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Return of C.O.A.L

Discussion in 'Handloading and Reloading' started by zchaparral, May 10, 2014.

  1. zchaparral

    zchaparral Member

    This is a bit of information I promised readers and responders a couple of months ago that I would submit. Sorry it took so long I hope some of you get a chance to read it. I hope it is helpful to someone.
    I am new to reloading as many here on THR are. When I started last January I started with an RCBS starter kit (a 15 year old in-law gift) which came with a Speer Manual 13th Edition and twenty years of used brass. After reading the manual I purchased an RCBS powder meter and case trimmer. I could go on and on and on from here about my trials and errors but I must cut to the chase.
    After loading several hundred starting loads of several calibers I ran into a problem. A certain 9mm bullet I had purchased online did NOT match a starting load C.O.L. I was working with. It would not manually cycle through my pistol. After a long pause and some thought I made a dummy round (no primer or powder) and seated the bullet until it passed the plunk test and matched some factory ammo I had. It was .025 less than the recommended C.O.L. That is when I first came across THR in my search for any answers. I was stumped!
    After many prompt helpful and different replies, thank you guys, I ordered another reloading manual, a Hornady 9th Edition. What I found in it was that the closer a bullet is to the land the greater the pressure. In my opinion (my new theory at the time) and in other words, if the bullet is into the rifling a couple of thousands it takes more powder burn i.e. more chamber pressure to get the bullet into motion. The further away from the rifling the bullet is i.e. a shorter C.O.L. the less chamber pressure because the bullet has uninterrupted space to build momentum.
    So I decided the only way I was ever going to be 100% sure, ok, maybe just comfortable with an answer was to test my theory. I purchased a chronograph. Unfortunately I am a city boy and would have to wait until the end of April, my annual spring break trip to the great Michigan North, in order to blast off hundreds of my homemade rounds for the first time. This required lugging my reload equipment.
    F@#$ing data already! Got it. Here it is. I am not a scientist, engineer, scholar or even an educated man. This is just simply one knuckle heads data. Take it for what you will.
    Pistol: CZ 9mm Luger P07 Duty
    Bullet: Nosler JHP 124 Gr. P/N 1041
    Powder: 4756 5.2 Grains
    These are four round groups. Everything is averaged. The first round or longest is the perfect fitting plunk test round and the first C.O.L. that would manually cycle through the pistol.
    OAL 1.075 velocity=1008ft./sec.
    OAL 1.065 velocity=976ft./sec.
    OAL 1.063 velocity=992ft./sec.
    OAL 1.056 velocity=1002ft./sec.
    OAL 1.050 velocity=1010ft./sec.
    OAL 1.042 velocity=1009ft./sec.
    OAL 1.038 velocity=1036ft./sec.
    OAL 1.025 velocity=1041ft./sec.
    All the spent brass had no deformation, separation, or splits. I did notice more barrel rise with greater velocities.
    I assumed when starting this experiment that higher pressure would necessarily relate to higher velocity. I’m not convinced of that any longer for different reasons. What I do draw from this is exactly what I needed. Although it should never be necessary to make such a dramatic O.A.L. adjustment when setting up a load; I feel 100% comfortable with making the proper length cartridge first to any firearm for any minimum powder load, regardless of recommended cartridge length. I don’t think I’ll explode (and I did think I might explode). What do you think? I appreciate all input and experience.
    p.s. sorry about format
  2. rchery59

    rchery59 Well-Known Member

    In your case higher pressure does mean higher velocity. The shorter the COAL the higher the pressure. Your COAL was shortened by .050 I'm sure the pressure in the 1.025 load is higher than 1.075, much higher.
  3. ArchAngelCD

    ArchAngelCD Well-Known Member

    To add to that, it's possible in a 9mm case to go overpressure with such a lowering of case space.

    It's possible to get the bullet into the rifling in rifles and that would rise the pressures but in handguns it's very rare to be able to reach the rifling with the bullet and still maintain proper neck tension because there isn't enough bullet in the case.

