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Understanding proper OAL

Discussion in 'Ammunition & Reloading' started by CarlMc, Sep 2, 2013.

  1. CarlMc

    CarlMc Safely north of Seattle Active Member

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    I have a particular powder/bullet combination that isn't in my books. I had a box of 500 bullets previously that didn't have the particular recipe in my books, either, and the load I came up with worked fine for training/stress relief, and when I got another box of bullets (availability is part of the problem lately) I ran into the problem again. This time I want to get a better understanding of what's important where, especially when the new bullet is a lot shorter than the same weight of the previous bullets.

    Powder charge mostly relates to bullet weight. For a particular powder, you can "ballpark" the charge if you know the bullet weight, but for all my reading, the OAL is the least explained.

    I can understand the OAL somewhat as it relates to rifling engagement, magazine fit, feeding, and compressed loads, but are there any other factors that I need to understand when determining the OAL?

    I avoided stating my particular specifications because I'm more interested in the process than approaching a specific answer.
     
  2. Otter

    Otter Oregon - mid Willamette Valley Active Member

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    Sounds like you have a basic understanding.

    OAL is usually measured off the tip of the bullet. That measurement is only meaningful with regards to magazine length. OAL measured off the ogive is what is really important, provided you have adequate magazine clearance. Where the ogive is in relation to the lands/rifling can improve accuracy if you find the sweet spot. As you are probably aware it takes a special tool to measure this.

    The sweet spot is determined by the harmonics of the barrel. Backing the ogive of the bullet off the lands or jamming into the lands can get the bullet to release from the barrel either at the top or bottom of the vibration cycle of the barrel which should produce optimum accuracy.

    It isn't overly complicated. My understanding is to find a powder charge that shoots well, and then fine tune the load by adjusting the seating depth. I've tried this in factory rifles with inclusive results, but I've tried it in target rifles and it can make a significant improvement.
     
  3. sneakboxer

    sneakboxer NW OR Active Member

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    OAL can/will also affect chamber pressure. A shorter OAL will reduce your case capacity and raise pressures. An OAL where the bullet is jammed in the rifling can also raise pressures. Some cartridges are more sensitive than others but if you keep it in mind and work up your load you should be OK within reason, just something to think about.
     
  4. AMProducts

    AMProducts Maple Valley, WA Jerk, Ammo Manufacturer Silver Supporter

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    You're kinda opening up a can of worms here, for exactly many of the reasons you state.

    I have a few "rules of thumb" that I use when determining safe load combos:

    1) If a bullet is of lower density, you can use load data from any bullet of the same weight, provided it does not go into the case further than the other bullet does.
    Corrolary to Rule 1: You can use load data from a heavier weight bullet to load a bullet that is the same form factor (and is thus loaded as deep in the case, reducing powder volume) safely, however usually the pressure will be lower and you may need to increase charge, but not before testing the load.

    2) Form factor is among the most important issues in determining OAL. Bullets with shorter ogives may interact with the throat and stick, either pushing the bullet further into the case, or making extraction difficult (the bullet may be lodged in the throat and will not be removed by cycling the bolt, at this point, it's cleaning rod time). Further bullets that are "light for form factor" usually hollow point, or hollow base will require smaller charges as the bullet will intrude further into the case and reduce powder volume.

    In my opinion, there are four important issues when determining OAL:

    1) The distance between the forward edge of the bearing surface (the cylindrical part of the bullet) and the throat.

    2) The actual OAL, which can be an important consideration for magazine fed firearms.

    3) The form factor of the ogive, and the size of the meplate, bullets that have unusually round ogives, or unusually large meplates (the front of the bullet), can have feeding issues in semi-automatics, and can interact with the throat of the gun prematurely.

    4) How much of the bullet is stuck inside the case, as it affects powder volume, and thus charge and burning characteristics. However, this is usually addressed by your load data.
     
