[Rik]

My suspicion is that any variation is due to the form rather than the equipment. Pulling hard against the stops produces a different effect than pulling softer. That doesn't surprise me.

Are the groups better when you pull hard, or not so hard?

I'm a little wary of the Larry Wise take on drawing against the wall from all that time ago, I think it was fairly firmly rebutted at the time. Results since have shown no problems with using the wall as a draw stop.