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Background
I was discussing the chances of an assault cannon penetrating armor values with my friend. He has a masters in statistics, and informed me that 4 shots with a BS of 4 does not give 8/3 hits, but instead gives 4 chances of 2/3 hitting. When factoring in the other variables of the AC penetrating due to rending, the numbers quickly become very complex.
Rising to the challenge, he spend a few hours computing out the exact chances, as shown below.



Results
The TL LC chances are linear. This is expected.
The TL AC has better chances in all instances except AV12, which is quite odd. When facing an AV12 wall, the LC is more dangerous than the AC. In all other instances the AC wins out.

On AV 12, the LC effects 2/3 of the time.
On AV 13, the LC effects 1/2 of the time.
That is where the 4 shots + rending of the AC pull ahead.