7mag vs. 7-08 sight in

[florida boy]

My 7-08 Weatherby Vanguard w/ 1:9.5 twist loves Hornady superformance 139 gr. sst's. I average .360" 200 yd groups.
I bought a TC Venture in 7 mag last year, same barrel length, it has a 1:10 twist. Started shooting a few months ago after mounting scope, got serious with sight in this weekend. I had used 150 Fed. Fusions to get close because they are cheaper and planned on hunting with the 139 Hornady's. I started at 350 yards because I wanted to see how the BDC reticle worked on the scope.
I got 4" - 5" groups w/ the 139 grain sst's and 1.5" w/ 150 fusions ???!!!
Moved to 250 yards today to verify zero did some tweaking on scope and Fusions are real close to zero, SST's all over the place ! Just wondering what everyone thinks, because I thought the SST's would shoot well, like the 7-08. Do you think it's the velocity of the Superformance ? I wouldn't think the slight twist rate difference would matter.
Sorry for the long post

 

[g0rd0]

7mm 1 in 10 twist with 150 0r 160 grn bullet around 3000fps should give you excellent accuracy.
I have 7mmrm 1 in 10 twist and it loves 150 grn at 3010fps (.750" at 100 yd)
with 139 / 140 I need to slow them down to 2600 to get the same results.

 

[varmintH8R]

+1. Your rifle is telling you what it likes!