Several years ago, I had opportunity to closely examine, AND MEASURE WITH PRECISION CALIPERS, the action of a full nitro .577 British boxlock rifle, 750 grain bullet version. At that time, I was working on several double shotgun to double rifle conversions I was making. One such conversion was a Spanish boxlock 10 ga. 3.5" mag., with Greener crossbolt---(these same type boxlock 10 ga. shotguns, actions, came from several different Spanish makers in the 1970s, and perhaps in the 1960s as well, some with and some without the crossbolts, though all, I think, from the same foundry). Comparing the British .577 nitro action dimensions with the Spanish 10 ga. action dimensions, I was delighted to see them very close to the same, with only the depth of the British action, below the watertable, slightly more. These big Spanish actions have proven, now with at least four or more makers, including myself, to hold up quite well to proof for full nitro .577, and, in at least one or two cases, to hold up well to proof for full nitro .600.