The official Mike Bellm's

Bellm TCs


TC Contender, G2, Encore/ProHunter Performance Center

 







Cases with a head size like that of .223 Rem. and smaller diameter, typically those using small rifle primers, are not much of a problem, but those larger diameter cases 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.

It is not the pressure alone you must be concerned about. It is the combination of pressure AND the diameter of the case and chamber. More precisely, it is the size of the surface area of the head the pressure is pushing on. It is the same physics as those involved in gas laws, hydraulics, engines, etc. Equal amounts of pressure applied to a larger surface area produces more force.

The chamber is a cylinder. The case is a piston. The larger diameter the cylinder & piston, the more force you get out the back end. You want LESS force out the back end, just the opposite of what you want from an engine for example.

Think of it like the small piston in your lawn mower engine compared to the pistons in your car engine. Both burn approximately a 15:1 air to fuel ratio with the same octane (burning rate) fuel, yet the larger pistons put out significantly more force. Or if too many pistons confuse the issue, think of it as a single cylinder 3 1/2 horsepower engine vs. a 5 horsepower engine. Increase the diameter of the cylinder and you increase the square inches of surface the burning fuel pushes on so it produces more force, and thus does more work.

In short, it is "pounds per square inch." The larger the case and chamber diameter, the greater the surface area at the case head and thus more force out the back end.......... just the opposite of what you want in the Contender.

You want the LEAST amount of force applied to the case head and thus the breechface, so to be able to run more pressure, which over time is what accelerates bullets, stay with the smallest diameter chamber you can and still have enough powder capacity of the right burning rate to get optimum velocity attainable.

As a benchmark reference, SAAMI maximum pressures for .223 Rem. for example are in the 55,000 psi category.

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.



Here is an example, Click here.
My 5.6x50-R Bellm is a clear example of how the above works.

.22-250 is about a 52,000 psi cartridge that is way over pressure for the Contender or G2 due to:
1) its larger diameter,
2) its thin brass, and
3) its highly tapered case.

It will stretch either the frame or the lower lip of the barrel lug and ruin one or both. .22-250 IS DEFINITELY A "NO-GO".

But by dropping back to the same case head/chamber diameter of the .223 Rem or 5.56 NATO cartridge, approx. .377" versus about .470" for the .22-250 running at 52,000 psi, you can run the full approx. 55,000 psi in the 5.6x50 R case and when blown out to maximum capacity then get the same ballistics as the .22-250.

My blown out 5.6x50R Bellm readily holds up to 36 gr. of ball powder and duplicates .22-250 ballistics, but does it with less force on the frame due to the small diameter of the 5.6x50R case.



The rule is to use the smallest diameter, straightest walled, and thickest case you can in the Contender that will give the ballistics you want.
Obviously, if there is no such case meeting that criteria, you are stopped short and have to go back to what is available that comes closest to what you want.



So what are the pressure limits of 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.

(Up to a certain pressure level, the case contains all of the pressure load and does not even set back against the breech..... as evidenced by bulged out fired primers.)

.223 Rem. types and smaller like .22 Hornet, approx. .375" head diameter, approx. 55,000 psi.

.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. The factors mentioned above can skew things above or below the max pressures indicated.

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, meaning thicker, .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, and because it is the larger diameter chambers that pose the most risk for shooters, 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, linked to at the bottom of this 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 maximum.

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 imprint appearance at the top edge of the primer, start using more caution. You are very near the maximum force on the breech the frame will withstand.

.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.

I will say this about the .307 Win. case. When chambered in the "improved" version aka .30 Bower Alaskan and handloaded to about 45,000 psi, it is probably THE very best .30 cal. cartridge for Contenders and G2's.

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.

Bellm/Shearer Max Pressure Index for Contenders, Click Here. Don Shearer's scholarly treatise is posted this page.



 

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