I tested it at the club with bisley magnums (i think, they were over 10 grains anyway), and the power dropped to 10 something.
This is the Si Pittaway video I watched, but he seems to have more options on the recticle view than the version of chairgun I have.
http://www.youtube.com/watch?v=dU3Rc5bVYjM
Sorry I thought you was shooting a PCP