# Ballistics Technician needs help

• Feb 18th 2011, 12:43 PM
Ballistics
Ballistics Technician needs help
As a ballistics technician in training I have recently taken a microscopy test which I passed. I would like to explain the nature of the test to a court in order to prove the credibility of my science. If asked in court I would like to use the statistical probability of guessing the answer to this test. I was initially given 20 bullets which had been fired in 10 consecutively manufactured firearms, 2 bullets fired through each. The bullets are marked in pairs i.e. 1-1, 2-2, 3-3, up to 10-10, so it is known which 2 bullets have been fired in each of the 10 firearms. This is so I can identify reproducible marks on each pair. I am then given a further 15 bullets marked alphabetically A through to O.

The test is to identify which firearm/firearms fired the 15 bullets. This can be any permutation; The 15 bullets have been selected completely at random and could have come from any of the 10 firearms. I may not have been given bullets fired from all the 10 firearms i.e. 6 from gun 3, 1 from gun 5, 1 from gun 6, 2 from gun 8, 5 from gun 10. I suspect the possibility of guessing the correct answer would be very large. Can anyone assist please.
• Feb 18th 2011, 02:09 PM
tom@ballooncalculus
Just in case a picture helps...

http://www.ballooncalculus.org/draw/prob/snake.png

The selection of bullets will be a 'snake' like this one, and the question is how many possible snakes are there to choose from.

For each of the 10 possible choices of firearm for bullet A, there are 10 possible choices for bullet B - so far that's 100 choices. And so on.

Does that help?

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