14 January 2002, 06:53
<Daryl Elder>
Brass flows toward the neck as the case is fired successively. This is why it's necessary to trim cases. The brass flows from the base of the case, above the web.
Sinclair International offers some measurment gauges to check brass flow and cartridge wall thickness above the web.
Varmint Al notes that case wall thickness above the web is a crucial issue in reloading, and provides a few pointers for checking this area. (Al is a retired mechanical engineer from Livermore Labs -- The atom bomb people.)
Some calibers flow more than others. One trait of Ackley Improved calibers and their steeper neck angles is that brass flow is checked to some degree, and this increases brass life. Obviously, high pressure, high velocity calibers flow more than those which function at lower pressures and generate lower velocities.
From the reading I've done, and it gets to be more and more daily, case dimensions are a poor means of measuring pressure. Pressure indices such as flattened primers, backed out primers, sticky extraction, etc. are a poor measure of pressure. Nonetheless, such indicators are better than nothing.
Seems like the whole game in producing accurate ammo and accurate data is eliminating variables. If I were to use head or belt expansion as an index of pressure, I'd eliminate variables by using once-fired brass.
But I'm NOT an engineer. I could be completely off base here.
First, it would be absolutely necessary to measure the head in exactly the same place, since lack of concentricity within .00005 is a certainty. Second, no instrument ordinarily available to the handloader can measure reliably within such tolerance.
I have a lot of respect for many of the proponents of case head measurement, like Ken Waters and others, but I just disagree that this can be an accurately measured indicator. I believe it's a bit like witching for water: The measurer sees what he expects to see.
On the other hand, I'll admit this: If the case head expands enough to be measured, then the load is definately too hot.
Probably a typo here, but the point about available instruments being able to measure this dimension AND variables in concentricity are both germane to the discussion -- be the dimension ten or hundred thousandths.
Now I'm wondering about the head dimensions on some of my loads!
I will say that Pressure Ring Expansion is my prefered method, but I do record Case Head Expansion as well for my Test Loads.
Stonecreek is correct about them being "old methods", however he is incorrect about them not being able to measure the values accurately. There are indeed a couple of tricks to doing it properly but they are not all that difficult to master.
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Good hunting and clean 1-shot kills, Hot Core
Then he'd run a "curve" or "slope" or some sort of "equation" out from his "data" and pronounce what was "maximum pressure" for such and such.
Well, gun metal fatigues just like brass. Overloading a gun and shooting it, then shooting again, and again with increasing loads until it fails tells you little about the load, and MAYBE a bit about metal fatigue in a gun.
I think we're all whistling in the dark on pressure, loads, and internal ballistics.
I'm convinced that pressure signs are unreliable, and calculations are nearly worthless unless and until mfgs start providing more ballistic data on their powders.
horsehockey, measuring the pressure ring works good enough for Govmint work!
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Ray Atkinson
[This message has been edited by Daryl Elder (edited 01-14-2002).]
rufous: So instead of measuring the belt of my 300 Win Mag cases you think it would be best to measure the pressure ring just in front of the belt?
HC: I measure and record both, but have found over more than four decades of using these methods that the Pressure Ring Expansion(PRE) is a bit more useful "Pressure Indicator" than Case Head Expansion(CHE). PRE works on ALL Cases where CHE will not work on cartridges such as the 30-30, 35Rem, 38Spl, etc. The reason for that is the SAAMI Pressure Limits for those cartridges is too low to cause the Case Head to expand any at all.
rufous: So I run the case through the fl sizer and measure it at the same point (Federal cases so one anvil on the "F") and then fire and measure it at the same place again? Is that right?
HC: For measuring CHE, that is correct. Measure across the Belt and record that value. Then fire the cartridge, remeasure in the "exact same spot", record that measurement and determine the difference which will give you CHE.
Also, I've found CHE to only be reliable on shots 2-6 or 8, depending on the strength of the Loads. This does involve a slight "learning curve", but everyone I've taught this method to has understood what is going on after one trip to the Range. By the time they've made 3 trips, they have been able to teach other folks.
I do agree with DB Bill concerning this though(as strange as that may seem). I've found people must be intelligent enough to work a "Radio tuning knob" in order for them to be able to do this. And rufous, after seeing your questions, I know you will not have any problem getting these methods to work well for you at all.
rufous: Hodgdon manual said 0.0007" on the belt would be considered max.
HC: I would recommend you stay with 0.0005" "average CHE" for both Belted and non-Belted cases.
rufous:Would it also be 0.0007" on the pressure ring or back to 0.0005"?
HC: No, there is no pre-estimated number for this value. And that is partially because of the questions asked by "MePlat" in his post concerning the variations in the actual Chamber and Bore dimensions(good for you MePlat).
What I recommend is for you to fire one box of factory ammo through your firearm, average the PRE for those 20 cases and use that value as your "Comparative Benchmark Standard". Then develop your Loads in those same 20 cases and as long as you do not exceed the Benchmark PRE, your Pressure will be within the limit set "for that Lot of cases" at the factory. PRE is normally reliable for shots 1-6 or 9 depending again on the strength of those Loads.
