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Let's use Glock for example... you could replace your OEM barrel with polygonal rifling with an aftermarket barrel which is button rifled. Now I've never been in a forensics lab, and I'm thinking about this from the perspective of a gunsmith/machinist, but the marks left on a bullet from traditional button rifling and polygonal rifling could be different.
The case could probably be examined for expansion in the case of an unsupported chamber versus a fully supported chamber which is seen in some aftermarket match barrels.
I'm not positive that there are different rifling profiles available in terms of number of lands, twist rate etc. but that's another possibility.
What was the movie? Since you only caught the tail end, perhaps the canonical answer can be found on the internet somewhere.
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