They're a custom action maker. They date back to when Aaron started the company (several decades ago), and was eventually passed into ownership of Zermatt Arms. Bighorn started as "a Savage action the way it should have been done" but have since gone well beyond that. Their "Origin" actions are probably the biggest bang for the buck (what I have) until recently . And the new competitor, while very nice, is still relatively new ("Coup de Grace" by Ted's "American Rifle Company"). I'd wager the CDG's haven't made it across the pond yet, so it's probably a moot point.
Regardless, the Big Horn/Zermatt TL3/SR3 actions are hella strong, and elegantly designed. Your grandson' grandson will probably still be shooting it 150yrs from now. They're that well made. Their adaptability (interchangeable bolt heads) and their CF/mechanical eject makes them tailor made for short cartridges that typically won't feed well, or eject well (17FB, 221FB, 6Dasher, etc.). The nitriding of the action adds a layer of protection as well as gives them a silky smooth bolt throw. The bolts are also typically already fluted (though it is shallow, as bolt fluting should be). Great, stout actions.
And yes, they use a standard Savage barrel thread pitch, so you can either have a barrel fitted with a shoulder, or use a barrel nut and buy a pre-chambered barrel and screw it on yourself with a headspace gauge.
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