Throat depth effect presure ?

mahony1977

mahony1977

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I loaded up some 308 according to my lymans manual starting load for IMR 4064 is 43 grns . I set the depth according to the manual , but i seem to be building pressure already , primers flatend & one case even has an indentation from the ejector pin hole .
 
You didn't say what rifle or what weight bullet. But if you mean there is a mark on the case where the brass is protruded on the case from the hole of the ejecter, you have a heavy overload. The book may say a starting load, but in this case it is an overload, so don't shoot anymore until you investigate the cause.
Make double sure it is the correct powder and that you are reading it correctly from the book, and you are looking in the book at the correct bullet weight for that listing of powder. Make sure the bullets are .308 diameter, and not 311, by any chance.
I know this sounds elementery, but that is where to start, because something is likely wrong.
There is quite a difference in rifles as to what pressure they take. It is not beond the realm of possibility for a rifle to have too much pressure, with what is shown as a starting load.
 
Ejector or plunger marks on the brass mean "STOP!"

Military brass is heavier thus it has less volume so high pressure signs appear with lighter loads. The rule of thumb is to reduce 10%, then work up. Some commercial brass is also quite heavy, and Nosler also advises that their brass has less volume than does their competitors.

Make sure that your scale is properly calibrated. I have reached the point that I prefer to set the weight on my balance beam scale with check weights rather than adjusting the scale to zero and assume it measures correctly.

edited to add . . .
If your specific rifle has a short throat, and your bullet is seated long enough to contact the rifling, pressure signs will appear with lighter powder charges.
 
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To answer your question, yes, throat depth does affect pressure. If the bullet is hard into the rifling, pressure will be higher.
But unless your rifle has a custom chamber (like an old target rifle barrel) it is unlikly a 150 gr bullet would be in the rifling.

Has your rifle powder been contaminated? Did you empty the powder thrower into the can one day and add the wrong powder?

Measure the bullets to make sure they are not some .312 look alikes.

I keep a 50 grain 223 bullet on my scales as a check weight that I use from time to time.

Have you pulled a left over round and checked the powder weight? With a balance beam scale it is not unusual to make a boo boo and be out by 5 grains.

Are you sure the bullets are 150 gr?

Has the 4064 dried out? As it dries it gets faster. Don't leave powder in the thrower.

Something is really wrong here. In the average 308 it woud take 48 gr (a full case of 4064 to get that kind of pressure under a 150 gr bullet.

An ejector mark means you are extruding solid brass into the bolt face. You are in proof round territory.
 
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Boomer said:
Ejector or plunger marks on the brass mean "STOP!"

Military brass is heavier thus it has less volume so high pressure signs appear with lighter loads. The rule of thumb is to reduce 10%, then work up. Some commercial brass is also quite heavy, and Nosler also advises that their brass has less volume than does their competitors.

Make sure that your scale is properly calibrated. I have reached the point that I prefer to set the weight on my balance beam scale with check weights rather than adjusting the scale to zero and assume it measures correctly.

edited to add . . .
If your specific rifle has a short throat, and your bullet is seated long enough to contact the rifling, pressure signs will appear with lighter powder charges.
Click to expand...


I'v checked my scale again & everything is ok , i think my problem's are being caused by a short throat . Should i try to seat my bullets a bit deeper to correct ?
 
GANDERITE said:
To answer your question, yes, throat depth does affect pressure. If the bullet is hard into the rifling, pressure will be higher.
But unless your rifle has a custom chamber (like an old target rifle barrel) it is unlikly a 150 gr bullet would be in the rifling.

Has your rifle powder been contaminated? Did you empty the powder thrower into the can one day and add the wrong powder?

Measure the bullets to make sure they are not some .312 look alikes.

I keep a 50 grain 223 bullet on my scales as a check weight that I use from time to time.

Have you pulled a left over round and checked the powder weight? With a balance beam scale it is not unusual to make a boo boo and be out by 5 grains.

Are you sure the bullets are 150 gr?

Has the 4064 dried out? As it dries it gets faster. Don't leave powder in the thrower.

Something is really wrong here. In the average 308 it woud take 48 gr (a full case of 4064 to get that kind of pressure under a 150 gr bullet.

An ejector mark means you are extruding solid brass into the bolt face. You are in proof round territory.
Click to expand...

I am useing a balance beam , bullets are right , powder is new . I think i will pull a couple apart to make sure , probably is a boo boo on my part . this is the first time it's happend , but this is also the first time i used 4064 in it .
 
Throat depth is not responsible for any major pressure change, and you have a major pressure increase.
I have loaded to hit the lands hard, clear a little or clear a lot, and I can notice no change, whatsoever, in pressure, on various calibres.
You have an excellent rifle, good brass, good everything. It certainly is a mystry. It would be interesting to try the reloads in another 308.
 
Contrary to what some people may believe,the distance to the lands can have a significant effect on pressures as shown in the link below.

http://www.shootingsoftware.com/pressure.htm

sampleseatingtrace.gif


Here's a classic good load for 6PPC. The only difference between all these traces is the first 3 were seated right on the lands. The last 4 were 30 thousands of an inch off the lands. Note the substantially reduced pressure and overall reduction in energy.

As the bullet broke it's neck tension and jumped to the lands it provides more expansion room for the gases reducing both the energy and velocity.
Click to expand...
 
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