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.