Brass
thickness at the web, brass hardness, surface finish of the chamber wall, and
the amount of taper on the cases all effect what pressure any given cartridge
can be operated at without damaging the frame. And of course any oil in the
chamber or excess case lube on the cases will also effect the point at which the
case sets back in the chamber.
.30/30 types, approx. .420" head diameter. A little over 45,000 psi.
.444 Marlin types, approx. .468" head diameter. Approx. 45, 000 psi.
.45/70, approx .505" head diameter. 28,000 to 32000 psi max.
Again, these are not hard and fast rules.
Take a .30/30 type cartridge with its thin brass and significant body taper, and
it may stick in the chamber and make the gun hard to open at something around
45,000 psi, yet the same head size on the thicker .375 Win case has a SAAMI max
pressure at around 48,000 psi and functions with no problem at all in the
Contender.
Similar results are obtained with cartridges based on the still thicker .225 Win
case, especially in the designs that have very minimal body taper.
Because I advise using strong .225 Win brass up through 6.5mm and .444 Marlin
brass for most applications from .25 cal. and larger, we will focus on these
types of cases.
And because I do little with .225 Win at present due to constraints imposed by
custom dies, from here out cartridges based on .444 Marlin are mostly what we
will be talking about. Between Don Shearer and myself, we have put a lot of time
and study into the characteristics of .444 Marlin brass and have come up with
some very good methods of determining a functional maximum pressure using it.
Don Shearer's refinement of my procedure will be posted as a separate page.
What I
want to do here is give some quick guidelines.
First, with any cartridge, hard unlocking of the barrel is a sure indication the
case head has set back hard against the breech face, and it is high time to
reduce powder charges. I find that from this point, you can usually get away
with a maximum charge that is about 5% below where the cases stick. Note that I
said "get away with..." No matter what is said, we all seem to insist on
stretching the envelope, so yielding to human nature, let's deal with that
nature as it is. 6% may be more prudent, or 7%. But if you were inclined to be
prudent, you probably would not be bothering to read this, right?
Focusing as I said on the .444 Marlin case, IF the chamber will allow it to
expand about .003" at the web, we find that with each increase in powder charge
producing an increase in web diameter, the point at which the rate of growth of
the web diameter levels off is also the functional maximum pressure.
Said another way, the new brass measures .465" at the web. Your starting load
gives a fired web diameter of .466." You up the charge a grain, and the web
diameter shows just a slight increase. You go up another grain or two, and the
web diameter is now .467. Note that I am not talking here about tenths of a
thousandth, but rather thousandths that you can readily measure with any decent
caliper... even some of the cheap plastic ones.
If you will very carefully watch the appearance of the SURFACE FINISH of the
primers as you go, this will also tell you a lot. Note that I did not say
flattening of primers... Flattening of primers is useless as an indicator of
pressure. It can be utterly, totally misleading, since much of the flattening
has to do with headspace, not pressure. This is another subject.
Forget everything you thought you knew about large rifle primer flattening
as an indicator of maximum allowable pressure in Contenders.
Ok, you increase the charge another couple grains, and your fired web diameters
are now up to .468." Another grain increase, no change in diameter. One more
grain, and still no increase in diameter. Gun still opens ok, and you try one
more grain. Still no change in diameter.
Go back
to the load that gave you the diameter before changes stopped occurring, and
call it max.
What happened? The pressure finally exceeded the elasticity of the brass, and
further growth of the case was stopped by the diameter of the chamber.
If you watched the primer surface finish very carefully, and I might add even
perhaps using a magnifying glass, you should have noticed the primers going from
shiny smooth to showing the texture of the breech face imprinted on the primer.
When you first get this "signature" of the breech face on the face of the
primers, you are within about 5% of max.
Note
also that primer cups are not all alike... some are thinner or softer than
others and will show the breech face signature at differing pressures.
Continuing your charge increases from the first appearance of the breech face
signature, at about the same point the case web stops growing, give or take a
little, you will start to see a little imprint from the rimfire firing pin hole
on the top edge of the primer. Once this rimfire firing pin hole imprint starts
to appear, you are within just a couple grains of absolute max. I.e., you may
well be able to go above this point several grains before the barrel becomes
hard to open, but don't do it... You are standing at the edge of thin ice, safe
enough for the minute, but take another step or two, and you'll be in over your
head. SO STOP WHERE YOU ARE AND BACK UP A LITTLE.
Note
that the rimfire firing pin hole signature does not appear on the primers of
fired factory .444 Marlin ammo which is supposed to be loaded to about 44-45,000
psi. This means that you are above the 45,000 psi level when the rimfire firing
pin hole signature appears.
On a daily basis I am running pressures up to the point where I get a full
expansion of the case so that
1)
I can be certain I have a clean chamber that extracts well and
2)
I have determined the maximum diameter the user will encounter at the web and
verified his size die will size this down by at least a thousandth of an inch.
So again on a daily basis, I am walking right out to the edge of thin ice... but
by watching the signs outlined above, I stay warm and dry. One shop frame went
nearly 10 years shooting thousands of rounds of wildcats based on .444 Marlin,
.307 Win, and .30/30 types, .300 Savage, .250 Savage, and .444 Marlin factory
ammo. It finally became questionable, and I sent it to TC for inspection; they
replaced it even though it was not particularly loose with most barrels.
Part of the basis for the above procedure is from careful comparison of the
primers from fired factory rounds in the KNOWN 45,000 psi class, namely .444
Marlin, .300 Savage, and .250 Savage. The rest is largely a matter of what works
and what does not.
The smaller diameter rounds based on .375 Win and .225 Win. will let you get
away with a more pronounced appearance of the rimfire firing pin hole than will
the .307 Win and .444 Marlin cases. Nonetheless, when that rimfire firing pin
hole first starts to make its appearance, start using more caution.
.307 Win. brass behaves much the same as .444 Marlin does, but since I do not
work that much with .307 Win, I cannot state emphatically that the case web
observations apply equally to both .444 Marlin and .307 Win. Similar,
that is all I will say until such time as Don or I have studied it more
thoroughly.
My approach to maximum pressures in the Contender is not a very scientific
approach, but the results are very workable. I prove it nearly every day test
firing barrels. Thus I am presenting it to you for your consideration.
Using the above approach, Don Shearer, Littleton, CO, refined and
validated the web expansion observations I had been using for years, and found
that with .444 Marlin brass, you can very predictably come up with a very
accurate Functional Maximum Pressure, based entirely upon the measurement of the
fired case webs.
