Home Forums Hunting & Shooting Ask The Gunwriters Prudent pressure in modern rifles

Don,
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<br>You wrote.."This large case-low pressure theory goes against the latest trend....It also results in less than full cases that might result in secondary explosions....."
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<br>I have been using this large case-low pressure theory with my hunting loads this year and have been well satisfied,trend bucker or not.
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<br>The key for me is to use a powder that is a "click" or two slower than normal. This results in 100% loading density or slightly compressed. I don't think anyone is suggesting one use a very slow powder with 50% loading density a let it rattle around in the case.
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<br>In summary,I too,will stay with full loads of slow burning powders. Just at a lower pressure and temperature
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Let me put a twist on this topic and assume that pressure and heat combined have the same effect on like metals, regardless of application. We can assume that a rifle chamber is the same as an engine combustion chamber and a like bore size can be affected by increasing its length or stroke. We can now make a comparison. BMEP is used in our trade as a measure of peak pressure. We can take an engine of the same bore size and increase the stroke length (chamber area). Now if we want to make the same power with the shorter stroked engine we must raise the peak cylinder pressure.The side effect of this if we are using like metals is the increased heat and pressure will cause increased erosion to the piston crown and valve seat surfaces. This fact remains the same regardless of engine size or horsepower.
<br>Enough of that. Ken is talking about throat erosion, this is a direct result of peak pressure and temperature and has nothing to do with attainable velocities, which are ideally gained by a constant pressure over the full length of the bbl.
<br>By reducing the 220 Howell to below the peak loaded pressure of a 22 Hornet, throat erosion should be less assuming the same cylinder temperures are achieved, which they likely are not as we are not trying to get the same power level. If we compare apples to apples and use the Howell cartridge to duplicate 22-250 or Swift velocities then I would agree throat erosion will be less and pressures can be reduced significantly and the same velocities will be achieved.

"This large case-low pressure theory goes against the latest trend of the PPC design of the short fat case."
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<br>Not at all. The ratio of case diameter to case length is an entirely different design consideration that doesn't matter here.
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<br>"It also results in less than full cases that might result in secondary explosions as Ackley found with reduced loads and slow powders."
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<br>Nope. You've assumed "facts" not in evidence. The larger case uses a slower powder -- with benefits that I didn't present in the discussion of the other comparison. The pressure curve in the .220 Howell compares very interestingly with the Swift's curve. The Swift's curve rises quickly to a higher and sharper pressure peak, then fades rapidly as the bullet scoots toward the muzzle. The .220 Howell's curve rises more slowly to a significantly lower peak, then fades much more slowly with bullet travel. The PEAK pressure (read "erosion") is lower in the .220 Howell, while its AVERAGE pressure (read "acceleration") is higher, clear out to the muzzle.
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<br>"Overbore" is a word used often, understood seldom. My cartridges are not over bore capacity for the powders that I designed them around.
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<br>"Bore capacity" applies to a cartridge in relation to available powders. A case that's "over bore capacity" is a case that can not be loaded with a caseful of any available powder without developing dangerous pressures. It's a matter of how fast and how high the pressure curve rises while the travel of the bullet down the bore bleeds-off some of the pressure. In a case that's over bore capacity, the travel of the bullet down the bore can not relieve the building pressure fast enough to keep the peak pressure (from a caseful charge) down to a safe level.
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<br>If there's a powder available that you can fill the case with, without producing dangerous pressures, that case is not "over bore capacity."
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<br>When I started loading rifle ammo in the early 'Fifties, IMR-4350 and H-4831 were the slowest powders available. Both are too fast for the .220 Howell -- i e, the .220 Howell is over bore capacity for these powders (just as the .223 is over bore capacity for Bullseye). It is NOT, however, over bore capacity for IMR-7828, Ramshot Magnum, and several other powders in this burning-rate class. I could fill my .220 Howell with Hodgdon's ultra-slow 50BMG powder and not even come close to optimum pressures. I couldn't pack, jam, or stomp enough 50BMG into the .220 Howell to raise pressures to a good operating level -- so the .220 Howell is clearly NOT "over bore capacity" for all available powders.
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<br>As I've already posted somewhere, this isn't just nerd theory. It's all long-proven fact, clearly presented in classic interior-ballistics literature that unfortunately isn't available to the average shooter. So of course it's all "new" to you and seems to run counter to what you already know about what goes on inside the rifle. But the fact that it's new, and negates some of your understanding, does NOT mean that it's wrong. I'm not putting anybody down. Just the opposite -- I'm trying to share what I'm learning in intense study of interior ballistics.

