Pushing pressure at longer OAL/CBTO

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  • Hohn

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    Bryan Litz's article on longer CBTO and the reduced pressure that comes with longer OAL has me thinking. The concept makes sense, but can it be quantified?

    For example, how much more RL15 can I run at 2.45" OAL instead of 2.39" for comparable pressures?

    I'm loading up some experiments that are both longer and charged more than book recommendations and I'm trying to get a feel for how hot is too hot for a longer-than normal OAL.

    From what I can tell, another 0.050" of OAL can really drop pressure a bunch. If the pressure can spike due to something like just having a thicker wall case, then surely more volume from longer OAL would work in the opposite direction.

    Putting some science to this:

    Accurateshooter.com lists case capacities for various makes of .223 brass as being anywhere from 30.6gr H2O from some old Lake City lots to as low as 29.8gr for some especially thick/heavy cases. That is a variation of 0.8 grains.

    How many "grains" of case capacity is equivalent to 0.010 more OAL?

    With a .224 bullet diameter, the volume of a cylinder of diameter .224 and length .010" contains the equivalent of 0.0317 grains of water. Thus, going 0.1 longer on OAL would be the same as having a case with 0.32grains more capacity.

    I've loaded some rounds that are 0.110 longer OAL than the load book shows. The math suggests this is the same as having 0.35 grains more capacity.

    The Hodgdon data for .223 uses a Winchester case which has a typical capacity of 30.1, same as Hornady.

    But if I was using Lake city with more like 30.4 capacity AND seated very long (2.5")? My long OAL gives me another 0.44gr capacity relative to mag length (2.26"). In LC brass, I essentially have 30.84gr capacity.


    Hence I propose a rule for .223: each 0.10" over book OAL is roughly 1.5% more case capacity and charge weight could likelwise go up 1.5% or so.

    Not a huge amount for .223 as we're talking about 2-3 tenths of a grain of powder for most powders, but it's not nothing and could very well let you push more speed safely.
     

    bigedp51

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    Using Quickload and the same charge of 25.0 grains of H335 the top case below at 30.6 and the bottom case at 28.0 case capacity will vary the chamber pressure 6,000 psi.

    And as to your question this is why the reloading manuals tell you to start at the suggested start load and work up. Meaning there are far to many variations in firearms and their components for the reloading manuals to be more than ball park guesstimates. Example the throat in my Savage .223 with a 1 in 9 twist is longer than the throats in my AR15 rifles.

    On top of this case hardness and construction in the base of the case will greatly effect your visual chamber pressure estimates. Meaning looking at the primer and measuring base expansion above the extractor groove.

    LjAQ7L9.jpg
     

    Clay Pigeon

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    I will say this Hohn, Quickload is money well spent, the more data you give it the more data and answers you will recieve. For what you are looking at doing it will save you hours at the loading bench and range working up loads.
     

    Hohn

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    Quick update. I got to shoot both my RL15 loads this weekend.

    24.5gr under 80gr ELD at 2.42: no pressure at all. This is over Hornady max by 0.4gr.
    25.0gr under 75gr ELD at 2.42: no pressure at all.
    This is over Hornady max by 0.1gr.
     
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    Quick update. I got to shoot both my RL15 loads this weekend.

    24.5gr under 80gr ELD at 2.42: no pressure at all. This is over Hornady max by 0.4gr.
    25.0gr under 75gr ELD at 2.42: no pressure at all.
    This is over Hornady max by 0.1gr.

    Not trying to be jerk but, why does this surprise you? Loading data is set by lawyers not real world information. .01 doesn't do squat on pressure, .04 won't either. Too many variables with components and chambers. I think what you are chasing is a waste of time. Just my opinion and you know about opinions.
     

    NKBJ

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    The part that surprises me is how accurate the cartridge is when little changes are proportionally big changes.
     

    Broom_jm

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    OK, so this is a very old and very slippery slope to dance around on. More than likely, the article referred to, (but not linked?) is written with all of the requisite caveats included, but I will list two major ones, here.

    1) If you don't have actual pressure trace equipment, you don't know what your pressures were before or after increasing the powder charge. You do not know if your new load is safe, only that you don't observe any obvious signs of being over-pressure. What most reloaders don't realize is that by the time you see incipient head separation (on newer cases) or a very flattened primer, you're actually way over pressure. You do not mention if you have used a micrometer to measure case head expansion, but that would give you at least a comparative basis between loads with different powder charges.

    2) Loading the bullet further out in the neck lowers pressures, until you get it to close to the lands, where it leads to a very sudden and dangerous spike in pressures, especially with an over-charged case! Another common gap in knowledge among reloaders is that the case determines the volume of the firing chamber. In truth, the chamber size is the chamber size; the case servers the dual function of delivering the three components and sealing the chamber against pressure. How far out you seat the bullet does not change the size of the chamber, it only influences the start pressure. That's why no reloading manual is going to suggest one set of load data for one cartridge OAL and another set for a different cartridge OAL. Another way of wrapping your head around this is to consider the 38 Special/357 Magnum, 44 Special/44 Magnum, 45 Colt/454 Casull/460 S&W Magnum cartridges. If you have a firearm with the longer chamber, and built for the higher pressures of the magnum rounds, you can safely shoot shorter, lower-pressure cartridges from them. However, you can't just seat a bullet really long in a 45 Colt chamber and call it a 460 S&W...yer gonna blow something up.

