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Need a little help, please -- Calibrating ballistic gelatin

  1. Oct 26, 2015 #1
    Pardon my ignorance, guys, but I have what should be a relatively simple question for physics buffs. I haven't studied any real physics since HS.......
    Here is the situation:
    I am performing some .45acp ammunition ballistic tests using a gelatin block. In order for my tests to be valid, I have to calibrate the block, which requires a standard 0.177" bb to be shot into the gel at 590 feet per second (+/- 15 fps) and penetrate 8.5 cm (+/- 1 cm). Easy enough. My issue is that my bb gun only achieves 350 fps. That is roughly 60% the desired velocity. My question is this: Is the desired penetration depth directly proportional? In other words, should I be looking for a penetration depth of 5.04 cm, or does deceleration act on the bb in a non-linear manner causing it to penetrate less than 60%?
    I appreciate any time you take to answer this and thank you in advance. Have a good one, guys.
     
    Last edited: Oct 26, 2015
  2. jcsd
  3. Oct 26, 2015 #2
    You mean a standard 0.177" diameter bb.

    There is a linear correction available for bb speeds close but not quite within +/- 15 fps of the target velocity, but it will not be proportional, especially for a velocity over 100 fps out of spec.

    We use a Crosman 2100 Classic pump bb gun (available from WalMart for under $70) with Avanti precision ground bbs. You can vary the number of pumps to hit the target velocity relatively accurately. The number of pumps depends on the ambient air pressure and is 7-8 near sea level and 10-11 at mountain elevations like Colorado (7000 ft).

    A few other tips:

    Follow the gelatin recipe very carefully. You need to weigh the gelatin and the water precisely or the block is unlikely to pass.

    The block needs 2-4 days in the fridge before it is likely to pass. 4 days is better.

    If you don't calibrate the block properly, your results are not really scientific and should not be compared with other results.

    If you are LEO or military in the US (or a contractor for an agency), contact me via a Private Conversation, and we'll walk you through everything very carefully (without any charge) to make sure you get it all right.
     
    Last edited: Oct 26, 2015
  4. Oct 26, 2015 #3
    Thank you. I appreciate the quick feedback. I didn't want to use the bb gun I have if I couldn't use it to accurately calibrate the gel. If I don't have gel that is to spec, I don't want to upload the results to my channel. I will wait until I can get it where I want it to be. The test is more of an experiment for fun, but I do want it to have some relevance for the sake of comparison to other tests. I have a tiny Youtube channel and thought it would be fun to do a test on a relatively new product so people can see some info on it early on. I will try and find the proper projectile spitter for the task at hand. Again, I appreciate the help and the quick reply. Have a good one.
     
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