Personally if I am seeing problems then I don't trust chrono readings from the muzzle. There's too much blast going on and if the chrono is seeing a pressure wave or looking through a pressure wave then it won't give you the right answer.
When I set mine up it's done 2ft from the chrono. To give you an idea of the problem we were chroning .38 and .44 subsonic live 6-7ft from the chrono.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep