This is all speculation and hearsay.
@Frank Green can speak more on this, but Bartlein builds tons of ammunition test barrels that are shot out of a test fixture with excellent accuracy results from my conversations with Frank. Manufacturers use these fixtures with test barrels for bullet and ammo testing. They arent shooting a chassis/stocked rifle, BR rig, etc for their testing, all are out of a test fixture. If there was an accuracy problem with a test fixture, they would be using something else....
We will find out once I have the test fixture in hand, I work off data and facts, not what someone told me some time somewhere.
On one hand I would say yes there is merit to a rifle and having it recoil etc....and it can aid in accuracy.
On the other hand things like rail guns or in particular the accuracy fixtures used by ammo makers, bullet makers etc...where they clamp on the barrel and the action etc...just hangs out in the wind and is pretty similar to rail gun set ups.
I’ve seen plenty of the accuracy rigs shoot groups down in the .2xxs and do it consistently and I don’t mean a 3 shot or 5 shot groups. Typically 10 shot groups.
I’ve also seen accuracy testing done where X amount of rounds have to be shot in x amount of time and out to distances of 300 and 600 yards. For example a 20 shot group and all of the rounds have to be fired within like 1 minute.
It’s pretty amazing what these accuracy rigs can produce.
Keep in mind that a given lot of ammo might not even produce accuracy better than .7moa. Heck I’ve even seen box match ammo that wouldn’t produce 1moa. The bullet could, the case wasn’t an issue or the primer but could be a powder issue.
So they use the accuracy rigs to remove as many variables as possible. From the components to the human error as well.