Do gas checks obturate?

[rosewood]

I picked up the Lee 200 grain .358 GC mold to use in my single shot CVA .35 remington. Sized to .358. Loaded up loads with H335 and H4198. Not 100% sure, but I swear the first few rounds with H4198 and with H335 seemed to be leading. I ran the bore snake through after a few shots and could see leading in the barrel. So pretty sure it wasn't burnt powder. Then I shot the hottest load in H335 and all of the sudden, all of the leading went away. Is it possible that the gas checks obturated and cleaned out the leading? Gave the barrel a good cleaning today and absolutely no lead was found.

Thanks,

Rosewood

[Tenbender]

Good question. I will be looking for replies from those who know .

[Nueces]

I believe it's possible that a gas checked shank can obdurate. I recall a gunzine article many years ago with photos of jacketed revolver slugs caught after firing without a barrel - just the cylinder. The bases were all swoll up, like they had mumps, from muzzle blast. I think the loads and bullets were something like early SuperVel ones - heavy charges and thin jackets.

[reed1911]

Yes, well sort of. The GC does not really have enough meat on it to obturate in the conventional sense, the bullet does and the GC stretches along with it.

[GRUMPA]

Yes, well sort of. The GC does not really have enough meat on it to obturate in the conventional sense, the bullet does and the GC stretches along with it.


Nicely put......I read the thread earlier and was trying to come up with something that made some sort of sense.

But......you said it the way I was thinking..

[44man]

Yes, they do but the primary function of a GC is to halt boolit skid. Any expansion of the boolit base and GC will aid that. Just don't slump the rest of the boolit.

[mdi]

Uneducated thought; a thin fairly soft copper cup with 30,000 PSI behind it will obturate. Also the lead in front of the check will deform with the pressure on the cup (like squeezing will make it fatter). So, my limited metal/mechanical knowledge says the instance you mention; the leading ceasing, is prolly due to the cup obturating and also grabbing the bullet to eliminate skidding...

[44man]

Sounds educated to me!

[williamwaco]

I don't know about obturate but I do know that they will remove leading from a .38 special or .357 magnum bbl with normal pressure loads.

[bstone5]

Gas checks on a plan base cast bullet gives a very good seal keeps blow by from happening and will also remove any lead in the barrel.

It takes some time to install the gas checks but will keep all the lead out of the barrel.

A gas ckeck on a powder coated cast bullet will result in no lead in the barrel.

[Harry O]

Gas checks are wonderful things, particularly with the .357 Magnum (and to a lesser extent, the 9mm). My opinion is, this is what happens.

The gas check does not exactly obturate. When fired, the pressure in the center of the GC moves the edges outward slightly. The GC flexes slightly. The edge that is moved out scrapes the leading off the bore of the handgun. In addition, it protects the base of the lead bullet so that the base does not differentially fail (the pressure of all magnums in more than the ultimate strength of lead) and cause the base to be off-square, meaning it is no longer flatas it exits the bore. Non-flat bases are bad for accuracy.

I have read that the GC keeps the lead bullet from skidding (making a gap along the rifling that the gas can get into, causing leading), but I am not sure about that. That may be so today. However, I remember using some GC's in the past that did not crimp-on. They could not keep the bullet from skidding, but they seemed to work as well as crimp-on GC's at stopping leading.

[mold maker]

Think of it this way. When installing the GC we compress the leading edge into the lead, which can doam the base slightly. When fired, the pressure is all against the base which removes any doam causing the forward skirt to expand slightly. This coupled with the obturation of the boolit creates a scraper out of the GC mouth. It is contained within the bore so any lead it encounters is removed.

[rosewood]

Uneducated thought; a thin fairly soft copper cup with 30,000 PSI behind it will obturate. Also the lead in front of the check will deform with the pressure on the cup (like squeezing will make it fatter). So, my limited metal/mechanical knowledge says the instance you mention; the leading ceasing, is prolly due to the cup obturating and also grabbing the bullet to eliminate skidding...


Definitely was hitting 35k with these loads. But then again, I probably hit 30k with the starting load also.

[rosewood]

Problem I have is, I didn't like the hottest load and want to use the lesser with H4198. Guess I am going to have to find/modify a Lee sizing die for .359 so I don't have to push so hard to avoid leading.

[Silvercreek Farmer]

Yes, well sort of. The GC does not really have enough meat on it to obturate in the conventional sense, the bullet does and the GC stretches along with it.

