What size X15 do I need for a 4 inch Model 10? I'm guessing medium? And a large for a 6 inch K-38?

Yup. Small is 2", med/small is 2-1/2", medium is 4", large is 6-1/2", extra lrge is 8-3/8".

Within that, the 32 autos go in the small, the 5" autos in the medium. These were the limited types of pistol available when the X15 was developed in the '60s.

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Red Nichols The Holstorian

I owned an early X-15. Can't say I would recommend it. I know there are better choices out there. I don't have any confidence in spring retention holsters.

There are those who like it a lot, too. On the other hand, vintage X15s must be approached with caution because there are several spring variants - some better than others - and these springs can both 'get tired' and be stretched to weakening by their prior owners. Best is the X2000 for example, because it uses a torsion spring. And better still were the vintage Hoyts, because their springs are extreme forms of torsion wire forms. I just hope that tales of them being hand bent are false because wire form springs need stress relieving / heat treatment after bending. Bianchi springs were professionally formed and heat treated by outside suppliers.

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Red Nichols The Holstorian

Thx for the replies. I have a medium and a large I bought a few years ago and I carried my Model 10 in one yesterday to the range. It doesn't conceal that well but it is very comfortable and just fine under a heavy jacket. And the springs are very strong!