Failure to obturate?


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Owen Sparks
April 26, 2009, 01:03 AM
I have noticed that mild loads in revolvers tend to have a black sooty smudge down one side of the empty brass case. Is this because the case is not obturating (expanding under pressure)? Would increasing the load stop this messy blowback?

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ArchAngelCD
April 26, 2009, 01:12 AM
If you're loading very hard bullets you will need higher velocities to prevent that "sooting". You are probably getting some leading too. If you don't want to increase the velocities you can change your bullet. Shooting a softer bullet will give you the same results as increasing the velocity on the harder bullet. (a hardness of 10-12)

Steve C
April 26, 2009, 03:54 AM
Is this because the case is not obturating (expanding under pressure)? Would increasing the load stop this messy blowback?

Short answer is YES. Generally it is best when developing loads to start at 10% below maximum or at the listed start load, which is usually the same 10% but the maths done for you. A start load should produce enough jpressure to seal the case in the chamber. If you are loading a "special" light load for a particular purpose like a light load for new or recoil sensitive shooters, you'll just have to put up with the fouling on the case.

ArchAngelCD brought up another issure regarding lead bullet base obturation and the need to match pressure with lead hardness to avoid leading. Hard bullets actually lead less (or not at all) with heavy loads while a soft bullet works well with lower pressure loads.

If you pour your own you can size them properly to avoid leading but with commercial cast you rely on the base of the bullet obturating enough to fill the barrel which requires an appropriate level of pressure for the bullets hardness.

Stainz
April 26, 2009, 06:16 AM
If the caliber is .45 Colt, you may have to exceed the SAAMI ratings for that to happen... unless you are shooting a custom or FA revolver with proper chamber IDs. Everyone else still uses the so-called 'black powder' ID dimension, which, being larger, still allows for black powder fouling. Of course, try to find bp loads in .45 Colt. Then you'll note that they are hotter - and a lot nastier - than the typical smokeless propellant .45 Colt load - especially 'cowboy' loads'. You'll note that smoke trail is usually on the vertically top side of the case.

You can also experience it with larger chamber IDs from just careless, in my opinion, manufacturing. My favorite example were my Ruger .32s - an SSM and a 4" SP-101. Both allowed case swelling to .337+", while sizers and commercial ammo was .334" or smaller. So, it's not always lite loads.

Stainz

Glockman17366
April 26, 2009, 08:52 AM
WOW!!!!!!!
I learned a new word today! Obturate!

harmon rabb
April 26, 2009, 08:54 AM
WOW!!!!!!!
I learned a new word today! Obturate!

me too :cool:

The Lone Haranguer
April 26, 2009, 09:00 AM
http://wpcontent.answers.com/wikipedia/en/thumb/8/8c/Cool_Hand_Luke_Martin.jpg/225px-Cool_Hand_Luke_Martin.jpg

Whut we got heah is ... failiah to obturate!

Beagle-zebub
April 26, 2009, 09:06 AM
I'm glad I'm not the only one who thought what Lone Haranguer thought.

logical
April 26, 2009, 09:06 AM
I think for cases, you should stick with "expand". Obturate has it's roots in "obstructing or blocking" and refers to a soft bullet filling a barrel because of the pressure squishing it from behind or from within the hollow cavity. cases get blown straight out...thats just expansion.

Walkalong
April 26, 2009, 09:10 AM
I think for cases, you should stick with "expand". Obturate has it's roots in "obstructing or blocking" and refers to a soft bullet filling a barrel because of the pressure squishing it from behind
Yep. Cases expand to seal the chamber and stop gas blowby/sooting in that direction. Bullets obturate to fill and seal the throats/bore to stop gas cutting in that direction.

Would increasing the load stop this messy blowback?Increasing pressure in the case will. More of the same powder, or use a faster powder that builds pressure more quickly. The faster powders are real handy for "light loads" in revolvers.

ArchAngelCD was asleep at 12:12 this AM. He knows the answer. :D

SaxonPig
April 26, 2009, 11:09 AM
Yes, low chamber pressure can result in sooty cases as they fail to expand and seal the chamber from blow-back.

At this point I feel the need to point out that I have noted this is a common condition with factory +P 38 Specials meaning the chamber pressure is too low to seal the chamber. So much for +P being so hot.

Matt-J2
April 26, 2009, 11:19 AM
Increasing pressure in the case will. More of the same powder, or use a faster powder that builds pressure more quickly. The faster powders are real handy for "light loads" in revolvers.

I don't think I can stuff any more BP into the cases! :p

ArmedBear
April 26, 2009, 11:58 AM
I've generally seen this when I'm loading light (non-magnum or "cowboy") loads in magnum cases.

.38 Special cases are built to obturate at a lower pressure than .357 Magnum cases, for example.

A .38 Special loaded to 800 fps will obturate fine. Put a similar load in a .357 Magnum case and you'll see a lot of blow-by.

