Published vs. Actual Max. Loads

[1st cav sgt ret]

and in a somewhat feeble attempt to add to what GonHuntin has said I can add that action type also makes a great deal of differance in the ability to load for a cartridge(which is something the factories must also take into consideration when developing their load data)the venerable 30-06 which has been chambered in lever actions,single shots,semi and fully automatic,and not the least bolt actions some of which have been around for nearly 100years and cannot take/handle high pressure loading,another example I have a m98 mauser that was rebarreled by Shillen(because they are only 18 miles from my house)and was proofed by them(6 rounds of high pressure test 70k ammo I was there and whinced every time it fired)before they would release it back to me another example of what im saying is my beloved 6.5x55swedes,I have a husquverna m38 made in about 1938 that I only shoot loads of about 46k psi in I also have a modern Ruger m77mk2 in the same chambering(they also have long throats for the long 160gr rn)that I load for(different bullets and brass and marking so I dont get them confused)to app 50k psi norma loads

[Mohawk]

Mark,
How embarrassing ! I must have been thinking of another one of our comrades at this board when I typed that greeting.

At any rate, it was pretty slow at the hospital yesterday, so I sat at length reflecting on some of things you've said. And it occurred to me in those musings that if overall chamber length (i.e. distance from the closed bolt face to the start of the bore's lands) has that much to do with variation in chamber pressure -- all other variables remaining constant -- that perhaps the fit of the chamber will also affect this factor. That is to say, that if we take two rifles, one having a loose chamber and the other a tight chamber (because I know there are differences), then will the chamber pressure of the latter not be higher given the same powder charge, overall cartridge length, and overall chamber length, since the latter provides for a smaller vessel in which the volatile gas can expand and move? Also, could a tight chamber inhibit the appearance of obvious pressure signs in the brass? For instance, wouldn't a tight chamber prevent things like case head bulging, neck cracking, or the impressions of bolt face features from showing up on the headstamp despite the pressures being dangerously high?

Regards,
Dave

[GonHuntin]

Dave

Don't worry, I have been called lots worse! You have asked some interesting questions.....all I can do is give my opinion......

....it occurred to me in those musings that if overall chamber length (i.e. distance from the closed bolt face to the start of the bore's lands) has that much to do with variation in chamber pressure -- all other variables remaining constant -- that perhaps the fit of the chamber will also affect this factor. That is to say, that if we take two rifles, one having a loose chamber and the other a tight chamber (because I know there are differences), then will the chamber pressure of the latter not be higher given the same powder charge, overall cartridge length, and overall chamber length, since the latter provides for a smaller vessel in which the volatile gas can expand and move?

Interesting theory, and, on some level, I'm certain you are correct, however, I believe the difference in case volume between a tight chamber and one that is sloppy would be very small......if I was guessing, I'd say less than a tenth of a grain difference in capacity......I'm sure there are ways to measure the pressure difference...... but I don't think it could ever be observed except maybe under laboratory conditions.....

Also, could a tight chamber inhibit the appearance of obvious pressure signs in the brass? For instance, wouldn't a tight chamber prevent things like case head bulging, neck cracking, or the impressions of bolt face features from showing up on the headstamp despite the pressures being dangerously high?

I do believe you are correct here and I have observed this in factory chambered rifles. I own a 700 ADL in 22-250......our board owner, Bobcat, previously owned one that was identical.......using the same loads, his rifle very noticeably bulged the brass cases above the web.... mine does not......brass used in his rifle would not chamber in mine without full length resizing......his rifle exhibited a sloppy chamber as compared to mine. I don't believe the pressure was excessive in either rifle, but the "generous" chamber in his showed greater case expansion above the web.....in this instance, measuring case expansion and relating it to safe loading practices clearly would not work.....

Furthermore, one advantage of the "ackley improved" case is less backthrust......this isn't a big deal in a bolt action, but, in the TC Contender for instance, it makes a huge difference. For example, I own a barrel rechambered by SSK to 309 JDJ......the 309 is basically a 30 X 444 marlin improved.....without the straight case walls of the improved cartridge, it would produce too much backthrust for the Contender frame.......how does that tie in to this discussion? In your example, you specifically mentioned " the impressions of bolt face features showing up on the headstamp".....it is my belief that a tight chamber would lessen the backthrust which is what causes the impressions you mentioned.

This all supports the fact that pressure equipment is the only reliable way to judge the safety of loads when the envelope is being pushed.....

If I intended to chase max loads on a regular basis......I'd invest in an Oehler 43 Personal Ballistics Lab as seen at the link. It allows pressure testing on your gun with your loads........

www.oehler-research.com/model43.html

PS: Dave, please check out the thread by OneMoreDeer and give us your input on barrel length as it affects bullet stablization.

Mark

[ninehorses]

Mohawk,

Back to your original question,

As a general rule I look at several reloading guides, and compare them against each other. Sierra, Nosler, Hornady, and Speer. With the Nosler being the first I go to. They use test barrels with a universal receiver. They also tell you who made it and the twist. Sierra tells the actual rifle the loads were worked up in. I also like the Hodgdon and Hornady web sites.

My most trusted indicator of pressure is case life. If I get ten or more loadings on a batch of cases, I feel that the load is safe in that rifle, and no other.

But you have to start some place!

[ninehorses]

As a follow up, a tight bore will also cause higher pressure. A hand lapped slick bore will also lower pressure and increase velocity. In other words it's not just the chamber. My 2 cents.

[littletoes]

Ninehorse wins the cigar! The tight chamber will increase pressure. This is as simple as changing brass. For instance, if you were to substitute Lake City brass for Winchester brass, and the loads were aproaching the higher end of the spectrum (Using the same load, without working up) you could experience significant signs of pressure. Just with the slight differance in case wall thickness, without changing outside measurements. The same can be said about chambers. Simple, the tighter you pack it, the higher it will blow! Now, as too Ackley rounds, think of it as a wedge. A wedge is easier to pull out than a dowel. Of cource brass flows as it expands, hence the reason for full length resizing. The less it flows, the longer brass will last. Back to the Ackleys-with no or almost no taper, the sides of the brass cartrige "grip" the walls of the chamber "tighter." (All brass cartriges "grip" the sides of the chamber, that is what keeps it from blowing back into your face!) but the chamberings that have no taper, grip it too well! Allowing them to be loaded dangerously high before any signs of pressure show up! Very bad carma! DANGEROUS! When they do show up, THEY SHOW UP BIGTIME! So watch those high loads. Another thing that should be mentioned, is that with most hot loads, you can load an as accurate load, but safer by just comming down a little. Try it. Good luck to all, hope this helps!

P.S.=Hope I didn't go too fast!