Please people, don't keep perpetuating myths.
Cut the film leader, because otherwise the film is liable to get caught in the pressure plate / film gate and tear some sprockets, as Juan just found out. I found that out the hard way myself also. Then the film chips can affect the travel of the shutter and really mess it up, or just outright jam it.
The ABLON template is the Leitz cutting guide. As with everything carrying the name Leitz, it is costly. You can do this with scissors in a few seconds without a cutting guide. Just find a ruler that is at least 4 inches (10cm) long. I actually have a Korean made Ablon copy. I didn't even know I had it, until I was clearing away a bunch of old darkroom equipment. With the guide and a scalpel, the film is cut in less than 5 seconds. I can usually load a Barnack under 1 minute with properly cut film.
Using a card to load film is quite a bad idea. You risk introducing grit onto your pressure plate each time, you screw around with the film gate with the shutter on "T" (for which you've got to remove the lens first), and you don't do the pressure plate springs any favors. Just cut the film for heaven's sake.
The Leica II style shutters do accelerate as they traverse the film gate. The trick is to get the second shutter to move a little less fast than the first curtain, so the overall exposure time is the same from one side to the other. It can be done, and Leitz's old standard was to be within about 1/3 of an f/stop from one side to the other at most. The Russian cameras FED/Zorki have the same design of shutter, and they can be adjusted also in the same manner (see the FSU header for a great sticky article on FSU shutter adjustment). If your shutter can't travel evenly across the frame at 1/20, then you need an overhaul.
Later shutters on the IIIF RD/IIIG have a separate cam for 1/1000 speed, but can also be adjusted to shoot at maximum speed without tapering. 1/1000 though, is the most sensitive shutter speed to show uneven shutter travel, by photographing the sky as someone else has mentioned. I like shooting at 1/1000 and 1/500 all the time, as I like to shoot with the lens wide open. It also minimizes any problems with hand shake when taking the image. What good is a camera that can't be fired at certain shutter speeds? To me, that means repair is needed.
I've used DAG or Youxin Ye as my destinations to send any newly acquired Barnack. They almost all need working on first, being at least 50 years old. My slides shot at 1/1000 don't show any change in exposure from one side to the other - after either of the two masters have overhauled the cameras.
