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.
From Brownell's:
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