Originally Posted by snipe1957
OK, with help from the people at Smith & Wesson,I am going to answer my own question.According to those guys,the difference in MOA between a 52g and a 75g round at 300 yards in a 1 in 7 twist barrel is approximately 1".I take this to mean that if the heavy round holds a 1"MOA then the light round should hold approximately a 2"MOA.Conversely,the difference in MOA between a 52g and a 75g round at 300 yards in a 1 in 9 twist barrel is approximately 1" with the light round being the most accurate.This is verbal information provided by Steve and his colleges at Smith & Wesson customer service.He says these twist are designed around the sweet spot of the .223,52g to 75g, round.
I don't think I've heard it described exactly like that before, but I shall not argue with the experts and S&W. The faster a bullet leaves the bore, the less twist is required to stabilize the bullet out to it's maximum useful range. With some "hot" 22 centerfire cartridges bullets will be turning at close to 400,000 rpms when leaving the barrel and twist of 1 in 14 or 16 is all that is required while the heavier bullets, which are typically used for longer range shooting leave the bore at a slower velocity and my require a 1 in 9 or even 1 in 8 to keep them stable at longer ranges.
There is a table at http://www.nfa.ca/content/view/129/197/
that will give you some idea of what I'm attempting to describe.
However the "tighter" twists that are required for heavier bullets also make the bullet more difficult to push down the bore and can therefore produce higher pressures. I think this is possibly what Steve at S&W may have been referring to as the "sweet spot". Less twist to produce higher velocities with lower pressures for lighter bullets, yet a "tight" enough twist to stabilize the heavier bullets for targets outside the range of a lighter bullet that will lose velocity much more rapidly and can be "moved" by considerably distances by even a 5 mph crosswind. At least that's my take on it.