excessive stretching of 22-250 cases?

[TonyH]

For years I've used an RCBS Precision Mic for checking headspace, case sizing, round OAL and so on. Still fiddling with my new 22-250 and I'm wondering if my initial choice of Win 760 powder was mistaken. Research suggested that although powder manufs' recommended max loads were around 38-39gr behind a 55gr bullet, I've found initial best accuracy with 39-40, using the 55gr V-Max; cases are sometimes sticky to extract (heavy bolt lift, in particular) but the Win primers still show some radius, not horribly flattened or anything.

But case measurement is instructive: new unfired cases run across the Precision Mic measure an average of -3.3 at a particular datum point; after one firing, they measure an average of +0.4 or so; I just measured a batch of twice-fired and they are an absolutely consistent +2.

So after two firings they are stretching overall 5.3 thou; that second firing produces around 1.5 thou stretch.... Frankly, the twice-fired get a little tight to chamber and I'm inclined to break out the Redding body-die.

Do my figures seem OK, or what?

Thanks, TonyH

[Andrew]

Determining pressure from primer shape isn't always the best idea. A sticky bolt lift and case stretching is far more indicative of excessive pressure so you might want to rethink that load. It has been an odd thing that I've noticed over the last 4 decades of reloading: Best accuracy is often found 5 - 10% under max, or, the forbidden fruit of reloading, just at or a smidge over max. I don't think this is a scientific phenomenon but just the Red Gods of Reloading trying to entice me into something I shouldn't do.

 

You might want to go back to the bench with this load and do some tweaking that involves generating less pressure. The cases will last longer, your rifle will last longer, and the game won't miss those few extra fps. JMHO, of course.~Andrew