Oh my .. here is how I zero all thermal scopes.
Since, I am using them for hunting .. I zero at 50yds ... that gives me a double zero somewhere between 100yds and 200yds depending on the gun and the cartridge. And between these double zero distances, typically the variation is rise of the bullet, is under 2 inches. And hence we are "flat" between 50yds and the double zero distance.
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So I use handwarmers for the targets ... and yes, handwarmers are too large for ideal ... but that is what I use. I shoot off tripod ... loots like this, except it is dark ..
I first setup a target ... like a 50yd target ... and then staple the handwarmer to the target ... then setup the tripod and the gun ... it takes the handwarmer a few minutes to get nice and hot.
Then fire my groups ... until I am hitting the center of the handwarmer.
Here first group was low ... second group was centered ...
So zeroing these thermals, is not the same (for me) as zeroing a precision rifle ... that I do at 100yds ...
But for hunting I zero at 50yds ... I do use the magnetoSpeed even for zeroing the thermals because I want to know where my double zero is .. and the magnetoSpeed tells me, without me having to walk out and shoot to find the double zero. Much faster and requires less ammo !
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And to zero any thermal scope ... I "chase the impact point with the reticle" ... so after each shot, I try to move the reticle (with the internal scope adjustments) so that the reticle sits where the bullet hit. And I keep shooting until I have a group centered on the handwarmer. Then I'm good. And before any real hunt, I recheck my zero.