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How to adjust a JHP recipe for FMJ's?

Discussion in 'Reloading' started by Kwesi, Aug 16, 2010.

  1. Kwesi

    Kwesi

    1,459
    7
    Sep 23, 2006
    TX
    I'm loading 9 & 10mm + 45 acp. Is there a safe standard, when the published recipe is for JHP's when I have FMJ's with same grain projectiles?

    What if the weights are different :
    124 JHP vs 115 FMJ
    165 JHP vs 180 FMJ

    Thanks!
     
  2. Daryl in Az

    Daryl in Az Enjoying life!

    839
    0
    Aug 31, 2007
    Lost
    The "standard" is to back off 10% from max load for the listed bullet when you change any other component, and then work back up. There is no standard or formula to figure for a lighter weight bullet, or differences in seating depth from a different type of bullet. There are just too many variables to deal with in reloading for that to happen.

    When you change bullets, and especially when you change bullet weights, you need to find new data.

    Sometimes, depeding on the cartridge, chamber, and powder being used, you can compensate for slight variations in bullet lengths by adjusting the seating depth. This is not always the case, however, and you need to know exactly what you're doing if you try it.

    It's easier and safer to find data for the correct, or at least a very similar bullet.

    Daryl
     


  3. FLSlim

    FLSlim

    1,340
    13
    Apr 12, 2010
    FL W Coast
    In general, I agree with D in AZ. If you change weights in you jacketed bullets, go back to the drawing board (i.e., load manuals) for start loads and work from there. When I go between jacketed HP and FMJ in the same weight I don't alter the powder charge unless it is near max. Even then I'm not sure it is a problem, but it would seem prudent to back down a few tenths and test a few rounds. A bigger issue might be OAL, as the same brand of JHP and FMP can seat at significantly different depths if your dies aren't readjusted. Keep one or more manuals handy; they are essential to safe handloading.
     
  4. fredj338

    fredj338

    21,710
    926
    Dec 22, 2004
    so.cal.
    When it comes to JHP vs FMJ, it's bearing length & OAL. Typically, the JHP will seat slightly deeper & have a slightly longer bearing surface. Seating to a longer OAL reduces pressures. Obviously reducing by 10% & working the load back up is prudent, particularly if you are already pushing max. If you are shooting midrange loads, there won't be much diff in pressures swapping any component out, within reason.
     
  5. Kwesi

    Kwesi

    1,459
    7
    Sep 23, 2006
    TX
    Thanks! I've run into this situration several times and was not able to find the FMJ recipe.
     
  6. W4CNG

    W4CNG

    816
    0
    Sep 19, 2002
    In the Gun Room
    I have several of the One Caliber Books that I have purchased in the last several years. They have lots of bullet weights and types including FMJ. SWC, JHP's. They also have many types of powder's for each bullet. A lot of the Hard backed books do not have this level of detail. It has helped me gin up favorite loads all of which go onto my master Excel Spreadsheet for tracking. Good Luck with your reloading.