A little bullet pinch, a big increase in pressure

[Clark]

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I necked down .308 brass to 243 and did not ream the donut or turn the necks. The load, 40 gr IMR4895 100 gr, was ~60k psi , but for bullet pinch.
The three rounds have the same charge, and took allot of force [~100 pounds] to close the bolt.

This bullet pinch causes pressure spikes faster than more powder, faster powder, heavier bullets, or deeper seating depth. A little bullet pinch makes big pressure changes.

I think that bullet pinch can be a problem with 9x19mm as well.
I was working up a Power Pistol overload with 158 gr XTP 1.169", and it
was ok with 8 gr, but the primer always fell out at 8.5 gr.

Because the powder was being severely compressed, I decided to correct
for bullet pinch.
I double compressed the powder charge [to prevent bullet squish] and
resized the loaded ammo with a 9mm sizing die with the decapping stem
removed.

I was then able to load up to 11 gr, and there we no more pressure
problems. This is stout load as the Alliant max load for 357 mag is 8 gr
and has a 1.59" OAL.

I am concluding that bullet pinch was what caused the pressure problems
at 8.5 gr.

 

[griz]

I think I have asked this before, but just so I understand:
You aren't located in the general area of the gun when these are triggered right?

 

[Gewehr98]

Now that's an experiment...

That I wouldn't want to undertake! Gun and shooter are ok, I take it?

 

[Clark]

Both the 9mm and the 243 were hand held by me.
I wouldn't want anyone else doing this, but I know how to reduce the risks.

 

[KP95DAO]

Clark,

When you resized the loaded round you reduced the bullet diameter. This would let you use more powder due to reduced resistance during travel down the bore. I doubt that there was any engraving of the lands due to the bullet, most likely, being reduced to a size so that it would freely slip down the barrel. Why don't you pull one of these bullets and give us the diameter? I will do the same and report back the diameter.

 

[Clark]

I agree partial resizing of the loaded round makes the bullet gets smaller inside the case, but the bullet forward of the case is not touched by the die and remains .357" which is a tight fit in the .355" grooves.

The reason I have to pre compress the powder is the effort of compressing the powder with the bullet to be shot will squish it out to beyond .360", which will interfer with the throat.

 

[KP95DAO]

I seated a 150 gr Sierra .357 dia bullet into a Fed 9mm case .300" deep.
I then ran it through my much used RCBS 9mm size die as you described. The bullet was then pulled.
The bullet, which measured .357 on its length before seating/sizing, measured .349 at the point of the case mouth (the sized round measured .372 at the mouth and .377 at the point of the seated bullet base). It then tapered to .352 just above the solid copper portion of the base. The base measured .354. The portion in front of the case was also sized down to .353. This included a portion (.050") below the canalure.
So, you end up with, not a bullet that would "fall through the barrel"; but, rather a bullet much reduced in size and with a wasp waist profile. Not unlike the old Wasp Waist rifle bullet that Herters used to sell.
This would still have a substantial effect on the internal ballistic profile.

 

[Delmar]

KP95DAO


I haven't even heard of anyone speaking of the wasp waist Herter bullets in years. A friend of mine gave me a box of 180 grain soft points-strange looking little critters. I loaded up a few for my 308-not bad in the accuracy department but it looks like a gimmic to me!

 

[saands]

Clark,

You wrote that you sized ".308 brass", but I see that the headstamps are NATO brass ... so the thicker military brass may be yet ANOTHER contributing factor ... this is a particularly interesting post as that 40 grain load should have been hot (probably over SAAMI max) but shouldn't have been ALL that wacko in factory 243 brass. I know that more than a couple of people make their 243 cases from 308 ... and I wonder if they actively sort the milsurp brass out of that process or if they are just a pressure spike waiting to happen ... I'll be sending e-mails with links to this post.

Thanks,
Saands

 

[Clark]

Years later, I now realize that it was not just variation in the case neck thickness that caused the bullet pinch.

Years later, I now realize that the powder was fast.

There was Copper fouling.

The Addams and Bennet bull barrel blank for $50 in 2000 was the roughest bore I have ever seen. It attracted Copper very quickly.

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The three sequential shots shown in the picture have the same load:
40 gr Surplus IMR4895, 100 gr.
This should be 67 kpsi, but that lot # of bulk powder is best modeled as H322.
Given that, the load should have been 83 kpsi.


What does it all mean?
Three shots in a row of the same load can give different results if:
a) The load is on the hairy edge of overpressure.
b) The Copper fouling was getting extreme.
c) It took 100 pounds of force on the bolt handle to close the action on the round.

 

[Walkalong]

Yea, I'd say that was a bit of over pressure.

 

[ftierson]

Originally posted by Walkalong
Yea, I'd say that was a bit of over pressure.

Yep...



Forrest