Failure to obturate?
- Thread starter Owen Sparks
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The case still obturates
The case still obturates on initial firing. But, the powder quickly burns out causing a drop in barrel pressure that may not be sufficient to keep the case obturated for the full duration of bullet travel. So the case starts to contract before the bullet exits the muzzle, resulting in soot along the case and sometimes around the case head.
This can happen with hot loads, too. If the powder is still burning when the bullet exits (creating a muzzle flash), the barrel pressure will drop and the case will contract. But because the powder is still burning, it may blow down around the case. I've experienced this with heavy loads and short barrels. I suspect this is what is happening with +P loads.
The case still obturates on initial firing. But, the powder quickly burns out causing a drop in barrel pressure that may not be sufficient to keep the case obturated for the full duration of bullet travel. So the case starts to contract before the bullet exits the muzzle, resulting in soot along the case and sometimes around the case head.
This can happen with hot loads, too. If the powder is still burning when the bullet exits (creating a muzzle flash), the barrel pressure will drop and the case will contract. But because the powder is still burning, it may blow down around the case. I've experienced this with heavy loads and short barrels. I suspect this is what is happening with +P loads.
rcmodel
Member in memoriam
I don't agree with that completely.
It is not unknown to get dents in bottleneck rifle cases from powder granules being blown back between the case and the chamber before the case expands enough to seal.
It's not a matter of the "sufficient to keep the case obturated for the full duration of bullet travel".
It's a matter of not enough pressure to expand the case in the first place before the powder burn is completed.
rc
It is not unknown to get dents in bottleneck rifle cases from powder granules being blown back between the case and the chamber before the case expands enough to seal.
It's not a matter of the "sufficient to keep the case obturated for the full duration of bullet travel".
It's a matter of not enough pressure to expand the case in the first place before the powder burn is completed.
rc
b·tu·rate (bt-rt, -ty-)
tr.v. ob·tu·rat·ed, ob·tu·rat·ing, ob·tu·rates
To close or obstruct.
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OK. Bullets do it "Forward", and brass does it "Rearward" (Vern
)
Bullets swage/swell up under pressure, and brass expands/strectches under pressure.
Both necessary things for optimum performance.
I still say the case does not obturate, it swells, obturating the chamber.
tr.v. ob·tu·rat·ed, ob·tu·rat·ing, ob·tu·rates
To close or obstruct.
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OK. Bullets do it "Forward", and brass does it "Rearward" (Vern
)Bullets swage/swell up under pressure, and brass expands/strectches under pressure.
Both necessary things for optimum performance.
I still say the case does not obturate, it swells, obturating the chamber.
woad_yurt
Member
Walkalong gets an A+.
I was watching this thread mainly because I'm an English teacher. Cool beans.
I was watching this thread mainly because I'm an English teacher. Cool beans.
Vern Humphrey
Member
Obturation is a physical action, and the case obturates the chamber by expanding.
I don't agree with that completely.
It is not unknown to get dents in bottleneck rifle cases from powder granules being blown back between the case and the chamber before the case expands enough to seal.
It is not unknown to get dents in bottleneck rifle cases from powder granules being blown back between the case and the chamber before the case expands enough to seal.
This is the revolver forum so I was addressing the original poster's concern regarding revolver cases getting smoked. But I imagine people won't mind if you throw in your two cents.
It's a matter of not enough pressure to expand the case in the first place before the powder burn is completed.
This is conventional wisdom, but I do not believe this scenario ever exists. The case is going to obturate very quickly, before the bullet even leaves the case, even with light charges. The time required to obturate a case is way less than the time required to push a crimped bullet out of a case. Therefore, the case is already pressed against the chamber when the bullet leaves the cartridge. It's not about the pressure, it's about the time. There is more than enough pressure to go around.
There is a way to test the theory. Throw a bullet in a camp fire. I know, I know; it's not safe. But say it happened by "accident" and you found the case. It will be split, or at least bulged, every single time. That's because case expansion happens before the bullet exits.
If you have a way to test your theory I would be interested in hearing it.
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