Home Forums Hunting & Shooting Ask The Gunwriters SAAMI Max Pressures

I was just wondering about the listed SAAMI maximum pressures.

Given a modern bolt action rifle in good condition, and the ability to accurately measure the pressure of loads is there any reason that a cartridge like the 223 Rem with it's lower listed max pressure (55,000 psi) could not be loaded to the same pressure as say a 22-250 (65,00 psi)?

I assume that the 223 has a lower operating pressure since it was developed as a military round and used frequently in semi-autos. Is the brass designed to handle the lower pressures and would it fail at 62,000 to 65,000 psi?

I'm wondering because using Mule Deer's rule of thumb that velocity increases at 1/2 the rate of pressure, and given that quite a bit of loading data grants the 223 with a 40gr bullet 3600fps, loaded to the same pressure the 223 would get really close to the 22-250. An 18% (10,000psi) pressure increase should theoretically result in a 9% (324 fps) velocity increase, granting the 223 3924fps with a 40gr bullet.

If one were to really stretch it and use Nosler's most aggressive data (3800fps with a 40gr bullet), the 223 would get right in the thick of things at 4142.

I definitely don't have the equipment to accurately test the pressures, and am not prepared to load my own 223 to those speeds, but is there any reason a person wiht the proper equipment couldn't?

The thing that puzzles me is if the 223 can theoretically push those bullets to that speed given the same pressure, why can't the larger cased 22-250 do even more at the same pressures?

Good questions all. My guess (and it is only that) is that .223 brass can't be built as heavily in the head area as .22-250 brass. But I will start asking around to see if I can find out. I do know that all the smaller-cased .22 centerfires have lower SAAMI limits than the larger ones.

Incidentally, my prsent .223 is a Remington 700 with 26" barrel. Using published data I get right at 3900 with a 40-grain bullet. This is indeed very close to a .22-250, with a lot less recoil and barrel wear, the reason I don't shoot the larger cartridges much at prairie dogs anymore. Our friend Steve Dogzapper gets a little bit more, I believe, out of his beloved .223 Ackleys.

Thanks,

Ol Steve is the main reason I built my 223 AI.

The answer to your last question is: on the paper yes, but in reality not � you are simply exceeding physical limits which are expressed by the simple rule: pressure = force / area.
As the case of the .223 is smaller (surface area) than the .22-250, pressure limits have to be lower.
So a .22 Hornet�s pressure limit (43.511 psi CIP) is lower than that of a .223 Rem (55.000 SAAMI) which is lower than a .22-250 REM�s (58.740 psi CIP, you say 65.000 psi SAAMI).

I ran QuickLoad (which is a bit flawy as it uses CIP or SAAMI pressures) with your data (I own the 3rd Nosler edition which starts with a 50 grain bullet � I wonder from which release you picked your data).

The pressure limit is
.223 : 27.7 grs N133: 3.622 fps, 55.012 psi (30 grs N133 yield 3.905 fps and dangerous 73.462 psi
.22-250: 36.5 grs N133: 3942 fps, 58.740 psi CIP or 37 grs: 4.038 psi, 63.552 psi SAAMI

When you set the fps/psi data in relation, you�ll find that the increase is
.223: speed = 7.8%, pressure = 33.5 % > 4x, and
.22-250: speed = 2.4%, pressure = 8.2% > 3x � both exceeding 2 as the mentioned rule of thumb.

If that were true, and the allowable pressure increased at a rate proportional to case volume (surface area), then the 460WBY should be able to handle close to 340,000 PSI. I don't buy it.

The simple fact that a small case like the 22-250, 220 Swift, 243 Win can be safely loaded to the same pressures as the 460 WBy, 378 WBY, 300 Ultra Mag, etc. seems to contradict thta theory.

I was also thinking maybe the brass is the limiting factor. I had some questions about Sierra's .250 Ackley data, so gave them a call. When I asked Sierra what pressure they loaded their .250AI to, they told me the same as the parent cartridge. Surprised, I asked if that was due to older M99s out there. They said no the M99 is a very strong action. The brass is the limiting factor. This also surprised me. I am still amazed that they are getting 3200 fps from a 100gr bullet at 46,000 CUP.



John



Just talked to Sierra again, and so had to edit to add this. This one really shocked me...... Sierra doesn't have a pressure gun. They use case head expansion measurements from a knife-edge micrometer. I guess when they told me previously that the .250 AI was loaded to the same pressure as the parent cartridge, they determined this by using the same case head expansion??? Based on what MD has found about the case head expansion method of pressure guessing, the 3200 fps from a 100gr bullet is not so surprising to me any more. Sorry to ramble, just found this to be very interesting.....



John

I did not say "a rate proportional to case volume (surface area)", CAS, but I admit, my statement was not clear at all - sorry.

What I mean is the surface area of the case not imbedded in the chamber (web area).



I think, other factors may play a role for pressure limits as well as case construction (belt) and potential use in ancient guns (.45-70, .30-30 etc.).

waitaminit,

I see where you're going with this.

I will probably get this wrong, but as long as we are measuring in Pounds Per Square Inch, case head size should not matter, where if we were measuring total force exerted, it would matter.

If we take a 1" square piece of cardboard and put a 10 pound weigh on it there is 10 PSI pressure on the piece and 10 pounds of total force.

If we take a 10 Square Inch piece of cardboard and put a 100 pound weight on it, there is still 10 PSI pressure on it, but 100 pounds of total force.

By the way, the 223 data quoted is from the Nosler #5 manual with a max charge of Benchmark driving a 40gr Ballistic Tip.

CAS, I am no ballistitian and may be wrong, but this is how I understand the matter: pressure goes to the total interior surface of the case, and only a part of it goes to the bottom part of the case sticking out of the chamber. As one has allways to assume a wrong headspace, one should not take the support of the bolt face into consideration, imho.



What I want to say is: you divide the pressure by the total surface of the case and multipy the force with the web area.

To get the pressure limit figure, you apply the factors for poor brass, case construction, etc. to the pressure calculated and check if the resulting figure is within the strength limits of the cartridge material ( 2600 brass). If not, correct pressure.



Does this make sense ?

CAS
I've been waiting for a good time to inject this , and nows is the time.........this is a response to the whole thread........after I got my pressure testing equipment and ran a few test, things started to click.......things like much of this data in reloading manuals is calculated, not tested......Sierra supposedly has NEVER had a pressure barrel........I know for a fact of one major powder company that doesnt either.....How do they come up with the data ? beats me......Ya'll just bare with me a little longer and I'll start publishing some results that are going to create quite a stir.........that quickload program ? way way off..........give me a few more days...
Charlie

Charlie,

Believe me, I've been anxiously awaiting your findings ever since you said that you had the new toy.

I can't wait to hear what you've found out.

Charlie

We are all really looking forward to your results. It should open up a whole new world, for good, disappointment or confirmation.

Keep us in the loop.

Thanks

A given pressure would cause less force with the smaller cases- all directions. So thats not the limit. The beast must be force and thrust. Not pressure.
I would think the limit with smaller cases is relative to bolt thrust. Maybe because there is less case to grab onto the chamber walls. If so, the bolt thrust generated would dip as the case area increased to grab the walls, until the area of the head is large enough to again, begin increasing thrust. Also, eventually too much case gets exposed outside of the confines of the chamber, and SAAMI limits start going back down. Just a theory, but it would tend to explain the non-linearness of published pressure limits.

C.Sisk writes: "that quickload program ? way way off......"

An interesting statement, Mr. Sisk. Can you please elaborate on this ?

Thank you.

Waitaminit
I'll give you some data in a few days, you'll see what I mean. <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />
Charlie