JimUCD:
YOU UNDERSTAND WRONG!
A "faster" twist - one full turn in a smaller number of inches is what is needed to stabilize long heavy bullets. The gyroscopic effect has to work harder on a long skinny object. Examples from .22 history:
16" twist - . 22 lr 40 grain; .22 Hornet 45 grain bullets.
14" twist - most .22 centerfires with 50-55 gr bullets, will handle up to 60 grain flatbase spitzer. Heavier if semipointed or roundnose, it is really the length that counts.
12" twist - early M16 barrel for M193 55 grain boattail in cold climate.
10" twist - I think Ruger used it in Mini-14 for a while, usually ok for SS109 62 grain and 68 grain.
9" twist - early .223 target barrel for 68-69 grain BTHP.
7", 7.5", 7.7", 8" twists - current .223 target barrels for 80 grain VLD.
7" twist - current GI barrel for M856 tracer, a 9" is enough for M855/SS109 ball.
"Overstabilization" of a light bullet in a fast twist is a bugabear seen only with very extreme differences. A very light bullet in a very fast twist might not be real accurate. You can actually blow up a bullet in flight by driving it too hard and an excessive twist for the weight will make it worse. But a little extra twist is helpful with mediocre bullets. A 9" twist will often be more accurate with cheap FMJ econo-ball than a 12".
My 9" twist barrels are accurate with everything from 50 to 68 grains and that is enough for me, but I am not a long range shooter who needs the really heavy bullets (see below.) I have a 12" .223 and a 14" .22-250 that are most accurate with a 60 grain flatbase and a 9" that prefers a 52 grain. The only way to know for sure is to shoot them.
An "understabilized" bullet will soon become an UNstabilized bullet and will not hit anything. A friend of mine has a .223 match rifle made when 68 grain bullets were the heaviest available. It has a 9" twist and is very accurate with them and has plenty of barrel life left. But it will not shoot the 80 grain bullets now preferred for the 600 yard line and is somewhat obsolescent, especially on a windy day.