Don is correct in using a sheet of paper in front of the target to determine if the head opens early or not. Now if your head also exits the back of the target, then a sheet of paper there will also determine if the head actually does open. You see, as the arrow stops or slows down drastically (as in the target) the blades will close on their own. The sheet of paper on the back will show you the cut as it exits, before the cirtificial force closes the blades. The sheet on the front does not have to be a few inches in front of the target as mechanicals will not have full diameter cutting on entry, but a uniform cutting hole.

Remember that mechanicals are not for everyone. There is a regular maintaince required with these heads. Rubber bands or O-rings need to be inspected and replaced when needed. Rubber will deteriorate in time when exposed to the environment, espically the sun. The heads which use clips (dimples) can only be shot a few times before the dimples will no longer have the ability to aply enough force to hold the blades closed.

Now if you get a good quality head, crossbows will not have any problem in shooting mechanicals ...... that is if you do your maintaince. I shoot the spitfires and when I checked them out, the manufacture told me (actually sent me their test results) that their heads are tested to 450 FPS and had no openings in flight, only on contact. But they also cautioned me about shooting the heads more then 5 times (that is their reqomended max amount) before replacing the clips.

Good luck.