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I measured one of my sporters.  From the butt to the center of the rear trigger guard screw is 12.125", the distance "A" in the pic.  That's a regular military butt plate on there.

I understand what you mean about drilling.  I have a drill press and all kinds of clever ways to clamp things where I want, perfectly square, etc.  Otherwise I'd never try some of the stuff I've done.  I really wish someone would give me a milling machine.