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I found this on the net, very interesting reading.


Testing the Mark V Action
This fifth and final prototype action had been built with the help of
a newly-hired engineer, Fred Jennie. It was even more streamlined,
and also incorporated a new simplified trigger and safety mechanism
to reduce tooling and manufacturing costs.
Was this the safest action ever produced, as Weatherby claimed? Roy
thought that, finally, it was time to subject his new action to vigorous
performance tests. Built on the prototype action, a rifle in .300
Weatherby Magnum caliber was used for the tests. Prior to any firing,
careful measurements were taken of the diameter of the bolt face
housing, head space measurements were recorded, and all other
pertinent dimensions.
The first test firing was with a cartridge that had been loaded with
82 grains of 4350 DuPont powder, using the 180-gr. bullet. This load
gave an average breech pressure of 65,000 psi. After firing, there was
no sign of pressure and extraction was easy. The same 82-gr.load was
fired in the Mauser action and a slight sticking of the cartridge case
resulted. Four more loads with the powder charge being increased in
two-grain increments were prepared, each one using 4350 powder and
the 180-gr. bullet. After firing the 84-gr.load, there was still no sign of
pressure and no sticking of the cartridge case, even though the breech
pressure had been increased to almost 75,000 psi. When firing this
84-gr load in the Mauser action, it caused a blown primer and it was
almost an impossibility to extract the cartridge case.
When the load was increased to 86 grains in the Mark V, the
cartridge case began to show signs of pressure, but it still did not stick
and extracted easily. Breech pressures were 85,000 to 95,000 psi. A
measurement of this fired cartridge case showed that it had stretched
at the belt by .0005 inches. When the 88-gr.load was fired in the Mark
V with pressures in the neighborhood of 100,000 psi, there was a slight
sticking of the cartridge case making the bolt slightly difficult to open,
but the cartridge case could still be easily extracted. In measuring the
belt again, it had now swelled .002 of an inch, from .533 to .535.
For the final test the cartridge case was filled with powder, leveling
it off at the top. This was 90 grains of powder. Mter firing, the bolt was
difficult to open, but without exerting too much pressure it could be
opened and the case extracted. There were now extreme signs of
pressure on the case, but no gas had escaped back into the magazine
and the primers showed very slight signs of leaking. The belt of the
cartridge case still measured .535 inches. Mter this firing, a difference
between the outside diameter of the head of the bolt and the inside
diameter of the barrel of .004 inches (or .002 a side) was noted. Even
with this terrific overload, there was still no swelling or damage to any
part of the bolt, receiver or barrel. The head space was checked
between each test firing and at no time was there any change noted in
the head space.
Weatherby was extremely excited after these tests, and felt certain
that the Mark V would be the safest action of all time!
Some time later, after additional working models of this prototype
had been produced, one last experiment was made. A 180-gr. .30
caliber bullet was lodged in the throat of the barrel. A standard load of
78 grains of 4350 and the 180-gr. bullet was chambered and fired, with
the following results: both bullets traveled out the barrel, pressures
were tremendously high, the primer was pierced letting gases enter
the interior of the bolt, hitting the firing pin sleeve which acted as a
piston, loosening the bolt sleeve cap somewhat. The bolt was opened
by hand. The cartridge case remained in the chamber, but no problem
was involved in tapping it out with a steel rod. The case appeared to
be in very good condition with the exception of the primer being
pierced.
In checking the dimensions of the barrel and action, it was found
that the barrel in front of the receiver ring expanded from 1.147 inches
to 1.1496 inches. The diameter of the bolt head expanded from. 7178
to. 7190. Head space increased from .2163 to .2174. All other dimen-
sions remained the same as prior to the test.
This experiment was repeated 15 additional times. The severest test
was when a 220-gr. bullet was placed in the bore, and a standard
180-gr. round fired in back of it. On this particular firing, the head
diameter of the cartridge increased to .545 inches. It was also noted
after these additional IS firings that the head space was set back a mere
.001. This was truly the ultimate in tests for the strength and safety of
this remarkable new action.
__________________
skipper