Its always better to slightly over stabilize them, imo i'd use an 8 twist then you can use 105 and 108 bergers, 106 clinch rivers, 108 BIB bt's, 107 smk's, and 115 DTAC's, I think swampy makes a 108 as well, and matrix has one in that ballpark, most of these bullet makers reccomend an 8 or faster but with 3200fps+ a 9 would work but an 8 is for sure and a 8.5 is a good compromise.
C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
D = bullet's diameter in inches
L = bullet's length in inches
SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)
Greenhill's equation is useful as a quick check, but what is missing in the equation is the effect of air density. Higher temperature, higher altitude, and higher humidity all reduce air density. Bullets are least stable in a cold dry high pressure atmosphere.
Berger recommends an 8" twist for their 105 grain VLD hunting bullets. I live and shoot in Arizona at over a mile elevation. I have pre-64 243 Win 70 with a 10" twist which shoots them very well. Despite that I chose an 8" twist for my most recent LR-243 build to be sure it will work in cold and low elevation conditions too.
Location: NY, #50 on the all time best States to live in! and droppin' fast! :)
Re: 105gr berger VLD
I bring the Greenhill equation here as a conversation point and just that, a quick check or a place to start. Certainly not an absolute. Backwoods83 also makes a good point about turning faster. For the next step, lets combine the two thoughts. For example, lets say a particular caliber/bullet combo works out to a 1 in 10, then I would be thinking 1 in 9.5 or 1 in 9.
I'm sure there would eventually be a penalty to pay for turning too fast such as centrifugal force tearing the bullet apart. One of my friends who shoots benchrest comps swore a gun he had, did this.
“The US Constitution does not guarantee happiness; only the pursuit of it. You have to catch it yourself.” ~ Ben Franklin, attributed.
Yes, you can over twist berger vlds with a J4 jacket for instance take a 130 6.5mm berger and run it at 3250fps from a 7 twist 264winmag and the jacket will come out in pieces and the lead core will go tumbling.
I'm not real sure never had a 7.5 twist but id guess anything over 3400fps would distort it pretty bad, some of my benchrest buddies shoot 8 twist 6x47lp with 105s at 2870 or so and they stabilze just fine at that speed at 1700ft elevation in decent conditions so 7.5 is a bit fast, you can probably shoot 115bergers if you can get them to 2940 or so.