Even then I've seen this sort with the scope mounted to an extended mounting afixed to the barrel and reaching back over the action, not something you'd want for benchrest shooting I'd think.
Actually, I do this on a number of my benchrest rifles. I use a barrel block, with the scope on the block, cantilevered back over the action. There is a certain amount of extra work to use a barrel block, and additional weight. I've used this setup on short range HV rifles (13.5 pounds), and 1,000 yard Light Guns (17 pounds). It could be done with LV (10.5 pounds), but the engineering would require some pretty good planning.
BTW, use of a barrel block often makes changing barrels harder. Clocking the barrel is not as easy as you might think for rifles having a conventional stock -- i.e., not a rail gun. How hard it is depends on several factors. The easiest kind of block for clocking simplicity is a clamping V-block.
Is there any consensus about how a barrel that is "too loose" will perform? Will there be verticals, horisontal shots - or just all over the place?
Think it through. Who would have done the exhaustive testing needed to come up with the answer? All you're apt to get is urban legends -- what I call the "what some guy said channel." But in passing, Tony Boyer did an interview published in
The 1999 Precision Sooting Annual (think that was the year), where he remarked that he had a rail gun with a loose barrel. It was an inconsistent performer. He found the loose barrel just about the time the rifle had lost its high-competitive use -- about 500 rounds. This should tell you that a loose barrel won't always make its condition known in an obvious manner, or a Tony Boyer would have determined it quite quickly.
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To me, this thread illustrated a common problem with coming onto the Centerfire Benchrest forum with certain kinds of questions. I don't doubt that Old Gunner has seen cracked P-17 actions. And oddly enough, they *have* been used for benchrest actions in 1,000 yard competition, as they would take the Rigby-sized cases. But they're not typical; they are an exception to the general BR action. BTW, the one's I've seen all used barrel blocks . . .
And who has done the testing needed to determine if the method of hardening that steel, with the various loads on the threads and shoulder from both tightening and firing, would cause the cracks *only* from tightening, or if there were multiple factors?
In order to give any sort of reasonable answer in the short space of an internet forum, you have to make a lot of assumptions. You have to generalize. This all works better when the questioner and responders can minimize differences, and one of the best ways to achieve is to rule out non-benchrest setups.