If I use a .220 Swift and you use a 30-06 necked to .224 you will use about 20 grs more powder per shot. At the 5000 shot mark I will have saved $285 which should pay for a new barrel!
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<br>I still have to find out more about this .220 H . So far it's hard to believe.
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<br>But more powder costs more, gives more recoil and more blast. And it does go against the modern trend of the low aspect ratio cartridges like the PPC and now the .300 WSM and SAUM.
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Then the perfect answer is dog-simple, Bro' Don --
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<br>You shoot a .220 Swift, and I'll shoot a .220 Howell, and we'll both be happy.

Ken,
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<br>I hear you loud and clear on this. It jibes perfectly with my experience over almost forty years of loading. In fact, right now I am looking forward to taking advantage of just this principle in a 270 rifle I am having rechambered to 270 Wby. It already gives me almost 3200 fps with a certain charge of H450 from a 24" bbl.
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<br>After the rechambering the same charge will obviously give me less velocity, because the case capacity will be increased. Increasing the charge will bring the velocity back up, even though the pressure may be lower. Going to an even slower powder, say Re 22, Re 25 or 7828, one should be able to get higher velocities still, without primer pockets expanding, etc.
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<br>The fly in this ointment, of course, is that the increased head area will increase the bolt thrust or force the locking lugs must bear.
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<br>Ted

Ken, thanks for sharing your thoughts, and please continue doing so. I have learned a fair amount off of this thread. You have a vast amount of knowledge, that is for sure. I didn't know exactly what over bore capacity was before, nor did I know that pressure was what is responsible for ruining bores, not velocity. Ken, are there any over bore capacity cartridges that you are aware of for normal speed rifle powders? Such as IMR 4064, IMR 3031, powders within that range more or less. Thanks.

"Ken, are there any over bore capacity cartridges that you are aware of for normal speed rifle powders? Such as IMR 4064, IMR 3031, powders within that range more or less."
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<br>Yes, the world's full of cartridges that are over bore capacity for the powders you mention, including those that aren't over bore capacity in the generic sense that there's NO powder that they can safely fire by the caseful.
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<br>Just for example, I plugged IMR-3031 into the computer to see what pressure a caseful of it would churn-up in my .220 Howell with an 80-grain Sierra -- over 113,000 lb/sq in.!
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<br>I should add that the optimum powder for any case depends on (a) the net capacity of the case, between the web and the base of the seated bullet, (b) the maximum average peak operating pressure desired, and (c) the weight of the bullet. In a given case, for example, the heavier bullets would generally call for a somewhat slower powder -- and the lighter bullets a somewhat faster one. How easily the bullet can be pushed down the bore -- or how stubbornly its mass resists propulsion -- affects how the powder behaves.
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<br>Since I designed my .220 Howell strictly for 75- to 80-grain (and heavier) bullets, I centered the design on IMR-7828 -- and sure enough, IMR-7828 is one of the two or three best powders for it WITH THESE BULLETS. I'm not interested in using anything lighter in it (yet), so I haven't considered anything lighter for it. It may NOT be over bore capacity for IMR-4831 or IMR-4350 or even something a little quicker, with the lightest .224 bullets. I suspect that it isn't, but I just haven't looked into that question at all.

Woah Nellie-- I have tried my best to wade through this entire post and in my limited way understand it. I don't. I sort of feel like the old story about the little girl who was going to give a school report on Penguins. Her preface was " I don't want to teach you more about Penguins than you want to know,".
<br>What is beyond my understanding is how in the world you are going to blow a third more powder with its attendant ejecta through the same size hole even at lower pressure and not wear out that hole as fast as less ejecta at higher pressure. There is more of it that has to go through there and it all has to go big charge or little high pressure or low. We are not talking about just sedately running it through either. Real time it is going through there A. D. Q. no matter what. That hole in the barrel is going to get real hot real quick either way and the hotter it gets the softer the steel gets and easier to scrub away.
<br>Just looks like to me if you run more of it at less or less of it at more sort of balances out the same.
<br>Must be magic but the higher forms of mathmatics are magic to me anyway. What am I missing here and please keep it simple. Remember the Penguins.
<br>BCR

Ahem, uh Dr. Howell,
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<br>I asked a question or two earlier in this thread not exactly pertinent to the thread although an earlier poster had mentioned the same phenomenum. Should I re-post these questions in a separate thread or was my post simply overlooked? . Thanks, badger.

