Revolver-obturation, where does it happen?

[Jim Flinchbaugh]

I finally took the time today to rid my 44 mag Redhawk of the thread choke at the
barrel /action joint. I could drop a gauge pin in and it wold travel down to the front of the threads and stop, it took a .002 smaller pin to drop through :shock:
I lapped till it was gone and the muzzle diameter pin slid right on through YAY!
Now, My question, my cylinder throats are: a .431- pin fits, a .432- pin does not.
The bore slugged out at .4307. In order to get +.001 over bore (or more) am I going to need to open up the throats to not undersize the boolits on firing or is the obturation later in the cycle like when it enters the forcing cone?
I'm was shooting straight COWW, -last time I cast some 50 / 50 wheel weights and pure to soften. That did help leading a little, but the thread choke was still leading badly.
240 grain, LBC WRFN boolit and Vit N340 cant remember the charge, (about 1200fps on the chrony)
Lube is 45-45-10 TL
I do not know enough about powders and pressure curves and peakiness and such, maybe there is a better powder? I like Vit cause its clean, but in years past I've shot a ton of 2400, 296, & Unique too.
I'm gonna shoot it as is, next time out, just wandering about the throat dimensions mainly (long winded aint I?:coffee:)

[GP100man]

Ever shot a revolver & it leaded 2" of a 6" barrel ??

That could have been where the pressure peaked & obturated the boolit .

Throat choke ,I feel peels lead off from the front driving band to the base ,then the boolit reobturates .

But there`s as many places it can happen as there are different powder loads & alloys ???

Your measurements are very close to what ya need , be careful opening the throats too much ya may run into chambering problems .

I`m intrested in your accuracy results from the fire lapping as I have a Redhawk with abit of throat choke (just a tad over .001"

How many shots did yours take ???& did ya embed the grit or buy the store bought lappers ??

[btroj]

No need to open the throats. They are larger than the bore, you should be fine.
Go shoot it a bunch. See what works, what doesn't. Until you actually shoot some cast in it how do you know it needs more work?

[Jim Flinchbaugh]

GP100man,
not fire lapped, hand lapped with a poured lap and compound.

btroj
I've shot plenty of cast in it, tired of scraping it out of the barrel :)
in fact, I dont think I ever shot a jword in it, maybe I should run a bunch through!
Will shoot a bunch more soon enough

[Bret4207]

If your throats are larger than the bore....then there will be no "obturation". The swelling we refer to as "obturation" occurs when an undersized boolit swells, usually on purpose, to fit the bore/grooves of a barrel. Starting out with a properly fitted boolit means there's no room for any swelling to take place. Now gas cutting and erosion, yeah, that can happen.

[btroj]

Bullet hardness, bullet size, and lube can all make a differnce in what you are describing. Load an undersized bullet in a revolver and you will get what you describe quite often.
Size .431 or 432 and you should be fine.
I also don't think dropping a pin gauge down the barrel is as good as slugging the barrel for measurement. A slug can tell you ALOT just be feel, does it get looser or tighter?
What are you sizing to? The fact the leading was lower with a softer bullet makes me wonder if your bullet isn't undersized and the softer one managed to obturate enough to help seal the bore quicker.
You may need to try a different lube also. I like the 45/45/10 but only for low velocity loads. I don't know that it is a great choice for heavier loads.

[Jim Flinchbaugh]

the slug definitely got tighter at the thread choke, no doubt about it.
It was slugged several times during this process, the pin guage was just confirmation on the bore size

[MtGun44]

+1 on no need to ream. Shoot throat diam or throat +.001 and you will be OK.

Bill

[Jim Flinchbaugh]

OK,
I got to the range today for the try out after lapping the barrel on the Redhawk.
I did not touch the throats, one thing at a time ya know?
Anyway, 100 rounds down range, and zero leading in the barrel.

Load was 6.5 grains 231 and 50 rounds each, LBT 240 wfn & Lee 240 grain tumble lube SWC . both lubed with 45-45-10
Happy days!

