It was sufficiently loud is made my ears ring for a few moments afterwards.
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I should have figured it out before. This is a 1913-built gun, but it's built to 1916-specs (curved bolt handle, gas ports on the side of the chamber area and bolt, in case of a pierced primer). Which means, of course it was rebarreled/refurbed at the arsenal at some point in time. That's probably when it was serialized as well (since the serials match on the receiver and barrel, but are different font and stamping depth than the cartouche).
Anyways, none of that really matters - except to say when the probably two big Spanish dudes torqued the new barrel on, it would have been nice if they'd chased the threads before hand. In their defense, Spanish '93 Mausers saw a lot of action over the first half of the 20th century and presumably the arsenal rebuilds weren't exactly focused on fixing every little problem. If it was close enough, it was close enough.