Why does my brass look like this?

[Jefe's Dope]

Out of 51 rounds fired, 6 came out looking like this.

Brand new Lapua brass. CCI 450 primers. H4350 powder.(3 were low grain count of 38.0 and 3 were random in the 39.5-40.0 grains. The higher grain count showed no signs of this. There's no other issues with this brass and there were seemingly no issues when firing this brass/ammo.

6 on top vs. 6 on bottom. All loaded and shot together. The powder on the rim on the top 6.

 

[pell1203]

Are you asking why there is more soot around the neck area with the lighter loads?

If so, my guess is that due to the light loads the brass at the necks did not expand quickly enough to fully seal the neck area against the chamber walls. This allows some of the gases and byproducts of the poorer combustion to bleed around the rim of the neck into the area between the outer neck and chamber wall. However, it appears from your pictures the shoulder expanded enough to block any leakage past the shoulder.

I believe that the heavier loads develop greater pressure and were thereby able to fully expand the cartridge against the chamber walls thus preventing the gas leakage seen with the lighter loads.

You don't mention the caliber or muzzle velocity so it is hard to determine how light or heavy your loads truly are.

 

[lash]

Pretty much what Pell said. That's not an uncommon situation and is nothing to worry about unless the leakage is blowing by the shoulder too.

 

[Black max]

I believe he is refering to the base of the case around the rim...

 

[demolitionman]

Looks like you've got gas seeping around the primer exiting into the boltface. Are your primer pockets loose? Are there any cracks or fissures around the brass? Take some close up pictures of the fired case with the fired primer still seated.

 

[lash]

Okay, I concede that.

Looking closer at the picture, I seem to notice one thing about the cases on the top. It may be the picture angle, but all 6 of the top brass pieces seem to have rounded shoulders vs. the sharper shoulder line on the bottom six cases. Is that actually the case or is it a bad viewing angle?

 

[L.R.Novice]

Can you confirm if the power is coming from the mouth of the case or the primer? Looks like power marks beyond the shoulder but tough to tell in the images. If it is, the bullet moved out of the case letting gas out before the case expanded to seal the chamber. What were your sizing steps? How much neck tension?

 

[SparkyHD1]

I would inspect your primer pockets. looks like gas escaping from around the primers for one reason, or another. the pros on here could probably tell you more if you could post some pictures of the case head and primers.

 

[Jefe's Dope]

Thanks for all the replies. 6.5 CM out of a 24" Tikka Tac A1.

They are all new Lapua brass w/ no sizing before loading. It doesn't appear to be loose primer pockets. I would have to assume they all needed to be fire formed and those were the ones w/ the loosest tolerances? Iv'e only seen this one time and with this new Lapua brass. All other once fired Hornady and Federal brass did not do this.

As stated three were the lowest grain fills and the other three came in at the middle grain fill but were scattered amongst others of the same fill and no powder/grease around the rims.

 

[Milo 2.5]

Give us more about load and the case used. You need to add powder, plain and simple, the carbon in the rim area is gas coming back toward you and stopped by the bolt face, you're borderline dangerous on the low end.

 

[Jefe's Dope]

Here's the target w/ load data and FPS info.

 

[Milo 2.5]

6.5 Creedmoor?

 

[spife7980]

Thanks for all the replies. 6.5 CM out of a 24" Tikka Tac A1.

6.5 Creedmoor?

 

[Milo 2.5]

Pay the fuck attention, huh?