A 156 is plenty of bullet weight in 6.5mm. Very few chamberings could take advantage of them excpet maybe a .264 WM, 26 Nosler, or 6.5-.300Wby, and ZERO current rifles or hardly zero barrel twists would support them. You would need at least a 1:7" to optimize them.
If you want a 170+, go to a .277 or .284.
I already did a twist rate, BC. .700, Cal 264, Wt. 175g., Lenth 1.580, Vel. 3000 FPS, 8 Twist, Temp 59 Degree, Altitude 1240
At these numbers, I only get a stability factor of 1.51. Stable. but....
If Berger states the 156 needs a
minimum twist rate of 1:8" (note MINIMUM, not OPTIMAL), I would highly doubt a 170 Elite/Hyb/VLD would need less than 1:7.5" MINIMUM, and 1:7" would probably be more realistic for a wide range of shooting conditions.
The 156 Elite is 1.510" long, and the 140 Elite is 1.40" long. Seems like a 170 would be about 1.60" long. 3000fps would be a big case speed. Like the .264WM/26 Nosler/6.5-.300Wby number. 1:8" would be marginal for a lot of hunters. a 175 would probably be closer to 1.62+" long. Making it a stability factor of 1.41 in a 1:8". Marginal.
That would be a game crusher though, as SD would be off the charts. But would take a lot more to outrun a 156 at a higher velocity.
Looking at numbers:
Typical late elk hunt, 35* and 6500'.
To maintain 1800fps and 1500ft/lbs minimum:
1:7.5" twist
156 @ 3066fps = 940 yards 1496ft/lbs, 2078fps remaining, 168" drop (this is my actual 6.5SS hunting load)
170 @ 2950fps = 940 yards 1498ft/lbs, 1992fps remaining, 183" drop (hypothetical speed based on what I have observed with a lot of testing between differing bullet weights in 6.5mm)
If you want to throw deer in there, drop minimum requirements to 1800fps and 1000ft/lbs:
140 Hyb @ 3156fps = 1210 yards 1801fps, 1008ft/lbs
156 Elite @ 3066fps = 1250 yards 1800fps, 1120ft/lbs
170 Hypo @ 2950fps = 1160 yards 1800fps, 1222ft/lbs
