[TheTinMan]

Flattened primers and 2,400 fps with a 168 SMK doesn't compute for me either.

From post #3 it sounds like you may not be pushing the shoulder back enough. "Just snug, no slop" is exactly what I'd be looking for in a bolt action, but you want the case headspace .003 to .005" shorter than the chamber.

"Lots of stretching" is a function of this rifle AFAICT. What sizing die are you using? I really like Hornady dies in general, but switching to an RCBS X-Die for sizing made a huge difference in how much the cases stretch. I literally didn't have to trim at all after using the X-Die for the 2nd time on a given group of cases. First time that ever happened for me.

 

[TheTinMan]

The RCBS "X-Die" for re-sizing comes in both normal size and "small base" size:
https://shop.rcbs.com/WebConnect/Ma...yDisplay&categoryId=C09J049N022&route=C09J049

If you're going to buy a new sizing die, why not get a better seating die while you're at it? Forster's Bench Rest Seater simply is a better mousetrap in terms of ensuring that the bullet is seated as dead-center as possible in the case:
http://www.midwayusa.com/viewProduct/default.aspx?productNumber=202919

Meanwhile, I'd really like to know the answers to these questions:

What is the optimum jump in an M14? How much is too much, before pressure rise due to lower case volume, like raising the compression ratio in an engine?

I've got an M25 that is generating unexpected chamber pressure according to a friend's calculations in QuickLoad. The jump to the lands appears to be about 0.040" in this rifle with a 168gr SMK seated to 2.82" COAL. Is that too much jump? (measurements done using Hornady OAL gauge)