    The more you learn about internal ballistics, the more you realize how complex and interconnected the various components and reloading techniques are. It is a fool's errand to think that you can (safely) use more powder simply by seating the bullet longer. Eventually you'll run into a rifle with a very short chamber or leade...and you'll regret ever reading that article.
     

    Mgderf

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    OK, so this is a very old and very slippery slope to dance around on. More than likely, the article referred to, (but not linked?) is written with all of the requisite caveats included, but I will list two major ones, here.

    1) If you don't have actual pressure trace equipment, you don't know what your pressures were before or after increasing the powder charge. You do not know if your new load is safe, only that you don't observe any obvious signs of being over-pressure. What most reloaders don't realize is that by the time you see incipient head separation (on newer cases) or a very flattened primer, you're actually way over pressure. You do not mention if you have used a micrometer to measure case head expansion, but that would give you at least a comparative basis between loads with different powder charges.

    2) Loading the bullet further out in the neck lowers pressures, until you get it to close to the lands, where it leads to a very sudden and dangerous spike in pressures, especially with an over-charged case! Another common gap in knowledge among reloaders is that the case determines the volume of the firing chamber. In truth, the chamber size is the chamber size; the case servers the dual function of delivering the three components and sealing the chamber against pressure. How far out you seat the bullet does not change the size of the chamber, it only influences the start pressure. That's why no reloading manual is going to suggest one set of load data for one cartridge OAL and another set for a different cartridge OAL. Another way of wrapping your head around this is to consider the 38 Special/357 Magnum, 44 Special/44 Magnum, 45 Colt/454 Casull/460 S&W Magnum cartridges. If you have a firearm with the longer chamber, and built for the higher pressures of the magnum rounds, you can safely shoot shorter, lower-pressure cartridges from them. However, you can't just seat a bullet really long in a 45 Colt chamber and call it a 460 S&W...yer gonna blow something up.

    The more you learn about internal ballistics, the more you realize how complex and interconnected the various components and reloading techniques are. It is a fool's errand to think that you can (safely) use more powder simply by seating the bullet longer. Eventually you'll run into a rifle with a very short chamber or leade...and you'll regret ever reading that article.

    Without knowing how to properly express the point, this right here ^ is my view of the issue.
    I'll be the first to admit that I'm novice hand-loader.
    I probably haven't loaded 1k rounds yet. I like baby steps when it comes to potentially dangerous practices.

    Yes, load data books are signed off on by lawyers, and the specs are much like the load limits listed on elevators.
    The weight limits given are there to ENSURE that you don't enter into a dangerous situation, as long as you follow the procedures outlined in the manuals.

    I may not have the experience with loading thousands of rounds, and thus lack the confidence (arrogance) of some, and lack the testicular fortitude to embark on such an endeavor.
     

    Hohn

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    You point is noted, Jim. I am a novice reloader, I readily admit. But I've done some homework.

    FWIW, I'm nowhere near the lands on my rifle. With 80 ELDs I don't hit the lands until 2.52" OAL. With the 75s, it's similar OAL (I'd have to check my load book). I think it's actually 2.52" also.

    I'm loading closer to 2.45" for my "long" loads. That puts me well off the lands and far from the pressure spike that results from being into them. Even at 2.5" I'd be 0.020" clear of the lands, and bullet tolerance is much tighter than that, making the risk of getting into the lands essentially zero.

    Also, the load data I'm working from is often for .223 and not a 5.56 chamber.

    Based on the throating of my particular rifle it is both a "longer chamber, and built for the higher pressures".


    Ultimately, the proof of pressure is measuring the case head enlargement and primer pocket loosening. I'm monitoring for those conditions as I work my way up.

    With 25gr of RL15 under my 75 ELDs at 2.42, I'm not seeing anything resembling pressure signs. No ejector marks. No primer flattening. No cratering. No case head enlargement.

    The Hornady data lists 24.9gr for the 75 ELD as the top. I'm comfortable pushing that a tiny bit with my rifle, but recognizing the risks of doing so-- powder batch variation, temperature changes, etc etc.
     

    judgecrater

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    "Hence I propose a rule for .223: each 0.10" over book OAL is roughly 1.5% more case capacity and charge weight could likelwise go up 1.5% or so."

    I think this is a vary dangerous assumption. I would never exceed max load just because you freed up some extra capacity.
     

    natdscott

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    Ok, I got the decimal point off. Still doesn't matter. Different loading manuals have more deviation than that, just saying.

    Robert, sir, you are very, very wrong.

    In .223 loading, 0.4 Grains can be HUGE. RE15, in partic'lar, is pretty touchy on the upper end.

