Primer Blow Out with NO Over Pressure Signs?

[BuffCyclist]

Yes, my LR rig will crater primers with very light loads and soft primer cups.


Also from what you said about tying to measure the OAL to the lands, I'd try loading a bunch, or just further seating the loads you have to 2.780 to make sure you're not jamming at all. If the pressure signs go away, you have your answer, but continue with caution.

But wouldn't seating the bullet 0.02" deeper cause other pressure signs?

[BuffCyclist]

Okay, so I decided to try the wood dowel method of measuring the bolt face to the lands again.

I seated a bullet in an empty case (FGMM once-fired in my rifle, BARELY sized the neck so it was a little tight but that I could easily push the bullet deeper by hand). Carefully put the round in the chamber, slowly slid the bolt forward until it stopped, then used a small bungee cord to hold it forward and up. Slid a wood dowel down the barrel until I touched the bullet and marked the dowel. Ejected the round carefully, then slid the bolt face forward again, bolt up, bungee and measured with the dowel again. Here are my measurements:

1 - COAL Before - 2.968"
1 - COAL After - 2.952"

Decided to fully close the bolt this time, and got nearly the same after measurement:
2 - COAL Before - 3.041"
2 - COAL After - 2.950"

Left the bolt in the up position again:
3 - COAL Before - 2.983"
3 - COAL After - 2.955"

I can continue doing this all day, but I have a feeling the COAL base to lands will be 2.950" +/- 0.01", so I have a feeling I am not jammed against the lands. And yes, while I don't have a way to measure the o-give yet by marking the bullet tip with sharpie and looking for the scratch mark from the rifling, it is at least 0.55".

The markings on the dowels are as follows:

1 - 3.015"
2 - 2.955" (closed bolt, not sure if this is accurate though)
3 - 3.016"
4 - 3.022"
5 - 3.010"

[Hoser]

But wouldn't seating the bullet 0.02" deeper cause other pressure signs?

Seating rifle bullets deeper causes the pressure to go down.

[BuffCyclist]

Seating rifle ammo deeper causes the pressure to do down, in rifles.

That is completely counter-intuitive (as long as the bullet isn't jammed against the lands). Seating it deeper means less case capacity, so when the primer ignites there is a smaller volume being expanded before pushing the bullet forward and before the powder ignites.

I've read a lot from you Hoser and know you are extremely knowledgeable on the subject, so I'll believe you, it just doesn't make sense.

I just googled it too and found an article by Barnes Bullets that corroborates this, but it also shows pressure spikes if the bullet is too far from the lands.

I have lots of time to figure this out, just found out my rifle is subject to the X-Mark Pro trigger recall, and sadly I don't have the funds for a brand new trigger right now so I think I'm going to have to ship the rifle back. :-(

[Zombie Steve]

In a 9mm case, seating deeper reduces case volume a lot. In a .308, your percentage of case volume barely changes at all. In a .30-06, even less. Big cases it just doesn't matter much. More important in rifles is that the bullet gets a running start at the lands. Keeps pressures down.

[BuffCyclist]

I've been researching this more (primer cratering) and found a source indicating that a dirty/fouled barrel can cause huge pressure spikes. The only thing here is that it isn't dirty. I cleaned the barrel when I got it (Hoppes #9, copper brush, patches, CLP) and have only put 20rds through it before yesterdays range day, so the 33rd round through the barrel blew out the primer, so I HIGHLY doubt that's the problem.

[TheBelly]

When brass flows into the ejector you are already pushing it. Primer cratering is not a very reliable indication of pressure.

Nail meet Hammer.

primer cratering is only indicative of one thing: there's a space in the bolt face for the striker to peek through and strike the primer. If there are pieces of the primer breaking/shearing off and ending up in the bolt, then that could be a problem.

my max load is 42.0 grains pushing a 175 gr bullet and a COAL of 2.805"

[TheBelly]

What's the max length from base of case to ogive that your rifle can handle?

[Hoser]

One more thing to consider...

Accuracy International rifles have a huge firing pin hole. With just about every mid range load and up, I get cratering.

May moons ago David Tubb blew up an SR-25 in 260 doing load development. The gun came apart but the brass/primer looked fine.

I measure case head diameter in extractor rim cut so I can tell if the pressures are high. Easier than measuring the insides of primer pockets.

[sako55]

It might be simple, but did you calibrate your scale?