Stress relief makes a difference heat related zero shift, as does barrel length.
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Funny as it may be - you are correct sir. They were 'consistent' groups. :)
I watched a bunch of the Faxon Q&A videos and from those guys and the stress relief thing came up frequently but never any details about what kind of stress relieving they were doing and if they were doing it 'in-house' or sending stuff out or what. Very vague on any details.
https://i.imgur.com/WVBhnLR.jpg
The Colt barrel on top was done by the nice lady at Cryo Plus. The bottom one was done by Diversified Cryogenics.
Sometimes you can slip a barrel in with some tools / cutters and the receipt will look like it was all for business stuff.
https://i.imgur.com/DXM1XBL.jpg
Goofy test protocol.
Two different rifles, two different sighting systems, two different barrel lengths (free-floated), with two different shooters. No telling if the triggers were different, either.
Add in a completely new and different SP-1, probably with a GI trigger, non-free floated, over a shit front rest.
... and I'm supposed to consider the results a close, valid test.
Zeroing is the same with any barrel. It's either center of mass or center of mess depending on the quality of barrel and ammunition.