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Custom action pressure ceiling.

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Unread 03-22-2010, 10:16 PM
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Re: Custom action pressure ceiling.

Originally Posted by jmden View Post
There wouldn't be less casehead expansion or primer pocket loosening with with a quality custom action vs. a factory action, would there? That area of the brass typically being unsupported, wouldn't it show pressure at the same pressures on a factory and on a more closely toleranced custom?
If the brass is unsupported by steel, it wouldn't matter what quality action or chamber it was in. The brass case head expansion would be controlled solely by the brass case strength and the pressures reached within the case during firing.

A very tight rifle chamber, if that chamber extended all the way back to enclose the cartridge case head? Maybe that would help. But the case heads aren't encased by the steel barrel chamber, at least on my bolt action rifles. That being the situation, unsupported & behind the steel chamber they will expand, and on a reliable basis.

I tend to believe the difference in a more stout action compared to a less stout action is more in longitudinal direction on the casing, than in the radial direction. By this I mean that a stronger barreled action assembly will stretch less in the case head to case shoulder direction, which allows the brass case less opportunity to stretch in that direction also. If the bolt is set back somewhat under peak pressures, this also allows the case to expand longitudinally from the case head to the case shoulder. After the pressure subsides, the brass has stretched so much that the lugs on the bolt remain loaded due to the lengthening of the case, similar to trying to close the bolt on a casing that hasn't been resized sufficiently. This accounts for some of the difficult bolt lift in a weaker barreled action than with a stronger barreled action.

Now the casing will also expand radially to a greater extent under greater pressures, and can do so under enough pressure and/or repeated firings such that the brass doesn't relax after firing, resulting in difficult extraction. But to the extent that radial casing and case head expansion is controlled by the barrel chamber or action, it is controlled more by the barrel chamber than the action. Again - in my opinion. Barrels are threaded into the receiver, and that threaded joint itself allows momentary barrel chamber expansion before the action that the barrel is threaded into could assist in preventing further radial barrel chamber & radial case expansion. The best way to minimize high pressure caused case swelling (and chamber swelling) in the radial direction is a larger outside diameter barrel - particularly the threaded tennon portion of the barrel where the barrel is machined down to its smallest outer diameter. This is possible with larger diameter barrel threads - which requires a larger diameter action and action threads. Or expressed another way, an action with a larger diameter barrel thread size will assist in reducing radial case swelling (of the portion of the case contained within the steel chamber), not so much due to the radial strength of the action itself, but because the larger thread size will result in a larger outside diameter of the threaded tennon of the barrel surrounding the cartridge casing. The larger OD barrel (greater cross section of steel) will better resist (and reduce) the momentary elastic swelling of the barrel surrounding the cartridge case at the moment of peak chamber pressure.

That's my take on it. My opinion. It's subject to change given sufficient explanation and cause otherwise.

Last edited by phorwath; 03-23-2010 at 12:46 PM.
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Unread 03-22-2010, 11:05 PM
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Re: Custom action pressure ceiling.

The best written reference I have come across regarding a discussion of "pressure signs" is Any Shot You Want by Art Alphin.

The discussion in the book is referenced by pressure barrel testing, and puts the classic pressure signs of sticky bolt lift, primer cratering, ejector plunger marks, and case head swelling to the test using actual pressure barrels.

His testing might raise some eyebrows, and cause some feathers to be ruffled.

In short....his testing revealed once "visual/measurable" pressure signs are detected by the shooter, they are not indications of "pressure" but are indications of being way over pressure.

If my book wasn't packed up right now, I'd type the applicable pages in a post on this thread and hope the lawyers couldn't track me down

That said, I used the previously posted technique by Kirby in a 30-338 Lapua Improved of using Norma cases for pressure load development, and then switching over to the Lapua case for final load I'm not looking down my nose at the discussion here...just offering a written reference to consult on pressure signs that actually puts a CUP/PSI/piezo rating with an indication.

If there are any other written references that use pressure barrels and actual laboratory testing data, and ties the classic visual/measurement indications that a shooter might use to determine pressure with a CUP/PSI/piezo rating, I would ask to please share that.

Good luck!
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Unread 03-23-2010, 12:44 AM
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Re: Custom action pressure ceiling.

Originally Posted by jmden View Post
All very good points and well taken, but does it answer Rocky's question? What about folks that are shooting wildcats?