Coincidentally enough, I was picking boolits out of my berm just the other day and came across a boolit that started with a gas check and lost it on impact. The boolit shank had expanded to nearly full caliber and was barely visible anymore which would support this theory.

[44man]

Yes. it is correct but I have recovered GC boolits with the checks totally leaded up because the boolit was too soft. It will not save everything. I can't get pictures up.

[vzerone]

Yes. it is correct but I have recovered GC boolits with the checks totally leaded up because the boolit was too soft. It will not save everything. I can't get pictures up.

I've found that too, but ironically my bore was lead free.

[rosewood]

Just measured my bore, it is measuring just under .358, maybe .3575 to .3578. So that is barely overbore, should I go to a .359 sizer?

Thanks,

Rosewood

[DougGuy]

OP to answer your question, you would need to recover some checks from the loads that leaded and look at them and see how much engraving each got from the rifling. I have seen gas checks recovered but never not once have I seen one that wasn't engraved so likely the answer is yes they obturate and IMO quite easily.

[geargnasher]

The medical and ballistic definition of the word "obturate" is to seal a tube. It has nothing to do with bumping or deformation. So, by definition, if a gas check checks gas, it has obturated. If it doesn't obturate the bore, then it leaks gas and isn't going to work very well.

Do they deform? Absolutely. Recover some bullets fired bullets and observe the change in shape of the trailing edge, the concavity of the cup, and the tails from the metal displaced by the rifling. The amount of shape change during firing can have a very significant effect on how a rifle bullet performs, and can be a good indicator of how well you're matching your powder burn rate to the application.

Gear

[BAGTIC]

I have always felt that they operate like a scraper ring on an engine piston. They scrape loose what the front part of a lead bullet has already deposited.

[Larry Gibson]

"Do GCs obturate?"

Yes they do, not always but at the normal psi's we use GC'd bullets they do obturate. Seems to be a lot of confusion with some about what "obturate" really means. Using a simple internet search we find the following definition;

obturate
[ob-tuh-reyt, -tyuh-] /ˈɒb təˌreɪt, -tyə-/
Spell Syllables

Word Origin

verb (used with object), obturated, obturating.
1. to stop up; close.
2. Ordnance. to close (a hole or cavity)so as to prevent a flow of gas through it, especially the escape of explosive gas from a gun tube during firing.

Seems simple enough but considering just the ordnance aspect the question of this thread is still not answered. Why? Because the definition answer the what of obturate but not the how it's done. The question of this thread asks the "how". Thus looking at several other ordnance/ballistic definitions we get a more definitive answer to the "how" question.

From Sierra's Exterior Firearms Glossary we find;Obturation: The sealing of a bore and chamber by pressure. During the firing process, pressure swells the case within the chamber, preventing gas from leaking back into the action. The same pressure, applied to the base of the projectile causes it to swell or upset, filling and sealing the bore.

Well now we're getting somewhere toward the answer. In addition we can look at the definition on Wikipedia:With reference to firearms (https://en.wikipedia.org/wiki/Firearm)and airguns (https://en.wikipedia.org/wiki/Air_gun), obturation is the result of a bullet (https://en.wikipedia.org/wiki/Bullet)or pellet (https://en.wikipedia.org/wiki/Pellet_(air_gun)) expanding or upsetting to fit the bore, or, in the case of a firearm, of a brass case expanding to seal against the chamber at the moment of firing. In the first case, this both seals the bullet in the bore, and causes the bullet to engage the barrel's rifling (https://en.wikipedia.org/wiki/Rifling).In the second case, it seals the case in the chamber and prevents backward travel of gases against the bolt. The thin brass case easily seals the chamber, even in low pressure rounds like the .22CB (https://en.wikipedia.org/wiki/.22_CB), but expanding or upsetting the bullet sufficiently for effective obturation requires sufficient pressure to deform the bullet material.

Pure lead is very soft, and canbe expanded or upset by most firearm cartridges, but the pressures required are higher than those encountered in most airguns. To allow obturation in airguns, pellets use soft alloys in conjunction with a thin, concave base designed to expand more easily. Some firearms ammunition, such as Fosterslugs (https://en.wikipedia.org/wiki/Foster_slug) and hollow base wadcutter (https://en.wikipedia.org/wiki/Wadcutter)bullets, also use a hollow base to allow the bullet to expand and conform to a barrel's irregularities, even as the chamber pressure drops as the bullet travels down the barrel (see internal ballistics (https://en.wikipedia.org/wiki/Internal_ballistics)). Forexample, it is not uncommon for revolver (https://en.wikipedia.org/wiki/Revolver) barrels to have a slight constriction at the breech end where they thread into the revolver's frame; a hollow base bullet will expand to fill the larger diameter of the barrel after passing through the constriction.

