7mm Twist rate

[Matt P]

Hi All
What twist rate is everyone using for 180 grn style bullet ???
Will a 9 stabilize @ sea level when it's cold ???
Actual experience prefered rather than "so and so's theroy says it should".
Thanks
Matt P

[bruce moulds]

hi matt,
i have used 9" and 8.5" twists and cannot tell the difference.
our range is minus six feet according to my gps, and it can get cold there, but not like the alps. but the alps are higher.
i currently hve a 9" and an 8.5" in the safe awaiting fitting, and don't care which one i use.
i don't run speeds more than 2850 for case life, so i'm not relying on speed to spin bullets.
i seem to remember that a 9.5 twist will give similar stability to a 180 berger 7mm as a 13 twist gives the bjd 308 bullet. you could check this on the safclass website stability calculator.
i have shot the 9 twist at 1100 meters.
keep safe,
bruce.
p.s.
if you are doing stability testing by looking at bullet holes, be aware that the flutes in a coreflute backer can slightly deflect a bullet.

[ratshot]

Matt
a 9 is working for me 175 smks and 180 berg running at 2950 out of a 28 inch lawton , did chase them up to 3100 but es was to high. could never get them to work at 300 and 500 just , but it does shoot 15 shot possibles at 1000 and 1100 , 1200 a bit harder. As you know it getz a bit cooler up here a bloody hot in summer still shoots the same. have not shot at water level yet. cheers

[DaveMc]

Gday Matt, I have used 9 twist on a few memorable cold mornings now down between 6 -10 degrees (Thats cold for me) at sea level with no problem with 180 VLD's.

Marty L has started using the Sierra 180 in his 9.5 twist up here at sea level but high humidity (40-90%) not too cold . Shooting great and no issues.

all <2850fps.

[bruce moulds]

dave,
does m l stand for mighty lobert?
bruce.

[DaveMc]

Yep, He plugged out a very nice 60.8 after keeping 2 * 6's as sighters (and proceeded with a couple more x's at end) with this load at 700m a month or so back in his 9.5. Although the 700m is a big target they were well in there. No winters day but at sea level and a 9.5. Time will tell but no problems so far. Marty has travelled a bit with his 9.5 and the 175 SMK's with no issues.

I know you don't want to hear theories (only real results) Matt but Bryan Litz has a good table at the bottom of his page here http://www.appliedballisticsllc.com/B_7mm_180Hyb.html
Marginal stability is 1.2-1.4. but his case scenarios are;

Best case: 1000 feet altitude, 100% humidity, 100 degrees F, and 3000 fps muzzle velocity.
Nominal case: sea level, 50% humidity, 59 degrees F, and 2800 fps muzzle velocity.
Worse case: sea level, 0% humidity, 0 degrees F, and 2600 fps muzzle velocity.

So worse case is -18 degrees C, o% humidity, 2600fps at sea level (I haven't seen too many places like that and most of our velocities are well above that. I would suggest his nominal case is closer to our "worse case" scenarios in Aus. and in reference to a certain place "high in the mountains far far away" - welll everything will be off the chart at that altitude.

[DaveMc]

On the other side I will point out that I have heard a couple of rumours of "9" twists not stabiising the hybrids. One was a gain twist.

Not sure why but I get the feeling there is more to it.

[Cameron Mc]

I will back up what Dave has said about the 9 and 9.5 twist barrels.
I have used both and can't tell the difference. I mainly use 1-9.5' twist like Marty. Mainly use 175 mk's but have used 180 vld's all with no problems out to 1000yards. Have shot at sea level and at home we have 300mtr elevation and cold in winter.
My best score at 1000 with 9.5 twist was a 96 on the 10 ring target at sea level.

Cameron
ps I spoke with Bryan Litz at the Nationals about twist rates. He said at the end of the day, you have to try these things in the real world, the theory is there as a guide.