7mm WSM long throating

[mooredj]

This is the modeled load data using GRT. I need to provide an explanation on why I show the freebore as 0.236. It is actually the traditional freebore as everyone knows it plus the chamber to bore transition distance added in. That distance is 0.0182 so that plus the traditional freebore of 0.218 equals 0.236. The load is slightly under compression but I believe GRT is slightly conservative based on actual data so I am hoping to use just a little less powder to get to 2900fps. Anything extra is a bonus. The 7 SAUM is a very efficient cartridge at 32.4% set up with H1000.

 

[mooredj]

This is what the cartridge and chamber drawings look like if anyone is interested. I have found that the ADG brass is thick enough that you can run it in a SAAMI chamber with no modification except to lengthen the freebore. The standard SAAMI throat angle is 1 degree. You could use a 1.5 degree throat reamer to lengthen a barrel chambered with a standard SAAMI reamer to match what is shown.

If you look a the "s" in the chamber dimensions column on the right, you will see it is 0.2360 inches. That is where I get that number from. Just easier for me to work with.

 

[MagnumManiac]

I love how some think that a coned throat is inferior when some of THE most accurate cartridges out there have this exact design…
As I said early on, pick a bullet, make a dummy round with the heal below the neck, compromise on length and do it.
A good all around figure for this cartridge is .200"-.220" of throat.
Simple.

Cheers.

 

[ARlife4me]

What I did when ordering a custom barrel is what was mentioned before......sending in dummy rounds (3) and ask to add xxxxx to the throat. My 6.5prc AR was chambered for Lapua 144fmj w/+.100. It gave me options with other bullets in different weights with it being close to the same ogive lengths.

 

[mooredj]

I love how some think that a coned throat is inferior when some of THE most accurate cartridges out there have this exact design…
As I said early on, pick a bullet, make a dummy round with the heal below the neck, compromise on length and do it.
A good all around figure for this cartridge is .200"-.220" of throat.
Simple.

Cheers.

Two things:

1) I had actually thought of using an even larger throat angle of 3 degrees until I did enough reading to figure out the benchrest community had figured this out already. They determined that after shooting through a barrel set up this way, you eventually end up with a 1.5 degree throat angle anyways so you are better off starting out that way.

2) Almost every new chamber design is being set up with a 1.5 degree throat. This is not by guessing. As noted, some of the most accurate cartridges use this design because it is forgiving and it works.

 

[mooredj]

Just some data I put together since I am jumping into the 7 SAUM arena. I am really working on getting the heavies to work. I wanted just a bit more powder capacity to makes sure I can get to 2900fps. I have set this up with a 0.218 freebore as designated by the column noted in green. The arrow on the bottom left points to the bullet seating depth. The arrow on the right is what I have settled on for shooting the Berger 195 EOL. I have been told what you need is bore diameter bullet seating and that is shown at 0.284. The cartridge neck length is 0.311. The bullet is seated in the case of the seating depth at 0.311 so that the bearing surface starts right at the case neck junction with the boat tail below that point. The boat tail is slightly up the neck when the bullet is seated at 0.284. I am taking a page from Brian Litz on seating depth for the Berger hybrids and assuming I will find accuracy at a jump of 0.025 with a seating depth of 0.284. If what he says holds true, I will keep shooting the same load with the same bullet seating depth until accuracy falls off at some point. In the meantime I will keep measuring the throat to see how fast it moves. Pay attention to the COAL on the far right. I will be shooting this out of a Zermatt Arms TL3 long action so no issues for me but a short action will be a problem. You really need a long action to stretch out the legs on the 7 SAUM with heavies. If I want to shoot the 180 Hybrids, I will not be able to seat to bore diameter. It can be done but I will probably need to single load to make sure the bullet stays concentric during loading. Okay for target and not ideal for hunting loaded in a magazine. This design does open up lots of possibilities using some of the other heavies.

You can also see what the SAAMI design looks like with the 0.90 freebore using a 1 degree throat angle. You can see how the 180 hybrids work but where using the heavies becomes an issue since you can't get the powder capacity you need to get them to shoot inside the 180 hybrids.

View attachment 555906

I posted this because I wanted to know the math behind what the gunsmiths work off of when you send them dummy rounds. Now I can just send them a chamber print, make their life easier, and I know what I will be getting when they send me the barrel. In this case, I have the option of either having a reamer made for me, that I can slide inside a barrel with a standard SAAMI chamber, and finish ream to my design dimensions, or just use a 1.5 degree throat reamer with a standard SAAMI chamber and cut the throat to my design dimensions. The reamer would be more expensive but would make it easy to get it right since the reamer would stop on the chamber shoulder. Obviously, easy does it with either method so you don't cut extra metal in the process.

I am hoping this is helpful to others.