    The 9mm case is very small and making changes that radical can raise pressures very quickly and even cause pressure spikes.
  4. zchaparral

    zchaparral Member

    That is one huge problem I am trying to understand. I have no evidence that a shorter C.O.L. causes higher chamber pressure, non compressed load of course. Can anyone cite a manual that states such? Thanks.
  5. ArchAngelCD

    ArchAngelCD Well-Known Member

    Oh reducing the case volume by seating the bullet deeper into the case will raise the pressures and in a 9mm by more than you think. You don't need a manual to confirm that, it is self evident IMO. also, compressing the load does not raise pressures, it just compresses the powder. The type of powder you use matters much more.
  6. boom boom

    boom boom Well-Known Member

    Now, I have not fired nor reloaded the Nosler bullet in question nor used SR 4756 powder. However, I have reloading 9mm and other calibers for a while with a variety of powders and dealt with the whole seating to a short COL issue before with Oregon Trail 124 gr. lead bullets and Hornady XTP's before with my shallow throated P6 Sig where I had to seat them to a short COL in order to chamber them. Spent a lot of time about 4-5 years ago researching this very subject so I could use these bullets safely with this pistol. What I consistently found is that every authoritative source claims COL is very important in small chambers regarding pressure--I could find no authoritative published manual, gunwriter, etc who said otherwise. Nevertheless, a lot of people on the internets keep claiming such things that bullet setback is a myth, shorter COLs don't affect pressure, etc. But, I have not seen one of them that cites any pressure chamber information either using piezo electric sensors or Copper Unit of Pressure test units.

    My personal recommendation for the poster is that he might want to follow Nosler's reloading manual as far as COL because they have pressure tested it for the best range for powder loads for that bullet that generated safe 9mm pressures.

    When dealing with powder, bullet, COL, case, chamber of the pistol, etc., you have a number of variables that affect the overall pressure exerted on the brass. Now, what the poster assumed above is that a weak if any relationship exists between COL and pressure generated.

    However, the Boyle's gas law states P=1/V where p=pressure, and v=volume. Thus, pressure is inversely related to volume. As you decrease the volume where the powder can burn, pressure rises (ceterus paribus). Now, compression of powder can cause pressure to actually decrease under some conditions but the effect is NOT linear because the compressed powder sometimes burns more slowly thus generating less gas volume in a given time period. Firing guns is almost a perfect example of an empirical test of the gas laws just like bullet trajectories are classic physics problems.

    The 9mm case is very small and so even setting a bullet deeper in the case represents a substantial decrease in volume which raises pressure within the cartridge and chamber (remember Boyle's law). People have survived such experiments because modern steel is pretty tough, temperature, powder variations, barrel throats, chamber condition, bullet profiles and makeup, primer variations in pressure, brass strength, neck tension, etc. leave a lot of factors beyond the powder load that affect overall pressure generated.

    Cites: For example, VihtaVuori in their reloading manual has a warning on overly deep seating bullets in small cartridges. They show a load they tried in the lab using 3N37 in the 9MM where a bullet seated at the book depth gave 230 MPa or 33,358 psi. They seated it 2MM (0.078") deeper and the pressures jumped to 310 MPa or 44,961 psi. This is a increase of over 12, 000 psi or about a 30% increase in pressure which puts it close to a proof load for a regular 9mm pistol. The chronographed speed only increased from 1120 to 1180 fps btw.

    Once you go beyond the ranges of the reloading manuals regarding their specifications, you can no longer assume a nice linear relationship between chamber pressure and velocity. I have read similar things regarding COL importance for small cases in Lyman's and Speer's manuals, and others--too lazy to look up the specific page numbers right now as most don't have that indexed. BTW, Lee Reloading manuals has a long chapter on safely reducings loads that deals with the pressure/velocity issue for rifles and dealing with lead bullets.

