Re: Barrel harmonics and chronographs
Yes, they obviously do. The extent of the affect on POI and precision can only be determined thru trial and error on each rifle - one at a time. So the only way to know is to strap one to your barrel and shoot it. The lighter weight/contour barrels are more affected than the heavier contour barrels.
I don't own the strap-on style because I expect it to affect both the precision (group sizes) and the point of impact on my #3, #4 and #5 profile barrels. But they're so handy and reliable that they could still be worth it to simply obtain a few MVs.
I still set up my light sensing chronographs. I have three sets of skyscreens set up on a single rail that provide me with 3 velocities for each bullet fired. It's a hassle compared to strapping a MagnetoSpeed on the muzzle, but I get three (actually four with the Oehler 35 proof channel) velocities and I don't have to worry about the affect the strap-on device has on precision and POI.