Point of information:
A 7 rem mag headspaces with the belt. The go/no go gauge for a belted magnum is very short and is interchangeable for the entire family of H and H style cases and only measures the distance from case head to belt area.
After the first firing the case can headspace on the shoulder.
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.
BELTED MAGNUM CARTRIDGE HEADSPACE GAUGES | Brownells