Loading for my 300 RUM, I worked up a load very similar to one that I shot in someone else's rifle. The only difference is that the original loads were right at SAAMI COAL of 3.600" and I was able go a little longer in my magazine at 3.680". In my rifle I do not have the same pressure signs with the primer flattening. Could this be a difference in the two rifles or am I truly running less pressure because of the extra 80 thousandths headspace? Same bullets, primers and powder charge (Retumbo).
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COAL and pressure signs
- Thread starter kellyz
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thatguyshm
Well-Known Member
Hmm, I don't know the math, but if you think about it, your combustion chamber has .308"x.080" more room, which could lead to your lower pressures. COAL can change pressure immensely, especially in a big boomer such as a RUM. If you load up, don't use the same charge weight with his at a shorter COAL. Thats why its always recommended to start 10% low and build up. Each gun is different, blah blah blah, hopefully you know all of that EXTREMELY important reloading info, so I won't repeat. Load safe and enjoy!
SHM
SHM
Gerard Schultz
Well-Known Member
There is a chart that shows the relationship between COL and pressure at this page. Have a look at the last diagram at the bottom of the page, the left hand illustration.
LoneTraveler
Well-Known Member
C.O.A.L. is just a measurement used to make sure shells will work through the magazine of a rifle. The distance from bore diameter on the bullet ogive to the lands of the rifles bore are what changes pressure quick. If the second rifle is a custom built rifle or a different manufacture may be the difference. If it is a custom rifle the gunsmith may have used a chamber reamer with a long throat to allow bullets to be seated till the bullet base is even with the bottom of the neck, And allowing a longer jump to the lands to keep pressure down in reason. The amount of free bore jump to the rifling can radically change pressure. Good Luck and Happy Reloading.
D
Deleted member 46119
Guest
Short answer, yes .080 longer COAL with all other things being the same will result in lower pressure.
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As far as I can determine in a 300 RUM 2k to 5k lower PSI. Without the load data I can't get more specific as bullet and charge have a great influence.
It will also reduce velocity.
It is quite possible that the extra .080 reduced the pressure below the ~56k PSI that shows pressure signs.
If your rifle has a "looser" chamber, more pressure lost.
Longer free bore, bigger expansion chamber before the bullet wedges into the rifling, less pressure but more speed.
More...
As far as I can determine in a 300 RUM 2k to 5k lower PSI. Without the load data I can't get more specific as bullet and charge have a great influence.
It will also reduce velocity.
It is quite possible that the extra .080 reduced the pressure below the ~56k PSI that shows pressure signs.
If your rifle has a "looser" chamber, more pressure lost.
Longer free bore, bigger expansion chamber before the bullet wedges into the rifling, less pressure but more speed.
Beyond what has already been mentioned you also have different lots of powders. Are the cases the same brand and prepped the same what about neck tension. Further your free bore could be different. Without know the distance to lands for both its impossible to say on that but their are many factors. That before even considering what differences there are in the barrel bores.
