Well, you found The Damned Crack and it is no more. That's good. What happens is that the crack lets go and the fore-end spreads, releasing the pressure against the sides of the action. For very best shooting, an SMLE action needs to be solid in the woodwork.
Fellow who taught me to bed SMLEs was 6 times to Bisley with the Canadianteam. He also coached the team and captained it once. What I try to do is pass on what he told me as I was working with an unfired 1918 Lithgow
which was shooting 14-inch groups at 100 yards.
The action needs to be tight in the wood. This means a THIN coat of glassbedding compound in the wood, then nestle the action down into it and wipe off anything that crawls out. Grease the action first with the thinnest coat you can of some kind of good, solid grease. I use Esso Unitol, that pretty blue stuff; it's easy to see and it coats super thin and it's easy to wipe off afterwards. Be sure that you get some bedding compound into the draws (what you call the 'lands'), as things have to be tight there, also.
The action should be solid for its full length and for the length of the chamber. Forward of that, the barrel should be free and you should be able to pass a dollar bill around it almost anywhere. There is a REINFORCE at the Inner Band and the barrel should be able to damp its vibrations at this point if the Magpie Screw (Inner Band Screw) is reefed down tight (which it shouldn't be, actually). This reinforce should be about half an inch behind and ahead of the Inner Band. There is also a reinforce at the MUZZLE. The factory made this reinforce 2 inches in length; the Canadian Bisley shooters reduced this to ONE inch.
Sgt. Bill Brown's rifle, set up like this, shot a 74x75 at 1100 yards at the PanAm Games in Jamaica, many years ago. My rifle, set up exactly the same, shoots 2-shot called groups from a dead-cold barrel of half an inch at 100 but, with me on the trigger, it does take a LOT of sandbags!
Hope this helps.
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