you use the lightest and stiffest string material you can find.
If you are interested in flight shooting and setting records, the US Flight Shooting nationals are held at the Bonneville Salt Flats toward the end of August. There are several Primitive classes as well as modern classes. It is a blast to be able to shoot far as you can without having to worry about losing your arrow.
-Alan
