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9mm diff bullet types = diff velocity

Discussion in 'Reloading' started by njl, Feb 27, 2010.

  1. njl

    njl Crusty Member

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    I've been reloading 9mm for about a year now, and have only used Precision Delta FMJ and Montana Gold CMJ, both 124gr. Loading those two brands of bullets with everything else being equal, I get about the same velocity, right around 1060fps.

    I just got a case of Montana Gold 124gr JHP, and chrono'd them today. Using the same brass, powder charge, primers, and keeping case volume about the same, I got higher velocity from the JHPs...just over 1080fps from my 17, and just over 1110fps from my 34. I knew this was a possibility, which is why I loaded 50 and held off doing more until chronographing and function testing them.

    I'm curious if others have had similar experiences when moving between FMJ and JHP and if anyone has any theories as to the reason for the velocity difference.
     
  2. HAMMERHEAD

    HAMMERHEAD

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    How did you keep the case volume the same? Seat by bullet base instead of COL?
    If it was the same, there must be some barrel resistance issue like a slight diameter difference or a bearing length difference.
     

  3. Bren

    Bren NRA Life Member

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    My first guess would be the COAL caused the difference. Maybe the JHP was a little longer bullet (makes since, since it still weighs 124 gr. with a bit missing from the center). I once made a run of match ammo that came out just a hair under the minimum speed I needed for the match - I just went back to the seating die and seated each bullet a hair deeper and got the velocity I needed. Same powder, same primer, same brass, same bullet, but seated deeper, will give you more pressure and more velocity.
     
    Last edited: Feb 27, 2010
  4. njl

    njl Crusty Member

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    The MG JHP bullets are about 0.02" shorter than the CMJ. I loaded the CMJ to 1.145-1.150. I loaded the JHP to 1.122. I figured that would keep the case volume about the same. According to my calipers, the CMJ are 0.355 and the JHP are 0.354 or 0.3545. Perhaps that or the amount of bearing surface could be it.

    I'm going to load some more and load them a little longer...maybe 1.130
     
  5. fredj338

    fredj338

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    It's your OAL. That isn't really much variation. Chronographing on diff days w/ diff temps will give you 20fps+ diff. You are probably fine right where you are at. The bearing surface length is also a factor. Next to case volumn & bullet wt. it is the other consideration for pressures & velocity diff.
     
  6. njl

    njl Crusty Member

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    The MG JHP bullets (just the projectiles) are 0.02" shorter than the CMJ bullets. Therefore, I figure loading the JHP to a COAL of about 0.02" shorter than I load the CMJ yields equal case volume.
     
  7. BHP9

    BHP9

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    It's more than like the difference in the bearing surfaces of the bullets.
     
  8. fredj338

    fredj338

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    NOpe! The JHP is shorter because of it's shape. By loading the OAL shorter, you are increasing pressures a bit. The mount of the bullet inside the case & the bearing portion, are the determining factor. Most FMJ have shorter bearing surface & longer noses. Most JHP have shorter noses & longer bearing surfaces. You might want to stretch that OAL for the JHP a bit or reduce your powder charge 0.1gr.
     
  9. njl

    njl Crusty Member

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    How do you equate loading the OAL 0.02 shorter to increasing the pressure? I'm already loading at the bottom of the range for recommended powder charge. I guess one thing I could do is load some extra long and see at what length they'll chamber, but I worry about not having much bullet for the case to hold onto.
     
  10. fredj338

    fredj338

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    Anytime you put more bullet into the case, you are going to increase pressures. How much depends on the powder & caliber. I still think 20fps+ is nothing to worry about.
     
  11. njl

    njl Crusty Member

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    But I'm not putting more bullet into the case. If the bullet is 0.02" shorter, then seating to a COAL 0.02" shorter puts just as much bullet in the case. I'm not worried that the JHP shot faster. A little surprised is all.