Typically, people who sell you a guarantee don't do it on factory ammo. They do it on "A" specific ammo, and that is typically FGMM. Federal uses the same bullets, primers, cases, and OAL. So building a rifle around those dimensions, is very simple. If someone squawks, then "Try a different lot of ammo", then it's the ammo's " fault".

And for $3,000 many who saved long enough to buy it, won't say anything against it anyhow.

I've shot several GA rifles, they assemble a nice product. But they don't have a sacred tub of rendered unicorn lapping lard. Any decent local Smith can easily do that quality of work, at a lower price.