[Piedmont]

To answer the original question obturation takes place in the case. Post #5 is in error. Obturation will take place in the case--they have to be loose enough to chamber, right? So if the alloy is soft enough to obturate at the pressures involved it will happen as pressure builds to launch the bullet.

[Bret4207]

Sorry, obturation, or rather what we call obturation, can happen beyond the case friend. It's a matter of the pressure curve and what is happening to the boolit. A boolit may or may not obturate within the case on ignition and it may of may not obturate when the major diameter hits the leade or throat. You just need to start thinking in pressure/time and visualize the boolits path through the gun.

[Piedmont]

No, maximum pressure is in the case. If pressure exceeds strength of the alloy you get obturation. If not, you don't. The exception to this is a hollowbase bullet, which if soft enough, can obturate both in the case and even bump up in a barrel. This is why hollowbase .455 Webley bullets can still shoot acceptably with undersized factory cylinder throats. But the original poster isn't using hollowbase bullets, so I didn't go into that.

[Wayne Smith]

Obturation is defined as the boolit expanding to fit the cylinder it is in. If that cylinder is the cartridge then it starts there, if there is no available expansion at that point it begins when there is room to expand to fit and pressure enough to do this.

In the entire world of firearms you will probably find examples of any possibility within this broad definition. One classic example of what Bret is talking about is the old black power cartridge rifles where the chamber/throat is smaller than the bore. These require soft lead boolits so they will still have enough pressure behind them to obdurate once they get into that larger bore. On the other hand the chamber/throat of the typical Steyr is a living room compared to the bore. Obturation (expansion to fit) there starts as soon as the big light bulb goes off at the other end.

Dr. Mann's The Bullet's Flight has some excellent pictures of this process in revolvers.

[Bret4207]

Bingo, what Wayne said. It can happen in a number of places. Ever see the pictures of a revolver with the barrel pulled off and the recover boolits? The boolit bases are far, far larger than the case mouth or even the cylinder throats. There's your proof that what we refer to as obturation, (The term is incorrect but it's in common use. I forget the correct term.), can happen well beyond the cylinder. I know it doesn't seem to make sense, but it happens.

As far as max pressure only being reached with in the case, better look at the graph readings of pressure curves. Revolvers don't show this like rifles but you can have your max pressure when the boolit is well into the bore.

[Jim Flinchbaugh]

As far as max pressure only being reached with in the case, better look at the graph readings of pressure curves. Revolvers don't show this like rifles but you can have your max pressure when the boolit is well into the bore.


Where does one find these graphs for different powders etc?
All this talk about this stuff and never a real reference to where it was found out.
This comment is stemmed more form m inquiry about "Dacron for dummies" which BTW still resulted in NO information on how, when & where to learn to use it.

This info is apparently top secret or doesn't exist.

Brett,
the comment above is not "aimed" at your post, it is an expression of frustration in finding information that is talked about but never given reference too its origination

[runfiverun]

jim.
the when to use dacron was covered.
it's simple.
use it when you have velocity variatons with medium fast burning rifle powders.
no more no less.

same with powders.
the slower it burns, the longer the pressure curve is.
there is no graph showing it burns 3.2 inches down the bbl.
it's a time graph.
it burns for x microseconds.
however far that happens to be in your revolver is different than in mine.
just like the max pressure will be slightly different.

[Bret4207]

Where does one find these graphs for different powders etc?
All this talk about this stuff and never a real reference to where it was found out.
This comment is stemmed more form m inquiry about "Dacron for dummies" which BTW still resulted in NO information on how, when & where to learn to use it.

This info is apparently top secret or doesn't exist.

Brett,
the comment above is not "aimed" at your post, it is an expression of frustration in finding information that is talked about but never given reference too its origination

You will see the graphs in various publications like Handloader or Shooting Times or from the powder companies. Like R5R said, it's a time/pressure thing that will vary from gun to gun and with different boolits or bullets and barrels. Lots and lots of variables!

Anytime I see someone say "That never happens" these days it peaks my interest. Why? Because I've seen dozens of examples of guys having what "never happens" happen! The more I see in this game the less I know.