Go get yourself a Hornady Lock-n-Load OAL gauge with a 260 Modified Case to find your jam length. Start with .020" off, and go from there. If you have a set of digital calipers, you can zero them out on the gauge. The measurements are usually referred to as BTO, or Base To Ogive. This is a much more accurate measurement with regards to seating depth consistency than measuring from the base of the case to the tip. Bullets can vary considerably in their overall length, but the ogive profile of a bullet is the characteristic you're looking for in regards to consistency.
Kevin
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