When measuring PRE, rotate the Pressure Ring between the anvils to find the "widest" diameter and use that as your PRE. Go slowly so as not to "force" the Pressure Ring through the anvils. I use the highest measurement that will not allow the case to fall out from between the anvils.
After a short while, this goes quicker than you can read it here. Just hold the 0.0001" capable Micrometers in your right hand and support the Case with your left hand while turning it.
rufous: If measuring the point just in front of the belt then I would think it would be clear that one would not want to use a new case since they are generally drastically undersized and would expand significantly.
HC: Well..., you are thinking ahead and not aware that you need to do your initial Load development in those "Once Fired" factory cases.
So as not to tell you a WHOPPER, after you become REAL FAMILIAR with CHE and PRE, you actually can use "new cases" to develop Loads without ever using a factory round, but that requires a thorough understanding of these methods that only develop over a fairly decent period of time. There are some very subtle measurement indicators that will tell you when to STOP, but they are not for this level of discussion. Just don't concern yourself with "developing Loads" in new Cases for right now with these methods.
rufous: So if once fired and fl resized case measures 0.5130" at pressure ring then after firing it should not measure more than 0.5135 or 0.5137" Does this sound right?
HC: No, when measuring PRE, you want to STOP once you reach the same "average PRE" as shown on the initial firing of the factory ammo, which in your example would be 0.5130".
CHE & PRE are not all that difficult to understand, are very SAFE if done properly and allow you to duplicate the Pressure deemed safe by the Factories that made your initial Cases. The factories spend the BIG $$$,$$$.00 on the Pressure Testing equipment and we duplicate their results with a set of 0.0001" capable Micrometers. Just not all that hard.
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Good hunting and clean 1-shot kills, Hot Core
[This message has been edited by Hot Core (edited 01-14-2002).]
My only addition to it is to measure each case at multiple points around the pressure ring since (in my rifles, anyway) the expansion may not be consistent depending on soft spots in the brass and irregularities in the chamber.
Waters has been using this method for at least 40 years and hasn't blown himself up. In fact, if you compare his loads to book max loads they're usually the same or a grain less.
No method is infallible, anyway. I measure PRE but also shoot the same rounds over the chrono. If I started getting 3200 fps with 180 gr. in the .30-06 I would suspect something was wrong.
John
Basic rule #1...if you want .300 WinMag velocity out of a .30, don't buy a .308 or .30.06. Buy the Win Mag but then don't try and push that to Weatherby velocity.
Basci rule #2...the only way to get "free" velocity is to get a longer barrel.
quote:
Originally posted by MePlat:
Then we must say that all the people who published load data for wildcats without the aid of a pressure barrel should not be believed. If it is not to be believed then what do we go on
Are you kidding? There are component manufacturers (Speer) who used to put out loading data for factory cartridges without, as far as I know, pressure testing with actual crusher gauges. I believe that this is one reason their recommendations in some cases ( )are much more conservative than they used to be.
Be careful, don't believe everything you read, and figure out for yourself whether it's a worse mistake to load lighter than one needs to or heavier than one should.
My apologies. I should have read the whole thread before I answered you.
Barring the sacrifice of the appearance of each of your firearms to some sort of a stick-on strain gauge, you are really kind of stuck with loading data from people who DO have pressure-measuring equipment and use it. Art Alphin recommends, for cartridges for which reliable people have produced loading data, not exceeding the velocity achieved by those people. Obviously, you need a chronograph, must correct for barrel length, start from below, panic & back off immediately at any pressure signs, which you can't count on appearing anyway.
As far as wildcatting, we know that many people do it, and some sort of extrapolation or, on a good day, interpolation may be done from known safe loads.
But if you are asking for a sure safe way for you to wildcat without pressure-measuring equipment, you're asking the wrong question. It's like a plastic surgeon asking for a sure safe way to do facial surgery without anesthesia. It could perhaps be done, but why? It is neither safe nor necessary.
This is not intended to badmouth those who do wildcat and know what they are doing. And maybe they will post here all the details of why their particular loads are known to be safe. But if you're looking for signs, I suggest you read Art Alphin's discussion of same in his reloading manual, available from Sinclair.
P.S. Obviously, this is a very conservative opinion. You may find one or two posters here who have a diiferent view. But I really think Art knows what he's talking about. Which mistake would you rather make?
[This message has been edited by Recono (edited 01-15-2002).]
Believe me I don't worry with that stuff, havn't got time..I load up til I get an extractor mark or sticky bolt and back off two grains or until she shoots good. BTW I'm not recommending this practice to anyone else..
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Ray Atkinson
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Ray Atkinson
Well said.
During World War Two, a buddy of mine (sniper) majored in tank crews because there he could get hot German 9mm ammo loaded for the Schmeisser. In his waist band, he carried a British .38 revolver. With a pair of dikes, he raised enough burrs on the rims of that hot 9mm ammo to give them a way to headspace on the rear face of his revolver cylinder.
One day, an insistent bowel sent him running into the latrine, where he dropped his trousers and sought relief. The momentarily forgotten revolver dropped to the tile floor --
-- and shattered.