    If you want more proof, check the internets for bullet setback (which occurs when you rechamber a bullet again and again in a semi auto.) If you want further confirmation, contact Hodgdon's directly--they are friendly folks. In regards to pressure signs, see Patrick Sweeney (gunsmith and reloader) 's Big Fat Book of the .45 ACP, Chapter 8, p. 254. "A standard rifle case running at 50,000 psi shows no pressure signs." He goes on to mention specifically 9mm reloaders who rely on lack of pressure signs in their brass for pressure guidance as unsafe.

    Be safe and good shooting.
  7. gamestalker

    gamestalker member

    AA nailed it!

    As for documented evidence of pressures rising when the bullet is seated deeper in cartridge such as 9mm, I'll quote from my Speer #10.

    (9mm) "Loads that produced 28,000 cup, went to 62,000 cup when the bullets were purposely seated .030" deeper"

    Compressed charges don't create a pressure increase, provided the powder used is intended to be used in such manner. Obviously, if you compress a charge of a powder not intended for such, it would likely be an incorrectly measured powder charge. In other words, compressing a powder does not control the pressure, the burn rate, or peak pressure obtained. I knew a guy who misunderstood this, he completely destroyed a beautiful Weatherby by compressing the wrong powder, in other words he excessively over charged and exceeded the published data. It stripped the lugs clean out of the recesses, and almost ripped them completely off the bolt.

  8. zchaparral

    zchaparral Member

    Thanks for all the great replies and they are great. Believe me I am cautious and afraid, that is why the subject is fascinating. This is on my mind. Ok, lower volume equals greater pressure. At a certain pressure the bullet starts to move out of the cartridge. When the bullet starts to move volume increases. As powder continues to burn, and build pressure, simultaneously volume increases and relieves pressure, surely not one hundred percent of pressure gain speed. Then the bullet contacts the rifling. This is where it truly has a chance to gather great pressures. The bullet surely can exit the case easier then engage the rifling. In other words it takes more pressure to conquer the rifling than simply exit the soft brass case. The math? I don't know. Powder burn rates are probably most significant.
  9. murf

    murf Well-Known Member


    internal ballistics is hard to understand. so, don't get discouraged.

    first, volume and pressure are indirectly proportional (i.e. cut the volume in half doubles the presssure, double the volume and halve the pressure). since the 9mm cartridge has such a small case (compared to the bullet) and runs at such high pressures (up to 35,000 psi), small changes in bullet seating depth can have great affect on pressure. that is why we urge everyone to start at the bottom end of the reloading data when assembling loads for the first time.

    second, the bullet is heavy and takes a bit to start moving. the first thing that moves when the powder starts burning is the case wall. it expands out to the chamber wall, so the bullet really doesn't have to "exit" the case.

    third, the powder will have completely burned before the bullet gets past the end of the case (hard to believe, but powder burns really fast and the bullet moves really, really slow).

    fourth, i believe you are seeing an increase in velocity (and therefore pressure) in your longer loads because the bullet is closer to the lead (where the chamber diameter necks down to the groove diameter) and has less distance to gain momentum before hitting said lead.

    fifth, the bullet is going fast enough, and the chamber volume has increased enough to have no affect on chamber pressure when it hits the grooves in the barrel.

    anyway, if you stick to your reloading manuals, i don't think you have anything to worry about.

    Last edited: May 16, 2014
  10. bainter1212

    bainter1212 Well-Known Member

    My reloading experience has been enhanced by the teachings of Mr. McClain. Mr McClain was my sophomore AP chemistry teacher.
    It was one of the few classes I enjoyed and paid attention to. As a result, I have always remembered the relationship between temperature, volume and pressure. I can relate this to everything from the weather all the way to burn rates and case volume to understanding internal combustion engines.

    Thanks Mr McClain. I may not have gone to college, but I remember my periodic table and the equations you taught to us.

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