I discussed the differences between the HF and HF2 in this thread on my local forum. Copying below:
Gun manufacturers who make higher power guns intentionally make their guns stockier in order to counteract the recoil produced by the higher band power (e.g., the thicker wooden stocks of Riffe guns). The drawback to this is that heavier guns are harder to maneuver. The HF/HF2 barrel designs represent an innovative solution by OMER to add extra stability to a gun
without adding lots of mass, thereby preserving the excellent maneuverability euro guns are known for. Whereas other brands make their guns beefier to provide stability, OMER figured out a way to achieve the same goal without making their guns heavier: the extra width of the HF barrel inhibits vertical travel, thereby reducing recoil when firing. Good idea, but it required some manufacturing innovation (the [ame="http://en.wikipedia.org/wiki/Hydroforming"]hydroform process[/ame]) and comes at a price.
A side benefit of the oval shape is improved horizontal tracking, though at the expense of vertical tracking, but this is specifically what provides the extra stability; vertical tracking not nearly is important as horizontal anyway (how often do you have a fish swim vertically?). Yet another benefit, which is actually really significant, is that the HF design and manufacturing process yields a barrel that is much stiffer than any other style of euro gun, even a carbon barrel:
This chart indicates that in OMER's testing, the HF barrel was 21% stiffer than a round carbon barrel with integrated track, and nearly 50% stiffer than a round aluminum barrel with track (for 100cm barrels).
As you know, the HF2 barrel shape differs somewhat from that of the HF barrel. I'm sure when we all saw the HF2 for the first time we instantly thought that it must be superior to the HF. Not necessarily so. The HF2 is meant to be a more maneuverable version of their hydroform design, while the HF is a more stable one. Mark Laboccetta explained to me that the length of gun you're after should determine whether you go with the HF or HF2. If you want a longer gun (110cm and up) go with the HF, as the extra stability and stiffness will be more beneficial with the more powerful gun, while maneuverability won't be as much of a concern since longer guns are usually used for longer shots on slower-moving targets. Conversely, since shorter guns are usually used for faster-moving closer targets, the increased maneuverability of the HF2 design will be more favorable when shopping for a shorter gun (100cm and below).
One other difference between the HF2 and the other Caymans is the muzzle (the HF shares the same muzzle as the carbon Cayman). The HF2 features a muzzle that has never been seen before in the U.S. market, but which has existed in the
XXV gun of the European line for a few years now.
The muzzle is meant to offer versatility, allowing four different muzzle configurations. If you follow the guidelines I mentioned and go with the HF2 in a shorter size, then the in-line single bands that come stock should be fine. Divers who want more range, though, who favor double circular bands and an open muzzle, should go with the HF.