Air hammers dont reach full velocity, but they do keep pushing. They dont need the "slap" of the spring to move metal. If that spring cant expand and open up the energy never gets released.
The beauty of making your own hammer is the ability to lower your anvil and step up the dies. It gives you that the versatility to be able to use tooling and not lose your blow. Beautiful thing if you know before you make it.
Helve hammers have the ability to "open " up, as I stated. I love Helve hammers.