Max Pressures in the Contender

Cases using small rifle primers are not much of a problem, but those using large rifle primers can get you in trouble due to difficulty in judging when pressures are safe and when they are in the danger zone.

Cases using large rifle primers, because of their larger case head diameter, exert more force to the rear with the same pressure as a smaller case head.  In actual practice for example, I have blown primer pockets in .223 Rem. cases with no discernable effect on frames.

On the other hand, shooting larger diameter cartridges I have stretched a few frames in my time.

So what are the pressure limits of these larger diameter cartridges in the Contender?

The following are generalizations only.  

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 dia.     A little over 45,000 psi.

.444 Marlin types, approx. .468" head dia.   Approx. 45, 000 psi.

.45/70,  approx .505" head dia.   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 alot.  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.  His scholarly treatise will be posted on a separate page.

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