7mm RM cases not fully fireforming to chamber

[Shootin4fun]

I bought a bunch of new Hornady brass to load for my 7mm RM. Virgin, the shoulders all run 3.107 on my comaritor. My chamber is 3.125; Fully formed cases are prove this.

Eager to try the 168gr Berger Classic Hunters, I loaded up a batch, starting low at 63.0g - 64.5 of H1000, .010 off the lands. (Berger's staring load is 62.5) I also also loaded some 139 SSTs with 59.5 of RL 22 which is pretty accurate but light load. My chamber is "tight" meaning I have found that velocities I get are usually notably higher than books, so nowadays I start fairly low. I've had pressure problems in the past and want to avoid that.

I fired the RL22 and H1000 loads on different days. The 168 Bergers are going out at 2780; the 139 SSTs at 2990. For the Bergers, not an anemic load but about 200 ft/sec less than what the Berger manual says for the max load. So I'm thinking this should be enough pressure to fully fireform the cases.

Lo and behold, the once fired cases for both loads are not ballooning out to full chamber shoulder length. They are all only making it to 3.21, not the full 3.125.

I have not experience this before. Am I shooting loads that are too light, or is it common for virgin brass to take 2 firings to reach chamber length? Will I see a velocity drop at a given charge now that the cases have expanded .010? I'm about to reload them and wanted to get some input from you guys before doing so. Do you think I'll need to bump the charges a bit to see the same velocities as the the first time out?

 

[Laelkhunter]

Lo and behold, the once fired cases for both loads are not ballooning out to full chamber shoulder length. They are all only making it to 3.21, not the full 3.125.

Is this a typo? Only making it to 3.21 not 3.125? 3.21 is larger than the 3.125, so I don't understand the problem.
I certainly don't think your powder charge is too light.

 

[doverpack12]

Regarding bumping your powder charges. Broz just had a thread going with his 30 Nosler and finally determined once he reached an accuracy node with new brass he had to back the powder charge off 1 grain in that case to maintain equal velocity and accuracy and the new brass. His summary was the new brass sucked up extra energy and dampened the pressure peak compared to once fired which basically matches the chamber already.

I would think your brass pretty much reached the chamber size and because it is new brass has better spring back than your other brass you are measuring accounting for the 0.005" difference to the shoulder (assuming a 3.12 and not the 3.21 listed above).

 

[Shootin4fun]

Is this a typo? Only making it to 3.21 not 3.125? 3.21 is larger than the 3.125, so I don't understand the problem.
I certainly don't think your powder charge is too light.

Yes, a typo. Should have been 3.121.

 

[dsculley]

When I got my die from Mr. Harrell at Harrell's Precision for my 6.5x47 he asked for three cases that had been fired three times each as it takes this many firings to fully fire form a case to your chamber.

Load a few and fire them a couple more times and see what happens.

FWIW

 

[Shootin4fun]

Regarding bumping your powder charges. Broz just had a thread going with his 30 Nosler and finally determined once he reached an accuracy node with new brass he had to back the powder charge off 1 grain in that case to maintain equal velocity and accuracy and the new brass. His summary was the new brass sucked up extra energy and dampened the pressure peak compared to once fired which basically matches the chamber already.

I would think your brass pretty much reached the chamber size and because it is new brass has better spring back than your other brass you are measuring accounting for the 0.005" difference to the shoulder (assuming a 3.12 and not the 3.21 listed above).

New brass spring back makes sense. My logic on bumping the charge was that since case capacity will be greater on subsequent loading, pressure would be lower, but Broz's experience and conclusions actually makes more sense.

I'm going to replicate the previous loads in those cases next. But another variable will be changing neck tension. I got lucky first time out with the lightest load producing a .1 x .4" 3 shot group and the next one .1 x .7". .1 was the vertical, but I'm not convinced I'm at the OCW because they had a .7" horizontal offset from the point of aim.

 

[gohring3006]

Sounds like you are on the right track.
I don't set my body dies up to bump the shoulders until they become hard to chamber. It usually takes three to four loads to get to that point.

 

[Shootin4fun]

Sounds like you are on the right track.
I don't set my body dies up to bump the shoulders until they become hard to chamber. It usually takes three to four loads to get to that point.

I only recently started checking my shoulders before resizing each time because I started neck sizing only. Until now I've been FL sizing and bumping the shoulder .001. I always thought they were full chamber size after the first firing.

 

[Deleted member 25294]

I bought a bunch of new Hornady brass to load for my 7mm RM. Virgin, the shoulders all run 3.107 on my comaritor. My chamber is 3.125; Fully formed cases are prove this.

Eager to try the 168gr Berger Classic Hunters, I loaded up a batch, starting low at 63.0g - 64.5 of H1000, .010 off the lands. (Berger's staring load is 62.5) I also also loaded some 139 SSTs with 59.5 of RL 22 which is pretty accurate but light load. My chamber is "tight" meaning I have found that velocities I get are usually notably higher than books, so nowadays I start fairly low. I've had pressure problems in the past and want to avoid that.

I fired the RL22 and H1000 loads on different days. The 168 Bergers are going out at 2780; the 139 SSTs at 2990. For the Bergers, not an anemic load but about 200 ft/sec less than what the Berger manual says for the max load. So I'm thinking this should be enough pressure to fully fireform the cases.

Lo and behold, the once fired cases for both loads are not ballooning out to full chamber shoulder length. They are all only making it to 3.21, not the full 3.125.

I have not experience this before. Am I shooting loads that are too light, or is it common for virgin brass to take 2 firings to reach chamber length? Will I see a velocity drop at a given charge now that the cases have expanded .010? I'm about to reload them and wanted to get some input from you guys before doing so. Do you think I'll need to bump the charges a bit to see the same velocities as the the first time out?

That load with h1000 and 168 bergers is really light.
Check out other data and you will confirm this.
I was shooting the 162 eld x with hornady brass and 70.5 grains of H10000, and getting right around 3000fps in a 24 inch barrel. Not a hot load in my rifle, I tested 5 cases that I reloaded 8 times and the primer pockets were still tight.