  5. CarlMc

    CarlMc Safely north of Seattle Active Member

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    This is awesome. Regarding seating depth, if have good (tested, or from a reputable source) load data for a given bullet weight, and know that said bullet is seated so far down, I can assume that I'm safe as long as I don't exceed that seating depth or that I reduce the total space available for powder?

    So I could assume the above would be the lowest I could seat a bullet? I could then seat the bullet somewhat higher than that level and still be safe, correct? At what point does the larger space create a detonation risk?

    Trying to keep this about the process, not the actual answer itself, but I could assume that for pistol, everything above the max seating depth is fair game providing it still operates properly? The shapes of the bullets is mind boggling in pistols, and I'm guessing there's room to play out in front.

    For accuracy in rifles, it's all about finding the right balance, so I can see how the same principles apply, but the importance varies.
     
  6. deadshot2

    deadshot2 NW Quadrant WA State Well-Known Member

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    When pressures are increased by inserting the bullet farther into the case, any issues are usually negated by the longer jump. This lets the bullet get moving and lowers that pressure spike. When a bullet is jammed into the lands you have not just the neck tension to overcome but the extra pressure required to get the bullet moving down the bore.

    It's generally accepted by those that "Jam" to develop your powder load with the bullet into the lands first and then adjust back into the case. You've already determined the best charge and the max pressure you'll encounter and are now just fine tuning to manage barrel timing. Just remember that many, if not most, powders can be used with compressed loads. That means that the bullet is as far back in the case as it can get with a full charge.
     
  7. evltwn

    evltwn Gold Hill Oregon Bronze Supporter Bronze Supporter

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    Not to hi-jack the thread, but is the listed oal in the reloading manuals the minimum or maximum?
     
  8. M67

    M67 NW Oregon Active Member

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    My manuals state that the listed COAL, is the minimum. Other manuals may vary.
     
  9. 2506

    2506 Seattle Well-Known Member

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    I think you may have that backwards. This is taken verbatim from Berger's website:

    "Most loading manuals (including the Berger Manual), present loading data according to SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute) standards. SAAMI provides max pressure, COAL and many other specifications for commercial cartridges so that rifle makers, ammo makers, and hand loaders can standardize their products so they all work together. As we’ll see later in this article, these SAAMI standards are in many cases outdated and can dramatically restrict the performance potential of a cartridge.

    Bullet seating depth is an important variable in the accuracy equation. In many cases, the SAAMI specified COAL is shorter than what a hand loader wants to load their rounds to for accuracy purposes. In the case where a hand loader seats the bullets longer than SAAMI specified COAL, there are some internal ballistic effects that take place which are important to understand."
     
  10. M67

    M67 NW Oregon Active Member

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    deadshot2: OAL's listed in the Lee 2nd Edition are "Minimum"

    As are the OAL's in my Lyman manuals.

    I have had quite the discussions with deadshot2 on this matter. lol
     
  11. Papa Jon

    Papa Jon outer N.E. Portland Member

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    The listing in my Nosler book is "maximum" overall length
    as an example :30.06 OAL is 3.340

    It gives this measurement for all bullets and loads

    From what I have read it is based on standard that fits in all magazines.

    Found some interesting info here: http://www.saami.org/index.cfm
     
  12. deadshot2

    deadshot2 NW Quadrant WA State Well-Known Member

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    Maximums are usually only listed for "magazine feeding rounds" for obvious reasons. The lengths listed in most manuals will be the minimum in order to prevent excessive pressures. Some powder manufacturers are adamant on the minumum COAL as they have some powders that can be very nasty if you go too short. One such is Vihtavouri 3N37. They want a COAL for 9mm loads that's too long to chamber in most CZ semi-auto's. Shorten the round to fit? Just make sure you have "ANSI spec Shooting Glasses". Trust me on that, I own a CZ75 SP-01.

    As for length, there is no maximum as long as you have enough bullet in the case to hold it straight while chambering. In this situation the COAL is dictated by how heavy handed the gunsmith was with his throating tool.

    SAAMI specs are designed for the manufacturers of firearms and ammunition so one will fit the other without damage occuring to the rifle or shooter.

