I shoot five shots through the LMBR2 take the average fps (normally 4fps spread with weighed pellets in a HW100 .20) then shoot through through it at 35 yards and take the 5 shot average .
Then use pyramid air calculator and feed results into IStrelok. Gives me a BC that works for dialling the swfa scope.
Obviously all the other info IStrelok requires has to be accurate for everything to work .