Perhaps to help OP find what you need - unlike bottleneck cases that headspace on their shoulder - so like 308 Win, 30-06 - many / most belted mags headspace off the belt - so 300 Win Mag, and 338 Win Mag use same gauges - nothing to do with the case body, shoulder or bore - they are stubby - just measuring from bolt face to the depth of the rim cut for the belt. The set that I have is marked "300 Win Mag" - JGS brand - but is same thing for most any belted case based off of the 375 H&H size ...
Many of us that reload, then transfer that "headspace" to the case shoulder - so do not use the belt as "headspace" - unless by accident - requires paying attention to how FL die is set, so that there is just a few thou clearance at shoulder - aka "shoulder bump". With belted cases can also get a "bulge" right forward of the belt - many FL dies will not adequately size there - becomes most apparent when using brass previously fired in someone else's chamber - I had to buy the Larry Willis die that deals with that - to allow several hundred previously fired 300 Weatherby to fit into my rifle's chamber - is not a headspace issue - is a cartridge diameter issue. I have loaded for my 338 Win Mag since about mid-1990's - several brass loaded at least 8 or 10 times - was not an issue - always started with new Remington brass - had only been fired in my rifle - so was a surprise to me to buy some "1-F" brass that would not fit after FL sizing.
