I believe it was about the time we were all leaving the Pond, or maybe we were already at the AR Forum; I guess it does not really matter. I was in the need of a new barrel for elk hunting here on the Oregon Coast, and I thought the .358 Bellm would be just the right caliber for the job.
Mike B. had just recently quit taking on rechamber jobs, so off my barrel went to David White. My barrel started it’s life as a factory Super 14” .357 Herrett. In a very short time, I had my new Super 14” .358 Bellm. I was sitting at my computer looking over the loading data on Mike’s web page, and got to thinking about my new rechamber.
Hmmm, I thought. I wonder how much difference there is in my barrel than the barrel that Don Shearer used to develop the data. I wondered if I could get a little more out of mine, or, Heaven forbid, maybe I could not get the velocities that Don had gotten with his. Therefore, I thought, what is the maximum load I can use safely in my new barrel. Along about this time there was a post on the Forum about a method of developing a maximum load for the Bellm Triads. I do not remember who first brought it to my attention, but thanks, whoever did. The method is called the Spring-Back Method. This testing was developed by our very own Don Shearer (Thanks Don!!)
What is the Spring-Back Method? In a nut shell, the method involves firing a series of 10, 5 shot strings, taking measurements of the web area of each case in each string and averaging the cases in each string for an average web diameter for each string fired. These measurements are out to one ten- thousandth’s. (A good Micrometer is necessary). The averages are then graphed. The graph is “ Powder Charge vs. Web Diameter”. When looking at the graph you will see the gradual increase in web diameter and then at some point you will notice where the brass has stopped it’s gradual increase in web diameter and this increase is slowed substantially. This point, where the graph curve change direction from horizontal to more vertical is the maximum web diameter and powder charge for this load. This is the maximum load or RED-LINE for this barrel.
For consistency in testing, new unfired .444 Marlin brass was used. After forming the brass in the FL .358 Win Die all the cases were fire formed. Then all the cases were prepared by trimming to exact length, primer pockets were uniformed, flash holes cleaned up. The cases were then weighed and separated by weight with like weights being placed in each given string. The bullets used were Hornady 200gr. #3510. The bullets were also weighed and separated by weight, and again, like weights were used in each string. Again, consistency is what I was after. One more step is needed in this testing. The regular .358 Win. sizing die does not size the web area small enough in diameter for test purposes. The web area has to be small enough to allow for expansion of the brass in the area of the web. At lease .003 smaller then normal chamber dimension. The test brass had to be sized down to .4620 diameter for testing. For this sizing operation, I used a shortened .444 Marlin Die, (Don calls this die a Super Sizer), shortened a little at a time on a lathe until I had the die so that it would size the brass to .4620 at the web of the case. CCI-200 Primers were used and IMR 4895 was the propellant. An electronic scale was used to weigh each powder charge and finally the OAL of each test round was 2.902
I started my loads at 45grs. and increased by .5gr increments up to 49.5grs. I will list the 10 string, 5 shot averages with Powder Charge, Web Diameter and Avg. Velocity.
1) 45.0gr. .4651 1969fps
2) 45.5gr. .4651 2026fps
3) 46.0gr. .4650 1983fps
4) 46.5gr. .4651 2023fps
5) 47.0gr. .4652 2047fps
6) 47.5gr. .4653 2081fps
7) 48.0gr. .4655 2106fps
8) 48.5gr. .4656 2131fps
9) 49.0gr. .4657 2157fps
10) 49.5gr. .4656 2186fps
When graphed it looks like this.
You can readily see in the graph were the brass decreases in expansion in the web area. When you draw two lines, one horizontal and one vertical, the intersecting point is the RED-LINE load. On the graph above, the 45 and 45.5 points are not used in determining the curve. These two loads were from a previous test. Therefore, if you start at the 46.5gr. point and draw a line up equal distance between the 48 and 48.5gr. Points, and then draw a second line form the point at 48.5grs. up and equal distances between the 49 and 49.5gr points. Where the two lines intersect would be the maximum load for this barrel.
In this case, my maximum load is 48.7grs. IMR 4895 with a Hornady 200gr. bullet. This will produce a web diameter of .4657 and an average velocity of about 2135fps.
With this information, I now know that any load I want to develop, when I get to a web diameter of .4657 in a fired case, that is the maximum diameter for any given powder, bullet combination.
From this point, I started working with H-322 and Nosler. 225gr. BT. I still need to do some fine-tuning on this load but it is looking very promising
I might add that I did this same testing on my .308 Bellm and developed the RED-LINE for it as well. Both barrels are great shooters and a pleasure to shoot.
Bob Miller
aka mranch