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Determining bullet seat depth

Discussion in 'Reloading' started by racer11, Aug 15, 2012.

  1. racer11

    racer11

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    Any loads I have done so far, .40, 44mag, 357/38 I have seated the bullet to the depth shown in the Lyman Hand Book.

    But not all bullets are the same length and the plated bullets do not have a canneuler (sp) to crimp on.

    How do I know that I'm seating at the correct depth and if not how do I determine what to seat a bullet at.

    Here lately I have been hooked on .40 reloads and I wonder if I'm using the correct depth,,,I know 40 is a critical load as well, and depth can create a unwanted pressure level. The 40 in the Lyman book is shown to be 1.135. I try for that measurement but they vary a few thousands + or-.

    School me..........
     
  2. fredj338

    fredj338

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    You start by matching the bullets & OAL as closely as possible to the book data. A diff of 0.005" is meaningless. When in doubt, load to the longest OAL yor gun will run.
     

  3. racer11

    racer11

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    Thanks Fred,,,,That gives me the guidelines I need
     
  4. shotgunred

    shotgunred local trouble maker

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    In pistols quite often the determining factor is going to be how long of a cartridge the magazines will hold.

    If you look at boxerglocker's 9mm loads you would see to different OALs listed. The reason his STI can take and feeds better with a OAL longer than his Glock will shoot.

    Sometime people pick a diffrent OAL for other reasons. I reloaded 40SW for years before I started 9mm. I chose to stay with my 40 SW of1.125 OAL for 9mm as well. Just because that is what I use in the 40SW and all my guns feed that OAL well.
     
  5. racer11

    racer11

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    With my .40 OAL I have 1.135, + or -. Now if I for some reason find that 1.125 runs better in my gun, don't I need to re adjust a lighter load (using the same powder) because of the smaller space in the cartridge causing a little higher pressure.
     
  6. freakshow10mm

    freakshow10mm 10mm Advocate

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    Yes that is correct. Generally I advise to drop down to the start charge and work back up, but unless you're near max charge loading to 1.135" OAL with fast powders, dropping down one full grain from your current charge and working up 0.2gr increments should suffice for being safe.
     
  7. fredj338

    fredj338

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    Depends on the load used. There isn't a lot going on pressure wise by seating 0.010" deeper. Things start getting interesting at 0.05" deeper. If your load is well midrange or lower, it isn't going to change much seating 0.010" deeper.
     
    Last edited: Aug 19, 2012
  8. racer11

    racer11

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    Ok,,,I understand....

    To increase the depth would start to increase pressure.

    But it seems like increasing pressure would always increase FPS every time.

    Why is it not always like that,,,,If I have this right,,,,increasing FPS is not always a cut and dried effect by increasing pressure.
     
    Last edited: Aug 17, 2012
  9. PCJim

    PCJim Senior Member

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    Racer, you are correct in that increasing the depth (reducing available case volume) will increase pressure. Increased pressure, up to a point, results in increased FPS. That said, the relationship between pressure and FPS is not linear.

    One of the signs that a reloader will use in determining a maximum charge is a change in the relationship of powder charge to FPS as measured over a chronograph. As an example, let's say that you have your favorite caliber and are developing loads in 0.2gr increments. You start with your low charge, recording average FPS readings, as you work up thru the higher charge increments. You notice a trend that for each .2gr increase in powder, your gain roughly 30FPS (these are examples only!). Then, you notice that the last .2gr increased charge that you fired only resulted in a 20FPS gain.

    That is where you stop, as you are now approaching a point where your pressure increases do not give you the expected FPS gain. You are hitting a velocity wall, and further increases in powder charge will only risk putting you, those around you and your firearm at risk.
     
  10. racer11

    racer11

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    That is a nice thing to be aware of because,,,,I was advised/suggested to switch from my BD powder to WSF. I took that advice and started working up a load with WSF powder. My FPS goal is under 1K and at least 950 or somewhere in between. I have ran a few sessions over my crono starting with a low load and working up .2g with each group of shots, I noticed a similar increase of FPS with each .2g's extra in my load group. I'm just a little shy of my intended FPS and I have not hit the "wall" as you described, it is something I will always look for in working up a load.

    Thanks for that bit of information about that,,,,you might of saved me some injury and a gun. :wavey:
     
  11. Zombie Steve

    Zombie Steve Decap Pin Killa

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    Where's Jack? He'll tell you all about measuring stuff.

    :tbo:
     
  12. shotgunred

    shotgunred local trouble maker

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    Who needs to measure anything. Brass is made the size it is so you know how much powder you need. Just fill it up and you are good to go.:wow:

    See no Jack needed.:supergrin:
     
  13. Zombie Steve

    Zombie Steve Decap Pin Killa

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    I thought you just pick a powder and keep adding until you get the speed that factory loads get. :whistling::supergrin:















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