Determine Correct Headspace
With the exception of the .240 Weatherby Magnum, most belted, magnum cartridges are based on the .375 H&H Magnum case. Therefore, they all use the same headspace gauges. The following is a partial listing of those cartridges which use the same Belted Magnum Headspace Gauges: .257 Weatherby Mag., 6.5 Rem. Mag., .264 Win. Mag., .270 Weatherby Mag., 7mm Rem. Mag., 7mm Weatherby Mag., 7mm STW, .300 H&H Mag., .300 Weatherby Mag., .300 Win. Mag., .30-.338 Win. Mag., .308 Norma Mag., 8mm Rem., .338 Win. Mag., .340 Weatherby Mag., .350 Rem. Mag., .358 Norma Mag., .375 H&H Mag., .416 Rem. Mag., .458 Win. Mag.
MIXING AND MATCHING BRANDS OF HEADSPACE GAUGES - Don't do it!! That's the short answer, here's why. It boils down to tolerance stacking. Each Headspace Gauge manufacturer works within a range specified by SAAMI. Manufacturer "A" may work at the high end of the range while Manufacturer "B's" gauges are in the middle of the tolerance range. Mixing the two could give an inaccurate reading. By sticking with one brand within a particular caliber, you will eliminate a variable. If you have a Forster, .308 GO gauge, get a Forster, .308 NO GO. Use Clymer with Clymer and Manson Precision with Manson Precision, etc. You can use any brand of headspace gauge with any other brand of chambering reamer. e.g. Forster Headspace Gauges with Clymer Reamers, Manson Precision Gauges with JGS Reamers, etc.
Big Book Catalog, Issue:65, Page:471
New Product Supplement, Issue:551, Page:006