I've got a vested interest in the question myself with a wildcat on a custom action coming. Thanks for asking it, RockMtnMT.
Perhaps I misunderstood Rocky's question . To determine what load and bullet gives what pressure in a wildcat cartridge the computer Program Quickload is quite helpful. To use it however you need to use bullets and powders which are in Quickload's database. The case capacity, neck diameter and case length are minimum information about the case and chamber. As a first approximation with the the powder type, case capacity, bullet type (including jacket material, mass, and length) bullet seating depth, and barrel length you can predict what velocity your load should have vs chamber pressure. Freebore is not part of Quickloads calculation, nor is primer energy.
Combining the information from Quckloads calculation with actual bullet velocity measured by a chronograph will give a good estimate of the pressure curve in chamber of a wildcat cartridge. There are at least six parameters which define how a powder burns. Just guessing that the powder you're using is similar to a powder which is in the database can lead to very wrong estimates of the pressure curves even though the two powders might give the same velocity.

There are also commercial strain gauge kits which can be fitted to most rifle actions to measure the chamber pressure curve directly. Accurate dimension of the chamber wall thickness needs to be knows as does the modulus of elasticity of the alloy the chamber is made of. When a rifle is fired the outside wall of the receiver expands linearly with internal pressure as long as the elastic limit of the receiver isn't exceeded, which it wont be for normal cartridge pressures. The strain gage measures how much the barrel stretches vs time when fired and a relative simple formula then calculates the pressure curve. From the pressure curve and the cartridge dimension the forces on the receiver and bolt can be determined.

As to what the design maximum pressure for a given receiver might be that should be obtained from the receiver's designer and manufacturer. Hopefully the designer did thorough calculations and the manufacturer followed the design, particularly with the metallurgy and heat treating.

Last edited by LouBoyd; 03-23-2010 at 12:49 AM.
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Unread 03-23-2010, 12:53 AM
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Re: Custom action pressure ceiling.

The brass is always the weak link, even with Lapua or RWS brass.

I am familiar with, and have read, the articles that document some pretty high pressures once case head swelling is measurable. Again not saying the brass is a means of quantifying the specific operating pressure of a given load - 62,000 psi versus 67,000 psi versus 73,000 psi. But repetitive loading of a single brass casing while working up a load will result in the trashing of the casing under repeated firings, and if one increments up cautiously, this brass casing will fail way before the rifle will. The brass case head and primer pocket will swell and become disfunctional before the rifle is damaged.

Therefore monitoring the condition of the brass should keep one from launching the bolt back through the skull - or even setting the bolt lugs back into the receiver. This is not as straight forward as "ABC - Doe Rae Me". Some would certainly be better off not reloading any cartridges, even to pre-established load data. Seriously. Others, with both the patience, understanding, and experience with reading over-pressure signs on brass casings and primers can develop a safe load with very little load data for a wildcat cartridge.

Heck, the safest thing of all is don't ever shoot a rifle, even with factory loads, because sometimes they'll blow primers too.

So for the newly initiated (probably not any of the members that have posted in this thread), again the methodology and idea is, once the brass is being taken out in short order with high pressure reloads then back off the powder charge until you're getting good brass life. The whole point is once those excessive loads are identified, reduce the powder charge to a safer level. Not to replace the spent brass with new casings and then continue to powder (and power) up time and time again, throwing away the spent brass after single shot usage.

I've measured case head expansion on all different brands of brass, Lapua, RWS, Winchester, Federal, Remington. Lapua and RWS will take a beating, but they will also expand case heads. It occurs in concert with the primer pocket swelling in each and every case I've experienced it. The guideline I've read and experienced is 0.0005" swelling of the case head with one firing is excessive pressure. Yeah there will be studies demonstrating that measured load pressures when the case head swells 0.0005" exceed SAAMI standards. Fair enough. The rifle didn't blow up & wasn't damaged. [Are there any studies that show a competent rifle ever let loose with less than 0.0005" case head expansion from one single case firing? Of course if the action or chamber explodes, the case head expands and may never be found - but you get the point.] Now... reduce the powder charge to whatever point you'd like. Almost every production rifle sold is seriously overpressured before it's ever released for sale. I know my Beretta Tikka T3 came new from the factory with gas etching of the bolt face caused from a pressure test load that let gas escape all around the edge of the primer. A perfect circle. For those that haven't seen this before, with new brass cases, this means the case head expanded until the primer blew it's seal in a single firing. Beretta didn't throw the rifle away. They sold it to me.

QuickLoad is a tool that was mentioned earlier and it certainly appears to give pretty good estimates of cartridge operating pressures. I don't own it or use it, but I've seen enough QuickLoad predicted load data and compared it to my reloading experiences to say it comes pretty darn close to estimating resultant cartridge/powder charge/bullet weight combination maximum loads & operating pressures.

If full blown pressure tested load data is available, use it. If there isn't, find a cartridge of the same caliber and capacity and use it as a guideline for both the optimum powders, as well as the approximate powder charges. If you wake up one morning and all load data is lost, you're the last one on earth, and you still have your eyesight, primers, cases, powder, bullets, and a means of weighing powder charges, then monitor brass cases and primers for signs of excessive pressure. Do so with a measure of caution and you should die of some cause other than 'rifle bolt through the brain'.

