Home Forums Hunting & Shooting Ask The Gunwriters 6.5 SAUM Hype

Some day someone will give us relatively affordable pressure measuring devices, like they did affordable chronographs. I suspect this will eliminate a lot of speculation.

I also suspect it will burst some bubbles for all of us.....the way chronographs did when we discovered our pets were not going as fast as we originally thought.

But I bet it won't stop the conversations though,because we will then discuss why pressures with the same components are different from one rifle to another....it will never end.

Yup... I keep getting an image of a dead horse and a bunch of guys with sticks in their hands....

rifleman700,

I should have been more clear. The formula (like the other on case volume) involves optimum powders for the case in question, those resulting in the highest velocity. It's not for the same powder, or powder charge, with the same bullet weight.

The big factor in different bore sizes is expansion ratio, since the volume of the bore adds more room for gas to expand. In a larger bore (say 7mm versus 6.5mm) this slows the pressure rise, allowing more "area under the curve" for more velocity with the same bullet weight.

Both of the formulas were worked up by me a number of years ago, empirically from loading data. The validity of the physics was confirmed afterwards by the late Don Miller, the guy who developed the rifling-twist formula included in Bryan Litz's book APPLIED BALLISTICS FOR LONG-RANGE SHOOTING. Don published a couple of articles on my formulas, I believe one in PRECISON SHOOTING and another in VARMINT HUNTER.

I started crunching numbers because there was so much speculation about how powder capacity changes potential velocity. A lot of folks assumed that "improving" a case resulted in the same percentage of extra velocity, and it doesn't--at least not at the same pressure.

The bore-diameter formula resulted from the same sort of curiosity. Obviously the .270 Winchester is capable of higher velocities with the same bullet weights than the .25-06, but I wanted a formula that would predict potential in various cases.

Like all formulas, there's some variation due to different bullets, but both provide a very close approximation of a case's potential, because they were developed from hard data from a bunch of different sources.

Not trying to nitpick here, but for the purpose of this discussion, heat and pressure go hand in hand.

The first law of thermodynamics states that heat and work are mutually convertible. As it pertains to gas in a pressure vessel, "work" is defined as change in pressure times change in volume. Therefore, heat IS pressure and pressure IS heat. During combustion, you cannot have low heat and high gas pressure or vice-versa

Therefore, pressure is a major factor in barrel life. Maybe not the only factor, but a comparison of pressure is a good indicator of relative barrel life.

Let's see, break out the physics book and read up on the first law of thermodynamics, or just "choot the chit out of it.?"

Tough choice.

But what that means is you'll ultimately "choot less chit out of it" if it's generating higher pressure (and therefore heat).

EDIT: or is it "more chit?"

No, I need to choot the chit out of my 6.5-06 AI first, and see if I can keep up with the SAUM guys as far as round count.

More chit/less chit, I could give a chit!!

20 pounds of H1000 used so far in my 6.5 SAUM and counting. I've got three more eight pounders on the shelf.....I'd like to know how many guys on here have actually wore out a rifle barrel...

I have. But it took me about 15 years.



Travis

What kind of pressures were you running?.....grin!

Lol!!

I have worn out two, they were a 7mmSTW and a 257 Weatherby. My fault, most of the time it is hot when we shoot and don't let the bbl cool down enough.Thats in 35yrs of shooting.

Higher than they were supposed to be I can tell you that.

That was my only rifle for a long time and work didn't allow much shooting. Glad that stage of my life is over. grin...



Travis

Couple.....

I've killed a couple, and my 243 is hanging on for dear life. If I ever kill the barrel in my Tikka I am going to cry and head to counseling.

RifleDude,

I acknowledged the duality of heat and pressure in an earlier post.

However, wear in a rifle barrel is a little more involved. There's also the duration of the heat, and the heat is mostly caused by burning powder, though some comes from the friction of the bullet heading down the bore and the primer.

I used the examples of the .308 Winchester and .300 H&H Magnum. The SAAMI maximum average pressure for the .300 H&H is 58,000 psi, 4000 less than the SAAMI MAP for the .308. Yet the .300 H&H will burn out a barrel noticeably faster, because the case uses 50% more powder, and the duration of the powder burn is slightly longer with each shot.

Of course, the .300 H&H does have a very long, sloping shoulder, which has an effect on the throat as well. But we could use the .300 Winchester Magnum as an example. It's MAP is 64,000 psi, only 2000 psi more than the .308, and it will also fry a barrel considerably faster than a .308.

One every other year.

So far maybe 5... but I have many more rifles than in those days...

If I had to bet, I'd wager that was a rhetorical question.........

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