I built mine many years ago and can't remember the twist for sure. I am 90% sure it is 1-9. I know it is not slower and I am pretty much sure I didn't use a 1-8. It is a very accurate cartridge and would recomend it to anyone shooting the 6.5 long range. My wife prefers shooting the 264 win mag because of less recoil. Both are fun but the speed of the STW is amazing for long range kills.
Predictions are difficult, especially when they involve the future
I have one gun with 1-8 and one with 1-9,both seem to shoot the 140 grain well with no problem as well as the 110 and 120 grain. I have shot over 1600 rounds,18 pounds of powder and my barrels are still good,limit my shots as to not get my barrel hot. My hottest loads,4018 fps with 100 grain, 3787 with 120 grain, and 3500 with 140 grain.
I was pretty sure mine is getting built with a 1-8" but now Long Ranger has me second guessing. Will check my billing statement and see. Has anybody seen or shot the Sierra 155gr Matchkings? Must have been discontinued?
if you could roll time back 50 years you would find the most popular long range hunting cartridge in pa. was the 6.5x300 weatherby. that due to one bullet, the 139gr. norma. irronicly,it had a full metal jacket and was illegal for hunting there.
powder selections were very limited also. h870 was widely used as was 4831.
the cartridge lost its popularity there mainly due to better 284 bullets becoming available.
the 7x300 weatherby is still quite popular there and is about identicle in performance as the 7mm stw.
i would be looking at load data for the 7stw and using that as at least a starting point reference.
wc872 has about the same performance as h870.
im currently using 7828 in my 7x300.
that should work for you also as will rl 22, rl 25 and h1000.
i would think retumbo a bit slow, but others seem to be saying otherwise.
i am currently playing with a 6.5x300 wsm.
all bullet weights shoot very well. best powders for velocity have been rl19 and rl22, followed by 7828. my twist on that is 8.5.
i wanted a snug no turn neck chamber.
it is too snug and gives me pressure before max. velocity without neck turning.
be aware of that possibility when looking for velocity.