Here's my snap action underlever double rifle (a .450/.400 3.25" NE):

It's important to distinguish between the Jones system and the snap actions such as this. This thread started as a Jones system discussion, I think.

On these snap actions the lever pivots forward in the same manner as the lever on a Ruger #1 or a Winchester 92. It does not swing to the side as in a Jones.

These snap actions are based on the Purdey bolting system. In fact, the bolting is virtually the same as the original Purdey system with the "thumbhole" in the trigger guard.

On my rifle the lever also cocks the tumblers, so the barrels just fall open and require no downward force to cock the tumblers. It is fast, indeed.

Curl