Oh... almost forgot.

There is one better way to establish a maximum working load for a complete, never before dreamed of, wildcat cartridge. Have your buddy build up the same cartridge and work the loads up in his rifle - first.

Last edited by phorwath; 03-23-2010 at 09:18 AM.
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Unread 03-23-2010, 02:52 AM
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Re: Custom action pressure ceiling.

All great posts and very informative. As for me, I'm taking them all to heart, but to a certain percentage. For example, if take Lou's post 100% to heart I would not be driving any more, nevertheless it'll waken you up to reality.

I do like what Phorwath posted and the info pasted from fiftydriver's previous postings. Makes a lot of sense. These two I liked the most but over all, very good thread so far.

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Unread 03-23-2010, 06:02 AM
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Re: Custom action pressure ceiling.

Originally Posted by RockyMtnMT View Post
I'll say it again, this is not some testosterone loaded, 'mine is bigger and badder than yours', how much farther can I push with a custom action than I can with a factory action question. I want to be safe when venturing into the unknown.
Message received. Being safe when venturing into the unknown is a worthy goal.

Is quick load a good predictor for a wildcat cartridge that has no load data?

Used within it's limitations as an engineering modeling tool set, QuickDesign (to get the wildcat cartridge geometry properly modeled and then transferred to QuickLOAD if it isn't already available in QuickLOAD) combined with QuickLoad is about the only tool set available for gaining a quantitative understanding of a new cartridge and setting up a load map before firing it. The power of that tool set combined with the two external ballistics programs that come with it is phenomenal.

By "used within it's limitations" I mean one has to respect the fact that it may not be perfectly precise in predicting pressures and muzzle velocities - but it is a whole lot more accurate than guessing. I don't know what it's track record is on pressure predictions because I don't have a way to measure actual vs predicted pressures. To the extend that muzzle velocity is an indicator of pressure (with out going into all the variables there) QuickLOAD is at times incredibly accurate at predicting muzzle velocity. Being within 2% is remarkably common. Being as much as 5% off is unusual. On that basis I've been comfortable backing off 10% from the predicted max loads in QuickLOAD and working up. I shoot all the loads through the chronograph and compare the measured vs predicted muzzle velocities as a way of assessing the quality of the QuickLOAD model.

To get that sort of accuracy it is necessary to get into the details - the user manual that comes with QuickLOAD is excellent and quite thorough. A good accurate measure of the water grain capacity of fired brass from the chamber in question is important. So are an accurate COL for the cartridge with the bullet being modeled, accurate data on the bullet itself, and barrel length measurement (bolt face to muzzle exit).

I haven't used it on a total unknown cartridge yet. Used in conjunction with load manuals from the bullet manufacturers it is frequently a significantly better predictor of muzzle velocity than the data in the manual. There have been a couple of occasions where I modeled a load from the manual and went "Whoa! That's too hot!", backed off and never was able to reach the manual max because it was too hot.

There have been other occasions where QuickLOAD would suggest powders that weren't in the manual would be better, and they were better (higher muzzle velocity). QuickLOAD has a "load table" mode that will scan every single powder in it's database, model it for your bullet, brass, and barrel length, throw out those that would exceed parameters you set (like percentage fill and peak pressure), and list the rest in descending muzzle velocity order.

I'm an engineer by education, training, and 35 years of experience in Aerospace where we spent most of our time trying to do what had never been done before. QuickLOAD is the closest analog to the computer modeling tools we used in the space program that I've seen for reloading. I can't imagine reloading with out it after having used it for the last three years.

It is one of the few programs that is always up on my computer. QuiclLOAD, QuickTarget Unlimited are there right now with load data for the 7-mag I got from my nephew. There is a very good review of QuickLOAD on

Did I mention I think it's an incredibly useful tool?

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Unread 03-23-2010, 06:10 AM
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Re: Custom action pressure ceiling.

I'll add a few things..
-For someone really good with QuickLoad, velocities and pressures can be predicted close enough. But it's like ballistic software, in that most reloaders are ill prepared to apply correct inputs.
You can take formed brass in-hand and measure it's H20 capacity. Then load in a cartridge of your caliber with same capacity. Run the what-ifs on it, validate with loads posted at ReloadersNest. Then when it's time to shoot, you work up from safe over a chronograph, to see your expected velocities -or a problem otherwise.

-Brass brand don't mean diddly. Lapua has not invented stronger brass or anything like that. It'll provide plenty of pressure signs as you go up.

-There is something to the mentioned requirement for enough barrel steel around your chamber. Those who chambered 'fat' cartridges in smaller diameter tennons are learning this lesson, with difficult extraction below the cartridge's potential.
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