Does anyone have any ideas for a low resistance marking system for a ballistics pendulum?

I am trying to build a ballistics pendulum for a low powered airsoft gun. I've found that even felt tip pens will skew the results. I'm supposed to come up with an actual marking system for this project, I'm not allowed to just draw a bunch of points and "eye-ball" the pendulum height.

I had some crazy idea that there might be some type of exposure paper that is sensitive to red laser (laser pointer) light, which I could use for this application (resistance-free marking). But then I realized that if this were feasible, people would already be walking around with this paper taking notes with their laser pens, no? (Plus I googled it without results).

Some kind of lightweight needle that is pushed up by the pendulum and then stays in place would be nice, but again I think there may be too much resistance (I'm going to see if I can find a clock to take apart...)

Anyway, even though this isn't really a physics problem, since it involves a physics apparatus (the ballistics pendulum) I thought you guys might have some input!

Thanks!