    When all is said and done, the best reloading manual for loads going into YOUR firearm is the one you develop for yourself. Keep good records and that then becomes the "bible" for your firearm.
     
    M67 and (deleted member) like this.
  13. Papa Jon

    Papa Jon outer N.E. Portland Member

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    All good info
    In working up a load for my 30.06 I found the OAL for my rifle. The length were the bullet just touched the grooves and then I backed it off .010". I will do my powder load development then I will fine tune the OAL
    I do need to pick up a chrono so I can keep track of MV.
     
  14. Otter

    Otter Oregon - mid Willamette Valley Active Member

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    Lots of good information but I don't think any of the previous posts cover this well enough for anyone to take action, including mine. Messing with seating depth can have dire consequences if you don't know what you are doing. I suggest anyone thinking about seating depth changes read this article. I believe it was written by Bryan Litz, an authority on ballistics:

    Effects of Cartridge Over All Length (COAL) and Cartridge Base To Ogive (CBTO) ? Part 1 | Berger Bullets

    There is lots of discussion about COAL on the Accurate Shooter website. I really encourage anyone thinking about messing with seating depth do some research before they take action.
     
  15. Jammer Six

    Jammer Six North Greenlake, Seattle New Member

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    You guys are all talking about different subjects.

    None of you even stopped long enough to ask the original poster what caliber he's loading or whether he's loading a straight walled pistol cartridge that headspaces off the mouth or a bottleneck rifle cartridge that headspaces off the rim or the shoulder.

    He made one glaring error in his first post, and another statement that required further questioning.

    Instead of doing that, you were all so eager to start telling him what you "knew" that you sound like the story about five blind men trying to describe an elephant.

    I suggest you guys start over, and find out what he's doing, why he's doing it, and how.

    Then you can probably help him.
     
  16. CarlMc

    CarlMc Safely north of Seattle Active Member

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    I explained that I was more interested in the process, and by stating a caliber right off the bat, the thread was likely to focus on that or similar calibers, which I was specifically not looking for.

    Please share my error. I wish I knew it first, but I came asking for guidance and it's easy to have asked a question based on a false premise considering how little knowledge there is out there on the topic.
     
  17. AMProducts

    AMProducts Maple Valley, WA Jerk, Ammo Manufacturer Silver Supporter

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    Usually, yes. So if you have two bullets that are of equal weight, but one is say a hollow point, you can use the same load data with a FMJ and still be "safe" however I'm sure the accuracy, velocity and other factors may change.

    As far as I am concerned "detonation" is a myth, I've done lots of stuff with powder that most experts claim creates a detonation risk and have yet to see it happen. However, there are certain powders that simply refuse to perform outside the normal loading envelope (H110 is a key example). Even setting charges inside a bomb type calorimeter didn't cause a detonation.

    That said, putting a bullet so it's right on the throat of the gun can cause a spike in pressure, as the powder is now trying to do two things, first it's trying to overcome static friction of the bullet, and engrave it into the throat.

    Personally, I've never achieved higher levels of accuracy by playing with the OAL, and usually just create more problems for myself in terms of feeding problems. I like to stick within saami specs for all cartridges I load, even my personal loads.

    There is a certain amount of play you can do with pistols, however typically loading the bullets longer creates feeding problems.

    Powder, charge and bullet combination play a significantly larger role in performance than seating depth, the people who are playing with seating depth are trying to squeeze hundreths of an inch off group size, not trying to make them all touch on the paper.

    This is actually completely outside what we are talking about, bullet seating depth and COL have zero to do with headspace, headspace is an issue 100% with the casing, it has nothing to do with the bullet at all. I'm not aware of any cartridge that headspaces off the bullet... Certainly not any that are in common use (perhaps gyrojet rounds? dardic trounds?)
     
    CarlMc and (deleted member) like this.
  18. Papa Jon

    Papa Jon outer N.E. Portland Member

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    For those of you that haven't looked at Otters link to Berger Bullets you should. This opened my eyes to a lot of information about the subject of OAL and COAL