    0.4 grains is the difference between "okay, but keep it out of the direct sun", and "blown primer pockets and scrapped firing pins".


    People listen to what you say here, and they listen FOREVER (or as long as Google can search). Please type carefully.
     

    natdscott

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    Hohn, here's what I think, as somebody who has pushed the .223 AR15 as far as it should ever be pushed (and then about 300 yards further):


    --It's not a .22-250, and it will never be one. Get okay with how awesome it is with what it can do (which is a whole awful lot) rather than worry about what it cannot.

    --Competitive loads have been hashed, re-hashed, and fried after 3 days in the fridge. We all continue to do so, but the AMU (best .223 shooters on the planet) continue to kick all of our asses with just 77 grain NATO-pressure stuff. They do so because they practice, instead of hash.

    --The 80 SMK/82 Berger or similar, in front of RE15 or Varget, can do about 97.24% of what the .223 can do in an AR15 as far as range and accuracy are concerned.


    I recommend your leave the other 2.76% alone, and I cannot in good conscience tell you how to get at any of it.


    -Nate
     

    Broom_jm

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    You point is noted, Jim. I am a novice reloader, I readily admit. But I've done some homework.

    FWIW, I'm nowhere near the lands on my rifle. With 80 ELDs I don't hit the lands until 2.52" OAL. With the 75s, it's similar OAL (I'd have to check my load book). I think it's actually 2.52" also.

    I'm loading closer to 2.45" for my "long" loads. That puts me well off the lands and far from the pressure spike that results from being into them. Even at 2.5" I'd be 0.020" clear of the lands, and bullet tolerance is much tighter than that, making the risk of getting into the lands essentially zero.

    Also, the load data I'm working from is often for .223 and not a 5.56 chamber.

    Based on the throating of my particular rifle it is both a "longer chamber, and built for the higher pressures".


    Ultimately, the proof of pressure is measuring the case head enlargement and primer pocket loosening. I'm monitoring for those conditions as I work my way up.

    With 25gr of RL15 under my 75 ELDs at 2.42, I'm not seeing anything resembling pressure signs. No ejector marks. No primer flattening. No cratering. No case head enlargement.

    The Hornady data lists 24.9gr for the 75 ELD as the top. I'm comfortable pushing that a tiny bit with my rifle, but recognizing the risks of doing so-- powder batch variation, temperature changes, etc etc.

    I have been reloading for over 30 years and studied exhaustively...which makes me more cautious than ever. There is nothing worthwhile to be gained from pushing the limits.

    Many chambers are cut exceedingly long because some folks like to dance on the razor's edge. Still, you have to understand that there is a period of time when the shank of the bullet is still in the neck of the case, while the ogive has passed into the leade of your rifling. Excessive start pressure during this time is a bad, bad thing.

    See above: You're decreasing free volume in the case while reducing the time allowed for the bullet to "uncork" the chamber. You're adding straw to both sides of the camel's back, potentially.

    Primer pocket loosening occurs either gradually or with one firing; the former indicates safe loads, while the latter strongly suggests you are playing with fire. Measuring CHE (with an actual micrometer) and comparing to factory ammo is a much better way of gauging the relative safety of your handloads, but unless you're using actual pressure trace equipment, you're still ill-advised to exceed the recommended cartridge OAL by any appreciable amount. If you're not seeing any case head enlargement, you are either not measuring it effectively or not measuring it at all. Do you have a micrometer and know how to read it?

    I'm not trying to undermine what you're doing, just introducing certain aspects of reloading when one works outside of "book" loads. Loading for half a dozen wildcats has led to a lot of "a-ha!" moments, including the understanding that seating longer is not an invitation to dump a lot more powder in the case.
     

    Hohn

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    I'd think loading long increases free volume in the case. Litz explains this.
    SAAMI-COAL.jpg



    Most relevantly here:

    The primary effect of loading a cartridge long is that it leaves more internal volume inside the cartridge. This extra internal volume has a well known effect; for a given powder charge, there will be less pressure and less velocity produced because of the extra empty space. Another way to look at this is you have to use more powder to achieve the same pressure and velocity when the bullet is seated out long. In fact, the extra powder you can add to a cartridge with the bullet seated long will allow you to achieve greater velocity at the same pressure than a cartridge with a bullet seated short.


    I'm not pushing pressure or loading hot for its own sake. I am trying to see if I can get to the next accuracy node in my rifle. I'm not trying to get 2900fps out of a 80gr bullet in an 18" gun.


    You'll see why I'm confused because you appear to be asserting things that are false. (reduced free case volume at longer OAL).


    It's simple really: I'm taking Litz's advice and I'm willing to go slightly over book charge weights when I'm also over book OAL. Indeed, I NEED to add more powder at longer OAL just to break even in terms of pressure and velocity.


    I also agree that pushing hot loads is usually diminishing returns. I'm not trying to reduce windage by getting another 100fps or so. I'm trying to get safely to the next accuracy node in my rifle with particular loads where the first node is FAR below book max.


    With your experience, I'm sure you understand this phenomenon.
     
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