I have used my Magnetospeed with both my 6.5x284 and 300WM. Both have barrels that are heavy, .78-.83" at the muzzle. Both show changes in POI and accuracy with the MS attached. The accuracy and ease of set up of the MS is superb though, and absolutely repeatable from day to day under any conditions. For this reason I will segregate my accuracy development from chrongraphimg which I did most of the time anyway. In my view, I'm still ahead of the game, and will save time and frustration over the variability I have experienced with my other chronographs. The key factor that needs to be observed with the MS is to make sure the unit is securely attached to the barrel, and does not slip during firing. This will effect readings. This can easily be addressed for heavy recoiling rifles by placing the extra rubber mount that is supplied, under the nylon strap on top of the barrel. When tightened the unit will not move. Unlike the optical chronographs that are set up 15' or more from the muzzle, velocities from the MS will give readings that are higher by 10-20 FPS depending on the load. This is likely due to the readings being taken off the muzzle. The exact value can be determined using a ballistic calculator. With this adjustment, the velocities from my MS match my Oehler 35 and actual velocity as calculated by using drop data.