You are quite right about barrel harmonics and vibration patterns. The Number 4 was designed for a full-floating barrel, action bedded solid, bearing-point about 2 inches long from 2-1/2 to 41/2 inches from the muzzle with 4 to 6 pounds of upward pressure.
The old SMLE was bedded very similarly but with the forward bearing-point 2 inches long and right at the muzzle. There was downward pressure on the barrel at about mid-point from the Inner Band; this served to damp vibration there. The Canadian Bisley shooters who were using SMLEs in the 1000, 1100 and 1200-yard matches used to cut the Muzzle Reinforce back to ONE inch, although not all did this. Their record in the score-books is pretty impressive, so I generally do it when I am playing with an SMLE, especially when it doesn't shoot as well as it should. Otherwise, the barrel of the SMLE was floated, also.
You are quite right: a LOT of commercial stocks are inletted for a "drop-in fit" regardless of what the RIFLE may want. You can tighten things up very nicely by bedding the rifle into Aca-Glas Gel or something similar, but it MUST be non-shrinking. I don't use the "parting compound" goop they give you: flammable, stinks, goes on too thick. I coat my parts instead with a VERY thin coat of lithium-based Grease. The resin does not adhere to it and I get a closer fit, which is important if you want the thing to shoot.
As to exact measurements, what I told you works. I am not qualified to remark otherwise. Depending on the rifle, you may have to adjust the sights for the MPI. If you are mounting a scpe, you will do that anyway, of course.
Most important points:
go slow and careful
don't get the fibreglass resin on the Cat, even if he DOES want to play with it
let her stand 24 hours before taking apart to remove excess FG
have fun.
Hope this helps.
When do we get to see pics? And a range report?
Good luck.
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