Sunday, June 3, 2012

Energy Dump & Penetration Data

I got a lesson is physics the other day and it's really opened my eyes to some things that are wrong with my testing.  You guys have seen my SIM-TEST blocks, so you know they are about a gallon and a half by volume and weigh in at about 14 to 16 lbs.  While testing some .45 ACP ammo, my penetration depths were coming in way less than those published by the manufacturer even though my SIM-TEST was calibrated to the density of 10% ballistics gel.  I did the calibration test by shooting the block with a BB going at about 600 fps.  The BB test is the generally accepted way to confirm the density/resistance of the block.  This concerned me quite a bit because if my results don't jive with those of others, then what am I really accomplishing other than to put more bad information out in the world through my blog.  I really needed to get to the bottom of this problem.

Two weeks ago I started doing two camera shoots.  If you've seen my Pow'RBall or Lehigh Defense water jug tests, then you have seen the two videos that had the two camera views.  Through my video editing software, I can now slow down the action and go frame by frame to see what's happening when the bullet hits the block.  I've actually used the video to "find" two bullets that skipped out of the block and into the berm.  This slow motion video analysis has been very helpful.

Let's take a look at a typical 380 shot with about 170 ft,/lbs. of energy.  At maximum, the block barely lifts off the table so the majority of bullet energy is being spent on penetration.  I feel pretty good about my 380 penetration numbers, but obviously they aren't going to be exact because the block does lift slightly off the table.

Let's see what happens when we step up to 9mm with about 275 ft./lbs. of energy.  That quite a bit more lift on the block and also a little squeeze action going on with that first water jug.  That's not good because the energy that should be driving that bullet forward is being wasted in lifting that 14 lb block up off the table.

This week was my first week to test .45 ACP that generated about 385 ft/lbs. of energy.  At one point, then entire 15 lb. block was up off the table.  I didn't pick that frame, but the one just after it that had the block lifted to it's highest point.  Again, look at all that wasted energy lifting the block and putting the big squeeze on the first water jug.

I did a quick google search on the FBI gel test protocol and found out that the proper gel block size is 6" x 6" x 16".  Quick math comes up to about 2.5 gallons and approximately 21 lbs.  That's quite a bit larger and heavier than the blocks I have been using.  In my mind, energy follows the path of least resistance and lifting my lighter weight blocks is easier than pushing the bullet forward into the resistance of the SIM-TEST media.  Thereby, invalidating my captured penetration data.  If I was shooting into blocks that were 12 x 12 x 16 I would probably see much less lift on the block and much greater penetration results. 

After having this epiphany, I have decided that I'm going to stop reporting penetration depths for tests that are done on calibers greater than .380 ACP.  I can still report side by side comparison penetration depths when I put two shots into the same block of SIM-TEST.  For example, if I shot 9mm 124 grain +P GDHP Short Barrel and 124 grain +P GDHP into the same block, I could measure the difference in penetration between the two shots. 

If you think my understanding of physics is whacked, please feel free to tell me so.  I'd also like to hear your thoughts if you agree or disagree with my logic and decision to stop reporting penetration.  I will still be capturing data about the expanded bullets and impact velocity/energy.

In reality, penetration data is always going to be sketchy because blocks of ballistics media are deemed "OK" over a pretty wide range of densities.  According to what I've read, the gel blocks are considered to be OK with BB penetrations ranging from 3.25" to 3.75".  SIM-TEST approximates ballistics gel density, but it was never intended to be a replacement for it.

I hope you enjoyed the pictures as much as I did.  It's pretty amazing to see the results of the energy dump in a slo-mo environment.

UPDATE:  I know this blog entry hasn't been posted yet so I wanted to add an update to this post.  Earlier today I was back at the range testing a bunch of different ammos.  Taking a tip from tnoutdoors9, I started taping the nose of the blocks down to essentially make them heavier.  I'm really astounded by the difference this small change made to my testing.  Take a look at the photo below that I captured from my video testing of Hornady Critical Duty 135 Grain +P ammo.  Block jump has been minimized!  The energy from the bullet is now driving the projectile forward and not being wasted in lifting the block off the test deck.  I'm really starting to feel better about being able to collect penetration depth data again. 

I did a short video on the bb calibration process.  I put it up on YouTube so you can see that it's not quite as easy as it seems to be.

3 comments:

  1. As I was reading through this post I was thinking, "He needs to check out how tnoutdoors9 tapes down the front of the block." It will help keep the denim in place, as well. Glad to see you figured it out.

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  2. Not a student of physics but it seems that if the path of the bullet was parallel with the platform the gel is resting on there would not be any impact jump caused by the bullets slightly upward trajectory. But tape is a good solution.

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  3. Block jump is nothing to worry about. The energy is transmitted in all directions, creating the temporary cavity we can see in the slow motion gel block videos. There is little resistance on the top or sides for the block to expand, but the resistance on the bottom of the block (the table) is great and causes the jump. It is not exactly "wasted" energy, as the bigger the jump, the bigger the temporary cavity and the greater the trauma to the surrounding tissue and likely greater injury and pain. I'll agree that this is secondary to the penetration numbers, but also significant.

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