7RUM case denting when firing (pic)

[Brain]

Loaded some Berger 168's over 94gr Retumbo. Case necks are partially sized and bullets were seated long to touch the lands. Base of bullet is at neck/shoulder junction inside the case.

Below is a pic of the result.

Never seen this before and I am assuming it is due to the small amount of bearing surface held by the neck not allowing the case to expand enough to seal the chamber before the bearing surface exits the case and gas is allowed to flow around the boat-tail and back into the chamber.

I base the above conclusion on not only appearance, but also measurement. Outside case neck diameter on loaded round prior to firing was 0.314. Outside case neck after firing is now 0.313. Internal neck diameter before seating bullet was 0.282. Internal neck diameter after firing is 0.282. Case did not grow in length at all, nor did body diameter change.

You can see in the pic that soot is evident on the case body along its entire taper. There is a definite ring at the case web where the soot no longer traveled rearward.

Am I on track with this? I have ideas for remedies, but wanted to get input from others as well.

 

[oldfamily]

Hope all is ok with you and your rifle, the first thing that comes to mind for me is there any thing that is in your chamber. I can see the neck and shoulder have blown out making it look like it did not have a good fit in the chamber. Was the bolt hard to close? Any other out of the ordnary feel with the rifle. Was this the first time you shot the rifle? I would not fire any more rounds until you figure this out. Hope you can find out what is causing this. Oldfamily.

 

[davewilson]

you're shooting too low of a charge. there's not enough pressure to seal it around the neck and it's escaping back causing the dents. i'd also be concerned with the clearance of a loaded round. sounds too tight to me.

 

[Brain]

oldfamily: I'm ok and nothing out of the ordinary happened from my perspective when I pulled the trigger. Nothing in the chamber, nothing wrong with the rifle, been shooting it for over a year. Contrary to the way it might appear in the pic, the case is not blown out, but rather did not expand enough to seal the chamber.

davewilson: Low internal case pressure is what I've concluded as well. What clearance are you referring to when you say, "i'd also be concerned with the clearance of a loaded round. sounds too tight to me"?

Thanks for the input fellas.

 

[6mm Shooter]

He is referring to the clearance between the neck of the case and the neck of the chamber. I like to see atleast .002". I have seen the same thing and it was caused by to low pressure loads. Either increase the load or go to a faster burning powder.

 

[bwaites]

From what I am reading, then, the case is not obdurating to the chamber wall?

This is happening because the charge is not high enough to raise internal pressure and cause the brass flow and obduration?

 

[Brain]

6mm: If the case neck clearance to the chamber was too tight, wouldn't that result in less gas being able to escape around it and also less pressure required for a proper seal? If I'm not thinking in the right direction here, please help.

Interesting ideas about faster powders and/or increasing the charge. After posting this topic I tested the same setup with increasing charges up to 98gr of Retumbo. They all did the same thing without any one more or less severe than the one pictured above. What is interesting is that this does not occur with the same powder charges when using deeper seated bullets. Using Hornady 162gr SST and SMK 175's, which seat deeper into the case neck than the VLD's, the problem is non-existent.

Could be that a faster powder would develop enough initial pressure to expand the case properly. I guess that's the next step to take if I want to seat VLD's at the lands with this barrel.

 

[davewilson]

as 6mm said, when you put a loaded round in the chamber, there should be clearance between the case and chamber in the neck area. .002 is also what i would consider minimum. you've stated a loaded round and fired measure the same in the neck. that's saying there's very little, if any clearance between the 2. potentially a very dangerous situation.

as for the other bullets not having the same consequences, every bullet is different and the others probably make more pressure due to baring surface length, material hardness or several other things. i still think you're too low with pressure. that's what causes denting like you have. you might try chronographing the speeds. most of the time this will tell you if you're in the ballpark or not. but first i'd make sure you have proper clearance in the neck area.

 

[dbhostler]

Brain, you are correct in your assumption. I had a Rem 7 UM do the same thing some time ago. You can do several things, use more powder, seat the bullet deeper in the case, increase neck tension or use a hotter primer. By the looks of your load, I would begin by seating the bullet deeper and increase neck tension.
db

 

[Brain]

Thanks for all the input. It's very much appreciated.


In this particular circumstance, yes, the fired case neck diameter was .001 less than loaded round diameter. This is not due to a tight neck chamber, but because the case did not expand. Chamber diameter in the neck area is 0.322 which gives me 0.008 of clearance with a loaded round.

Unless I'm hunting, or shooting a match, everything gets shot over a chronograph and recorded. 94gr = 3330, 3322; up to 98gr = 3372, 3379. I believe these to be acceptable velocities for a 26" barrel and in line comparatively with other loads I have used in this rifle.

I too think pressure is too low. Before I try a different powder I am going to seat the VLD's deeper into the case for more bearing surface engagement. With the 168 VLD's there is only 0.073 of bearing surface making contact with the neck. The 162 SST's have the entire length of the neck (0.287) contacting the bearing surface. This should allow for more dwell time and resulting pressure increase for the case to expand before the mouth of the case is void.

I'll post results when I get them.

Thanks again.

 

[dbhostler]

FYI this subject has been discussed several times before. It's been a few years ago, if I remember right, I brought it up when I began loading for my new 7UM Sendero. If you have a mind to, you can dig far enough and read the posts.
db

 

[6mm Shooter]

Try some H-1000 or IMR 7828 and see if it still happens.

 

[Pdvdh]

FYI this subject has been discussed several times before. It's been a few years ago, if I remember right, I brought it up when I began loading for my new 7UM Sendero. If you have a mind to, you can dig far enough and read the posts.
db

Is it possible to search back more than one year? If so, how? I see a max of one year when I perform searches. Anybody know?

 

[KRP]

Go to the advanced search and under "Find Posts from" put in "A Year Ago and older". Using "Any date" will work also but you are limited to 500 results so you may need to narrow the search and/or specifically search the older than 1 year posts.