To prevent excessive deformationin high pressure rifle (https://en.wikipedia.org/wiki/Rifle)and magnum pistol (https://en.wikipedia.org/wiki/Pistol)cartridges, lead bullets are often covered in copper or another harder alloy. These bullets are generally designed to swage (https://en.wikipedia.org/wiki/Swaging#Firearms_and_ammunition) to fit upon firing.

The same principles apply to artillery ammunition; guns are traditionally categorized (in English-speakingcountries) as "BL" and "QF" guns, or "Breechloading (https://en.wikipedia.org/wiki/Rifled_breech_loader)" and "Quickfiring (https://en.wikipedia.org/wiki/Quick-firing_gun)" guns. "BL" guns are the older style, and typically use propellant stored in a number of fabric bags, the number of which can be adjusted to vary the range in some cases, as well as a separate shell that is rammed before the powder charge is placed into the breech. Large-caliber naval guns are traditionally of the "BL" type. Lacking a cartridge case to seal the breech, these guns require a complex and strong breech mechanism to safely seal the breech during firing. "QF" guns were a later development, usually of small to medium caliber, and came about from the need for rapid firing guns to counter torpedo boats and other small, nimble threats. A QF gun fires metallic-cased ammunition, generally with the shell fixed to it, as a single unit, just like a firearm cartridge. This allows a simpler breech mechanism, such as a sliding block breech, which when coupled with the easier ammunition handling and ramming procedures, allows a much higher rate of fire. Larger guns are most often BL guns, because the ammunition is so large and heavy, that even adding a metallic cartridge case would add a great deal of weight and make handling more difficult, even if you still loaded the shell and propellant separately. There have been exceptions, however; some German naval guns during WWII used fabric powder bags with just thin brass base section, to provide obturation to the breech without adding too much weight,similar to a modern plastic shotshell with a "low-brass" base for sealing and extraction.

I'm not a great fan of Wikpedia but that definition is essentially correct.

Some basic examples in context with cartridge firearms; when the primer fires it's sidewalls expand to the sides of the primer pocket and seal....thus it obturates or has obturated. When the psi is sufficient it expands the case sidewalls sealing them against the chamber walls....thus the case obturates or has obturated. When the undersized bullet, cast or jacketed, expands or is swaged down to fill the grooves sealing the bore it obturates or has obturated. If the cast bullet has a GC on the base and it expands or is swaged down to fill the grooves and seal the bore it obturates or has obturated.

There are numerous other similar definitions available should anyone care to research the topic.

If the GC on a bullet is sized larger than the groove diameter and is swaged down to seal the bore it also swaged the shank down and it obturates or has obturated.

If the GC on a bullet is undersize of the groove diameter it can be swaged out by the upsetting bullet shank to seal the bore. We can say the GC then obturates or obturated as did the shank or a greater part of the bullet.

So, yes, GCs do obturate.

Larry Gibson

[mdi]

When fired, the pressure in the center of the GC moves the edges outward slightly. The GC flexes slightly. A fair defination of "obturate".

[44man]

One thing to watch with any GC is never have them sized larger then the boolit. They will expand the brass before proper tension can be had on the boolit. Make them the same size as the boolit.

[rockrat]

I think I would try a .359" or possibly a .360" dia boolit.

[rosewood]

I think I would try a .359" or possibly a .360" dia boolit. The boolits are dropping at about .360, but I have to install the gas checks. Guess I will order a Lee .358 sizer and hone it out to .359 since I don't see anyone that sells one in that size. I want to keep my .358 for my other guns that need it.

Thanks,

Rosewood

[Menner]

Get a hold of Buckshot here on the form I just got a 359 and 360 lee type sizer from him. this is the 4th and 5th sizer that I have got from him top notch work
Tony

[44man]

Larry has it right. But you need to understand it is not the total solution. I worked with many muskets shooting Minie' balls with hollow bases. We shot competition and some guns could not hit a 4'x4' board at 50 yards. I found poor fit that depended on skirt expansion to seal. Did not work. Of course it did seal gas but there was more to it. I lapped molds for a better fit to the bore so a hard thumb push was needed. Then a gun that would not hit anything could ring a 200 meter gong. you need guidance not just a seal.
Same with a revolver, if you think just base expansion is enough, not so.