? about calculating FPS

Discussion in 'The Ammo & Reloading Forum' started by mongo, Nov 18, 2009.

  1. mongo

    mongo New Member

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    I am trying to determine velocity based on a specific load. I am shooting 155gr lead cast bullets using Hodgdon Clays. I cant find any data on the bullets and have read to use jacketed data minus 10% to start.

    I was trying to calculate the average FPS based on the difference in valocity betwen the minimum and maximum recomended loads on Hodgdons website for a 155gr Hornady XTP. With a minimum load of 7.7gr of HS-6 the velocity is 1035 and with a max of 8.5gr the FPS is 1137. That is a difference of 102fps. The difference between 7.7 and 8.5 is .8. If you devide 102 by 8 you get 12.75. So I was assuming that for every tenth of a grain, there is roughly a 12.75fps increase or decrease depending on which way you go. Can I assume that or is that just nuts??
  2. cycloneman

    cycloneman Well-Known Member

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    Right it's just nuts. You need a chrony. No way around it. Under 100 bucks can set you up. Your chrony will tell you what is happening in your gun with your load.
  3. cycloneman

    cycloneman Well-Known Member

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    A chrony is a great tool for what your looking for. I use it to see what is coming out of my long range rifles. You would be suprised. The only way to know for sure.
  4. mongo

    mongo New Member

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    Yea just a little short on funds and have a steel match this weekend, so I was hoping for other avenues. It is on my wish list. Thanks
  5. woolleyworm

    woolleyworm Active Member

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    I'd bet there will be a crony somewhere at the match, you may be able to get a few test shots. My suggestion would be to load up 3 different loads, test them prior to the match for accuracy and pressure. Then chrony the loads if possible. That way if you have one that doesn't make power factor, you have options.

    btw what caliber are you shooting? .40?
  6. mongo

    mongo New Member

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    I have to be under 150PF for the match and total rounds required if I dont miss is 150. Thats 450 rounds I would have to take with me minimum. Keeping it under the 150PF should be easy shooting a 155gr bullet. I am going to load a low level batch and test it befor the match. I have no access to a chrony so I will wing it.
  7. woolleyworm

    woolleyworm Active Member

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    450 rounds is nothin for an evening :D:D but I would agree, if you only have one good load out of the bunch, that leaves 300 left for mindless plinking at best.

    Other question would be this, do you know anyone else that is going to be shooting the match also? You may be able to get a recipe from them. May not work as well in your gun, but should be ok for power factor, barring big differences in barrel, etc. Still double check their data too.
  8. Bindernut

    Bindernut Well-Known Member

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    fps isn't directly linear to powder charge so as others have already posted it'll be hard to guesstimate your load's velocity. Your math should get you in the ballpark, but a chronograph will tell the tale.

    I would also guess that someone at the match will have a chrony handy. In my area, the officials would always had one in their kit. BUT...by the time you get to the match, if your pet load is over PF, then it's too late to run home and load up another batch. :(
  9. mongo

    mongo New Member

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    I just loaded 64 rounds at 7.1gr of HS-6. Minimum recomended on the Hodgdons website is 7.7 using the XTP type bullet vice the lead cast I use. So I subtracted 10% and came up with 6.9. I'll go out and do some testing and see how it goes. Thanks for your help.
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