.44 Magnum case walls can be so thinck that I have to wrestle with my press when resizing the brass. They REALLY resist obturation at low pressures. The fired cases generally fall right out of the revolver, too, no ejection required.

Walkalong
April 26, 2009, 07:53 PM
Brass expands, brass does not obturate. ;)

Don't even........ just look up the definition of obturate. :neener:

ArchAngelCD
April 26, 2009, 09:56 PM
Yeah, staying up all night doesn't mean I'm actually awake although I'm usually alright until after 4:00 AM!! LOL

ArmedBear
April 26, 2009, 10:55 PM
Brass expands, brass does not obturate.

Well, the brass doesn't expand, either. The actual brass does not change in volume at any point.

The case, made of brass, really inflates and doesn't actually expand. It doesn't get bigger; the case wall stretches and gets thinner, while the diameter of the cylinder it forms gets larger.

I'm not defending my misuse of "obturate". It really doesn't matter.

The point is, heavier magnum cases take more pressure to stretch them out to fit the chamber tightly, than thinner-wall cases like the .38 Special.

PRE 64 JOE
April 26, 2009, 11:17 PM
obturate, I think my brain hurts cause I just figured out what y'all means.

woad_yurt
April 27, 2009, 09:39 AM
Walkalong:
The OP does have obdurate brass. There's no denying it.

Noxx
April 27, 2009, 10:15 AM
At this point I feel the need to point out that I have noted this is a common condition with factory +P 38 Specials meaning the chamber pressure is too low to seal the chamber. So much for +P being so hot.

I think it's more a function of +P cases being stronger. Obviousl the ballistics don't lie, 38+P is higher velocity.

Anyhoot, I noticed the same thing when I started reloading, my first loads were very cautious, and I would tend to get a lot of case staining. After coming up to "factory" type loads in pressure and velocity, most of the problem went away, except with certain powders.

ArmedBear
April 27, 2009, 10:59 AM
...and when you worked up to just pouring in as much 296 as would fit in the case, and stuffing a bullet on top of it with a massive crimp to keep it in, your cases came out of the guns looking like they just got tumbled?:D

What's a few blown primers and cracked cases when you can avoid cleaning them?:p

(DISCLAIMER: THIS IS A JOKE. IT COULD BLOW UP YOUR GUN. DO NOT TRY THIS AT HOME!)

Walkalong
April 27, 2009, 11:11 AM
The case, made of brass, really inflates and doesn't actually expandI'll buy that, along with strecthing as it "inflates" :)

The point is, heavier magnum cases take more pressure to stretch them out to fit the chamber tightly, than thinner-wall cases like the .38 Special.Absolutely.

Noxx
April 27, 2009, 01:29 PM
...and when you worked up to just pouring in as much 296 as would fit in the case, and stuffing a bullet on top of it with a massive crimp to keep it in, your cases came out of the guns looking like they just got tumbled?

296? Are you made of money man>?! I use Titegroup, it doubles as a desert topping.

logical
April 27, 2009, 01:41 PM
Well, the brass doesn't expand, either. The actual brass does not change in volume at any point.

The case, made of brass, really inflates and doesn't actually expand. It doesn't get bigger; the case wall stretches and gets thinner, while the diameter of the cylinder it forms gets larger.

You are being obtuse. Using the term "expand" to describe a brass case swelling is a perfectly correct use of the word. Nothing in the definition of expand says it changes in actual volume (I think you mean mass anyhow). A bird expands it's wings, a person expands their lungs, a ballon expands whem blown up.....

Words matter.

logical
April 27, 2009, 01:46 PM
double post

Vern Humphrey
April 27, 2009, 01:52 PM
The proper terminology is "rearward obturation" which occurs when the brass expands and fills the chamber, and "forward obturaton" which occurs when the bullet fills the bore.

If you have sooty brass, you're not getting rearward obturation.

logical
April 27, 2009, 01:53 PM
Vern...you just plain made that up and you know it.

I've never called for a thread to be locked but this one should be locked, burned and buried.

liljohn
April 27, 2009, 05:53 PM
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
April 27, 2009, 06:01 PM
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

Walkalong
April 27, 2009, 06:18 PM
bĚtuĚrate (bt-rt, -ty-)
tr.v. obĚtuĚratĚed, obĚtuĚratĚing, obĚtuĚrates

To close or obstruct.

---------------------------------------------

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. :evil:

woad_yurt
April 27, 2009, 06:26 PM
Walkalong gets an A+.

I was watching this thread mainly because I'm an English teacher. Cool beans.

Vern Humphrey
April 27, 2009, 06:40 PM
Obturation is a physical action, and the case obturates the chamber by expanding.

Walkalong
April 27, 2009, 07:01 PM
Obturation is a physical action, and the case obturates the chamber by expanding.
Another A+ :D

liljohn
April 27, 2009, 10:53 PM
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.

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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