15 thou. headspace on M1917... what to do?

[Patrick Chadwick]

In 3 firings, cases have stretched beyond max length.

Sounds like you really have got a rifle with a larger than specified headspace.
But if you were a reloader for original blackpowder cartridge rifles, you would be used to the idea that every rifle has to be treated as an individual. Especially if you want to get the best performance out of it.

But a case that becomes so long that it jams up at the mouth is bad, because this causes extra pressure by hindering the expansion of the mouth and the release of the bullet. - If you force in a slightly overlong case, the mouth will be crimped! So I would trim the cases to the standard length. Once you necksize they will grow very little. I have lost count of the number of times I have reloaded my match set of Geco cases (cases from the same lot, weight matched to within 1 grain), but it could well be into double figures, and only now are some reaching the maximum length. Good cases will last so long with necksizing that there is little point in keeping count. Just retire the whole set if any splits occur (I have never had one with my 30-06 cases).