COAL and chamber pressure

[pslater]

In reading Shawn Carlock's account of developing the 338 Edge +P round I discovered his account of some type of 'taper boring' the land/throat to allow the bullet to be seated farther out of the case thereby lowering chamber pressures and allowing for more powder and ultimately, more velocity. Needless to say given the velocities he was able to attain, his theory seems correct.

My Browning X Bolt .308 has a 2.90 COAL with a 178 gr. Hornady A Max touching the lands as measured with a Hornady OAL gauge. My Lyman manual says 2.78 is the max COAL with this bullet and 2.90 is too long to fit in the magazine. The 2.90 COAL leaves .45 of the bullet in the case and part of the boat tail protruding into the shoulder area - well more than the suggested 'one caliber of bullet engagement'.

I want to experiment with using this rifle in single shot mode. If I set the COAL at about 2.88 (.02 off the lands) might I experience a drop in chamber pressure or velocity that would allow me to add enough powder to restore the velocity? Right now with the 2.78 COAL, I'm getting nice groups with 44.0 gr. of Varget.

I don't have a way to check chamber pressure but I do have a chronograph.

 

[steve4102]

No.

Lengthening the throat reduces pressure, not because you can seat the bullet farther out of the case thus reducing case capacity but because a longer throat allows for more bullet jump. The father the bullet has to travel before it engages the rifling the less the pressure.

Think 5.56 vs 223, 6.8 SPC vs 6.8 SPC Spec II, and think Roy Weatherby.

Bottom line, the longer your OAL the closer to the lands, the closer to the Lands the Higher the Pressure.

 

[243winxb]

Interesting

Shawn Carlock its talking about a "stepped throat". Take note there is no pressure testing equipment mentioned. http://www.defensiveedge.net/index.php/338-edge-p Almost like machining in throat erosion, before the powder burns it out ??

 

[gamestalker]

Steve4102 is correct, more throat, lower pressure, less throat higher pressure. Unlike pistol cartridges, in which pressures grow high the shorter the OAL, rifle bottle neck gain pressure the closer the bullet gets to the lands.

This brings up another element of interest. I always work my powder charges up at the longest OAL, or with the bullet at the closest distance off the lands. With my powder charge worked up from the highest pressure OAL, I can adjust back off the lands with little concern of encountering a sudden spike, where as starting short and increasing OAL can produce sudden spikes in pressure.

GS

 

[rcmodel]

Roy Weatherby invented long throating to reduce start pressure in 1950 or so in his new line of Weatherby Magnums.

And they still use it.
As well as Remington, and most others today.

rc

 

[pslater]

Thanks Gentlemen. My knowledge was incomplete. It sounds like I should just live with the .12 throat jump I've got now. The accuracy is certainly acceptable.

Thanks again...

 

[Jim Watson]

The "stepped throat" sounds interesting.
Now if we could see what he really meant...

He needs to rediscover gain twist rifling.
Bartlein will make the barrel.

 

[pslater]

By 'gain twist' do you mean a barrel with variable twist rifling?