Primer eject 308

[towman32]

What powder scale are you using?

I was using the Lee perfect powder measure but I did notice its not perfect. Since then I've been using an electronic scale. It would probably be good for ball powder.

[Unclenick]

Towman 32,

If you go to my post, as Steve suggested, you'll see this image, reproduced with kind permission from Jim Ristow at RSI, maker of the Pressure Trace instrument. It shows an example of pressure increasing about 22% when the bullet is in the lands. An earlier study by Dr. Lloyd Brownell showed that this is a curved but continuous increase in pressure as the bullet approaches the lands, and not a sudden jump upon contact with the lands, as some suppose. So you are probably shooting at pressures that are getting uncomfortably close to the proof pressure range. This is causing the case head to be expanded until the primer pockets are loose.



When you take load data from a book, the case brand, seating depth, and primer brand all have to match to be sure the data is valid. In .308 there is a lot more variation in case capacity from one brand to the next than for any other chambering except 300 Win Mag. It's enough that some of the heavier and less capacious cases can need as few as 2 grains less powder than the most roomy ones do in order to have the same peak pressure. That's why you are hearing such different maximums reported in the other posts. Case capacity difference is the source of about 10-15% variation in pressure in this cartridge, depending on the powder used. So you really need to work loads up carefully in .308 Win, watching for pressure signs. Loose primers are a high pressure sign.

What you read about best accuracy coming from touching the lands or being very close to it is an untrue generalization. In the Precision Shooting Reloading Guide (Precision Shooting Publications, 1995), Dan Hackett relates how, in changing the type of bullet he was loading for his 40X KS 220 Swift one day, he turned his competition seating die's micrometer adjustment the wrong way. This resulted in 20 rounds loaded 0.050" off the lands instead of 0.020" off the lands, as he'd intended. The dogma he had subscribed to at the time said nothing over 0.025" would shoot well. Faced with pulling down the loads or just shooting them in practice, he opted for the latter. To his amazement, this gun, which had previously averaged 5 shots into about ½” and never smaller than ⅜” at 100 yds, averaged ¼” including two bugholes in the high ones with these “incorrectly” seated bullets. He claimed to have learned a lesson about conventional wisdom from that.

In your case, you have a bullet that has VLD design features, including a secant ogive (Rt/R=0.58 from Litz's measurement). Secant ogives are more finicky about jump to the lands than tangent ogive bullets are. From Berger's article on making VLD bullets shoot:

Originally Posted by Berger Bullets

"The following…is consistent for all VLD bullets. What has been discovered is that VLD bullets shoot best when loaded to a COAL that puts the bullet in a “sweet spot”. This sweet spot is a band .030 to .040 wide and is located anywhere between jamming the bullets into the lands and .150 jump off the lands. "

I recommend you read the whole article and use the same approach to finding best seating depth for your bullet. Try about 39 grains of 4895 so you can shoot safely touching the lands. Once you have the best seating depth, you can then gradually raise the powder charge to tune the groups to their smallest size with the bullet in that sweet spot position.

[towman32]

WOW! That was a lot of info. Thank you! How would one figure out case capacity? Are some cases more desirable than others? I have an assortment from PMC/hornady match/rp/lake city/FC/win/CBC/wcc/aff/GGG/mke/ppu/RA/PSO etc.

[steve4102]

How would one figure out case capacity?

Use fired cases from Your rifle.

Weigh a case.

Fill it with H20, flush across the case mouth.

Weigh case again.

Subtract and that is your case capacity in grains of H20.

I don't do this with every case, I do a sample of (5) from each of my different headstamps.