https://www.google.com/search?client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&channel=s&hl=en&source=hp&biw=1280&bih=578&q=6mm+Whit+Oak&btnG=Google+Search&safe=active#sclient=psy-ab&hl=en&safe=active&client=firefox-a&hs=lMg&rls=org.mozilla:en-US%3Aofficial&channel=s&source=hp&q=6+WOA+cartridge&pbx=1&oq=6+WOA+cartridge&aq=f&aqi=&aql=&gs_sm=e&gs_upl=4979l5823l2l6470l5l5l0l0l0l3l445l1295l0.3.0.1.1l5l0&bav=on.2,or.r_gc.r_pw.,cf.osb&fp=24943182f94f819c&biw=1280&bih=578
It's a 6.8 SPC necked down.
Really fantastic cartridge...
