gstaylorg said:The MagnetoSpeed affects both POI and group spread with every rifle I've attached it to (.223 and .308). If you're not shooting 100 yd groups in the 0.25 to 0.3 MOA range (or smaller), you may not notice it. But it's definitely there; anything with sufficient mass hanging off the end of a barrel is going to affect the harmonics to some degree, this is simple physics and cannot be debated. If you don't notice a dramatic change in group size, it probably won't be an issue for you. But that doesn't mean it isn't there.
I (and others) posted a series of tests at another shooting forum (SH) on this topic quite some time ago. The best evidence suggests that the shift in POI is due to muzzle blast off the sensor deck. Any type of muzzle brake that diverts expanding gases away from the sensor deck will lessen the POI shift effect proportionally to how much gas is diverted. However, it will not alter how the device affects barrel harmonics, which will be proportional to the mass and position of the device.
Don't get me wrong, I love my MagnetoSpeed and wouldn't be without it. But I do not do load development with it attached. Rather I should say that my velocity rounds and group rounds during load development are done separately. I do not find it ideal in terms of time, effort, and reloading components. But I'm not willing to accept the results of both velocity and group spread with the device attached. YMMV. I'm looking forward to the debut of the LabRadar device, which seems like it may be the best of both worlds as chronographs go.
Out of interest - if you used Eriks' 100 yard approach for powder only (with magneto attached) - would you end up with the same powder node? While its not being argued that the group size is larger does it still reflect the same POI differences between charges and therefore the same powder conclusion is arrived at?
You may not have looked at the targets this way - I'm only curious since you have undertaken more testing.