Gotcha, Badger -- yes, I missed your question altogether.
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<br>"It is a Model 7 Youth in .243Win that only starts to group decently with a warm load. Littlebit & I were talking about it and theorized that this rifle may have a bedding problem. It takes 42 grns of IMR 4350 behind a 100grn Partition to make it MOA or better. Interestingly, the POI shifts drastically with reduced ... loads ... and grouping opens to more than 4". The POI is approximately 4-6" lower at 100 yds with a starting load (about 37.5grns IMR 4350) Your thoughts on this?"
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<br>I think you may be right -- that you have a bedding problem. That much variation in POI can't be blamed on the difference in muzzle velocity, and the variation of group size strongly suggests bad variation in the barrel's vibration pattern.
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<br>You may also have a related problem -- the torque on the action screws may be partly to blame. They may be too loose, too tight, or too unevenly tight.
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<br>If it were my rifle, I'd glass-bed it, probably with pillars to boot, and make sure that the fore-end didn't touch the barrel anywhere. Then I'd invest in a good torque wrench (I bet you know exactly where you can lay your hands on one!) and mess around with the torque on the action screws.
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<br>For a bedding compound, a number of the bench-rest gang recommend Charley Robertson's Pro-Bed 2000 (http://www.scorehi.com/scorehi/home.htm). I bought some but haven't used it yet, so I don't know how much better it may be than the good ol' Acraglas and Acraglas gel I used so much of in years gone by.

Boggy, ol' buddy, I think maybe the solution to your confusion may lie in the distinction between heat (in Btu) and temperature (in �F). Consider two logs of the same species of pine (your choice -- longleaf, shortleaf, slash, loblolly, or whatever else), exactly the same length and diameter.
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<br>Imagine (a) shredding one log into excelsior and striking a match to it, and (b) letting the other lie in the woods and rot. Both logs will put out the same number of Btu, but of course over strikingly different spans of time. Only a brass monkey or a cigar-store Indian lacks the imagination to see that the �F levels of the two are drastically different.
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<br>The high peak temperature concomitant with a high peak pressure, whatever the weight of the charge, is what affects the throat adversely. Lower that temperature, and even the far minor scrubbing action of the powder granules has less effect on the surface of the steel. Also, with the higher peak pressure, the increase in the temperature of the steel is more localized -- more tightly focused -- in the throat and early rifling.
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<br>At a lower peak temperature, a higher heat output (Btu) can be absorbed along more of the barrel's length without anything like the adverse effect of the higher peak temperature at the throat.
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<br>I've heated our homes with fuels from corn cobs to coal, natural gas, bottle gas, fuel oil, and several kinds of wood. It's been easy to notice that woods put out more heat (Btu) at lower temperatures (�F) and thus heat the house more evenly, and that some of the pitchier pines put out about the same amount of heat at higher temperatures. Good stove-fuel hardwoods (e g, oak) burn at much "cooler" �F than the pitchier pines but put out as much or more heat.
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<br>As I think I've already mentioned, the larger charge of a slower powder in my .220 Howell produces a lower peak pressure (therefore temperature) more gradually, and the gas thus produced maintains its pressure higher and longer. The peak is lower, longer, and less sharp, and the downslope is much less steep. So the larger charge does indeed produce more HEAT but at a significantly lower peak pressure and TEMPERATURE.
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<br>I hope this makes this mess a bit clearer to you -- but I bet that now somebody else in our crowd is more confused than he was.

Thank you. I am going to try a different stock (Big Stick is sending me a Synthetic one) & will see how it shoots before doing any bedding work with a new stock. Just want to see if the same characteristics with regards to POI shift & loads apply. badger.

I think I got a handle on the why as to how a .300 super magnum loaded at 55,000 psi burns out barrels faster than a .308 loaded to 60,000 psi. You're saying that basically, the .300 super magnum at lower pressure produces lower temperatures more spread out over time, rather than a higher temperature that burns out. I think I get it now......

If you said what you meant and meant what you said (" ... a .300 super magnum loaded at 55,000 psi burns out barrels faster than a .308 loaded to 60,000 psi."), you got it exactly backward. It's HIGH PRESSURE and its related high temperature that burns-out barrels faster. So the smaller cartridge loaded to a higher pressure burns barrels out faster than the larger cartridge loaded to a lower pressure.

I,too,think "I got it".But then I think I saw Elvis at the Fort Worth Stock Show yesterday,too!

Ken, I mean the .308 would burn out barrels quicker, thanks for noticing!

Thank you right on for the answer you gave, Ken. I sort of understand that explanation. Sort of the same difference heating steel with a rosebud tip and a pinpoint.
<br>Still not sure it is going to make a lot of difference in the cosmic scheme of things but at least I know what you are talking about and it stands to reason.
<br>BCR

I'm glad I stumbled in here, I think I actually managed to learn something again.
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<br>If I understand this correctly, high pressures erode, but what we perhaps did not explicitly state, is that the "over bore" cartridges have high pressure for a long(er) duration. A 308 has high pressure, same as 30-378, but is at near-max pressure (temp) for a much shorter duration than the large cap case. In other words, time x temperature are the function.
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<br>If you take the extreme temperature down, you can increase the time exposure, and still reduce erosion.
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<br>At least, that's what I think I just learned. FWIW, Dutch.
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Dutch,don't you dare leave here!Ken often talks smarter than I can listen.An interpreter is exactly what I need.I realize one can "dumb things down" only so far and still transmit meaningful information,but a large gap is still left with me when Ken is thru dumbing something down!much obliged.