seating depth and jump for jacketed and cast bullets

[7000grain]

Friday night reloading thought: seating depth and jump for jacketed and cast bullets

Seating depth is just how you control bullet jump. Jump is what actually matters.

For jacketed bullets, jump mainly affects pressure and consistency. For cast bullets, jump affects alignment and whether the bullet gets damaged before it enters the throat.

Starting points that usually work:
For jacketed bullets, start about .020 off the lands.
For cast bullets, start at light contact or very close to the rifling.
If the rifle needs to feed from a magazine, seat to feed reliably and accept some jump.

How to adjust jump:
Make changes in small steps. About .010 at a time for jacketed. About .005 at a time for cast. Change only seating depth, not powder charge.

What to watch for with jacketed:
   •   Groups tightening or opening up
   •   Vertical stringing changes
   •   Sudden pressure signs as you get closer to the lands
   •   ES and SD changes on the chronograph

What to watch for with cast:
   •   Random flyers instead of round groups
   •   Leading near the throat, often from too much jump or poor alignment
   •   Pressure signs showing up early if seated deeper with soft alloy
   •   Accuracy falling off as velocity increases

If jacketed loads show pressure jumps, add more jump. If cast loads lead or throw flyers, reduce jump or improve bullet fit.

Bottom line: jacketed bullets care about pressure and consistency. Cast bullets care about fit and alignment. Seating depth is a tuning tool, not a magic number.