Buy a roll of opaque cooking paper or grease proof clear. Make a square to cover the aiming mark divide in to quarters with a pen line. Place a new aiming mark on the ET at a medium to long range. Fire three rounds into each quadrant of the aiming mark with just two of the total number as close to the centre as possible. Record the X Y coordinates for each shot if the log can not give them to you. From the record mark where each shot is on the grease proof cooking paper place this over the aiming mark for best fit. Things that you will notice. X does not mark the centre of the target. The acoustic centre is mobile as the day goes on and more tests are done The centre area could be up to 30mm wide. You will see this in time with more tests done. The size or mobility does not affect accuracy. The wider the shot is out from the centre greater than half way out you will notice the error is almost always in the shooters favour. This is also where you see if the mics are true. A wide error indicates there is a problem in closed systems. A wide error or constant error on an open system is the doppler effect when the mics are correctly placed. A good closed system will record inside less than half the diameter of the bullet some even better. It is all in the construction of the frame, timber used and age of. Frames do warp under tension, which is why we have moved away from tension to solid sound chambers. The first shots fired on a brand new target should be a test which is kept as the base line info for that target.