07-01-2020, 01:10 AM
tdbru Wrote:Mel,
go to either hornady's ballistic calculator 4DOF or the berger twist rate calculator. put in the bullet, MV and twist rate and see what the gyro stability number looks like. in Lyman's book on long range shooting they have an article by, hope i get the name right, Dave Emary who use to be the chief ballistician at Hornday. in that he says get a twist rate that you get gyro of 2 for what you're using. i'd been shooting Sierra 142s in a 260 Rem with 1-8 and the gyro was less than 2. and my groups weren't super either. tried the hornady 130 eldm's and the gyro went to either 2 or like 1.98 for my mv and the groups got tighter. maybe something to what Mr. Emary is saying. anyway, due to what i read, right or wrong, and very limited experiences i'll limit my 1-8 twist to the 130gr. VLD bullet length or shorter. it's actually bullet length, not weight, that drives the twist rate. for my 142s i should have been using 1-7.5 or 1.7. don't recall off the top of my head. your mileage may vary of course, since each barrel is a law unto itself as far as how it groups due to many variables. but the gyro number is a calculation. if i recall both Hornady and Berger say if you get above 1.5 you're good. but they're both trying to sell bullets. if Hornady's former chief ballistician thinks 2.0 is where to run, i'd tend to use that recommendation, or weight it heavily. the long range guys, like F-class or PRC are running very long 6.5 VLD bullets with screw thread twist rates. very long, like in the 150gr. range VLDs in 6.5mm. so it depends on what bullet length you want to run as to what twist rate to run. however, that said, the very long 6.5 bullets will come out of the Grendel quite slowly, so you'll need LOTS of scope MOA range or a 40-60 moa rail or both to shoot then long long range like you do. and you may have quite a powder compression issue if you load to fit in your AR mag or you may have to cut back on the powder amount quite a bit since a super long VLD seated to mag length will stretch back into the case quite a ways. so that will be a compromise as well. most cartridges have an "optimum" bullet weight for the bore diameter and case capacity. with the 6.5 Grendel i feel that that optimum range is in the 110gr. - 130gr. range, which will work well with the 1-8 twist you picked i think. my opinions of course and feel free to disagree, won't bother me at all. just bringing some things up to discuss or think about.
-tdbru
I'm really happy with the 130rdf and 135a-tip the stop prolly being the longest of the bullets I plan to shoot ,heck it's even longer then the 140's I'll run those threw some calculators and see what comes up only one I wanna try heavier is the 140rdf