The Firing Line Forums - View Single Post - IMR4831 vs H4831 Target Loads 30-06

Bart B. · November 16, 2013, 12:33 PM

I've no idea what criteria publishers of load data use to establish a minimum charge weight for a given cartridge, bullet and powder. But it's easy to find out what your's is with bottleneck cases headspacing on their shoulders.

All bottleneck cases headspacing on their shoulders have some amount of shoulder setback when the firing pin drives them hard into the chamber shoulder. It varies from 1 to several thousandths inch. The case shoulder stays hard against the chamber shoudler as it expands against the chamber wall starting right behind the shoulder (where brass is the thinnest) then works back to the pressure ring in front of the extractor groove. Cases with low shoulder angles (17.5 degrees on .30-06) and smaller surface areas do so more than those with higher angles and larger areas (40 degrees on .30-06 AI). If there's not enough pressure built up to push the back end of the case hard against the breech face, the case will have a shorter head-to-shoulder dimension than it did before it was fired. Subsequent reloading of such a case without resizing it to push the shoulder back forward to where it was (or specs for the case when new) results in the same thing happening again.

Continued shortening of case headspace results in excessive case head stretching back from the body at the pressure ring. That can be felt with a dog-legged sharp edge put inside the case to feel the thin spot in the case wall. See the link below for an example:

http://accurateshooter.net/Blog/casehead02.jpg

One way to find out what a minimum charge for a set of components for bottleneck cases is to use a case headspace gauge (RCBS Precisioin Mic, Hornady LNL, or equivalent made at home) to measure cases before and after firing. Start with your normal maximum load you've worked up to, then load 15 rounds of its fired cases with each one incrementing 1% less than maximum. If your charge weight for you maximum is 60 grains, then load one with 59.4 grains and mark its charge weight on the case, the next with 58.6 so marking it, then 58, then 57.4 and so on down to 51 grains of powder. And every case has its charge weight on it.

Measure the maximum one, record its headspace, write that down, shoot it, measure its headspace then write than number down next to what it was before firing. Move on to the next lightest round and do the same thing. Pay particular attention to when cases end up a thousandth or more less headspace. Check their primers to see if they stick out past the case head. As soon as primers end up pushed out past the case head, that round had a bit too much powder in it to fully expand the case to the chamber. Any round with lighter charge weights will end up with shorter case headspace as well as more of the primer sticking out of the case head. Break down those with lighter powder charges; don't shoot them as they'll end up not good to reload maximum charges with.

So, whatever minimum charge weigh still let the primers remain flush with the case head is your minimum load for that rifle and that set of components. This usually starts around 10% below maximum.

November 16, 2013, 12:33 PM	#13
Bart B. Senior Member Join Date: February 15, 2009 Posts: 8,927	Finding Minimum Load Data for Your Stuff I've no idea what criteria publishers of load data use to establish a minimum charge weight for a given cartridge, bullet and powder. But it's easy to find out what your's is with bottleneck cases headspacing on their shoulders. All bottleneck cases headspacing on their shoulders have some amount of shoulder setback when the firing pin drives them hard into the chamber shoulder. It varies from 1 to several thousandths inch. The case shoulder stays hard against the chamber shoudler as it expands against the chamber wall starting right behind the shoulder (where brass is the thinnest) then works back to the pressure ring in front of the extractor groove. Cases with low shoulder angles (17.5 degrees on .30-06) and smaller surface areas do so more than those with higher angles and larger areas (40 degrees on .30-06 AI). If there's not enough pressure built up to push the back end of the case hard against the breech face, the case will have a shorter head-to-shoulder dimension than it did before it was fired. Subsequent reloading of such a case without resizing it to push the shoulder back forward to where it was (or specs for the case when new) results in the same thing happening again. Continued shortening of case headspace results in excessive case head stretching back from the body at the pressure ring. That can be felt with a dog-legged sharp edge put inside the case to feel the thin spot in the case wall. See the link below for an example: http://accurateshooter.net/Blog/casehead02.jpg One way to find out what a minimum charge for a set of components for bottleneck cases is to use a case headspace gauge (RCBS Precisioin Mic, Hornady LNL, or equivalent made at home) to measure cases before and after firing. Start with your normal maximum load you've worked up to, then load 15 rounds of its fired cases with each one incrementing 1% less than maximum. If your charge weight for you maximum is 60 grains, then load one with 59.4 grains and mark its charge weight on the case, the next with 58.6 so marking it, then 58, then 57.4 and so on down to 51 grains of powder. And every case has its charge weight on it. Measure the maximum one, record its headspace, write that down, shoot it, measure its headspace then write than number down next to what it was before firing. Move on to the next lightest round and do the same thing. Pay particular attention to when cases end up a thousandth or more less headspace. Check their primers to see if they stick out past the case head. As soon as primers end up pushed out past the case head, that round had a bit too much powder in it to fully expand the case to the chamber. Any round with lighter charge weights will end up with shorter case headspace as well as more of the primer sticking out of the case head. Break down those with lighter powder charges; don't shoot them as they'll end up not good to reload maximum charges with. So, whatever minimum charge weigh still let the primers remain flush with the case head is your minimum load for that rifle and that set of components. This usually starts around 10% below maximum.