I second thisAccurate makes a "10 round" .300WSM mag (holds 7 WSM cases). You can add an alloy mag extension that might get you to 10.
AICS .300 WSM/6.5 PRC Magazine
The .300 WSM magazine expands the short action double stack line to accommodate WSM calibers, SAUM calibers and the new 6.5 PRC. ITEM CAP W L H .CAL .CAL MAX OAL 00100- 0006 DSSF 3RD .97 3.055accurate-mag.com
MDT Short Action Mag Extender - Short Action Precision
MDT's new Short Action Mag Extender lets you add +2 capacity to your AICS metal short action magazines and gain an advantage in high round count competitions. NOTE: MDT 10 Round Mags Require Spring Kit Found Here NOTE: Not Compatible With Poly Mags NOTE: Not for MDT 6BR/Dasher Mags or Mags w/...www.shortactionprecision.com