Re: 69 SMK success in 1x12
The optimal twist for the 69 SMK is actually closer to 1x10", and there's still some wiggle room to go a tad slower under the right conditions. I suspect that you've got several factors in conjunction here that were all (or mostly) working in your favor here. For one, that supposed 1x12" barrel you've got on the M700 could be significantly faster than that, perhaps a 1x11.5" or even closer to a 1x11". This is common to buttoned barrels, with the true twist being the result of the helix on the button itself, as well as the hardness and uniformity of the barrel steel determining the final measurement. In some cases this may work in your favor, in others, not so much. Luck of the draw here, and it sounds that this time it worked for you. You might try this again, but I suspect that your results here are borderline, and may be impacted by atmospheric conditions and location (altitude) in future shoots. Maybe it'll repeat, maybe not. Worth a try, but don't be shocked if you see different results down the road.
As for stability downrange, yes, bullets tend to become more stable, not less. If it's stable at 100, it'll remain so all the way down range until you reach the transonic zone. At that point, all bets are off, as some bullets make the transition quite nicely while others don't. The reduced velocity will work against you here, but again, if you're stable at short range, they will remain so downrange.
Hope this helps!