I'm not sure about the exact odds, but I just realized there seems to be some sort of pattern with these findings.
I've noticed that most of the El Patron findings appear to occur when a crew is present. Now, this may be either because of the fact more pirates = faster kills, or it puts the types of drops under rapid fire. This means, perhaps with one lone pirate, theres a 5% chance of getting a rare weapon, as they make up 5% of the weapons in the Caribbean (again, just throwing random numbers out there.) Now, if there are, say, 5 pirates, Darkhart spits out the probabilities of weapons faster. This means, as he losses probabilities of some weapons, it theoretically makes the likelihood of getting rarer weapons higher, as he loses the probability of putting other weapons in drops. (NOTE: I'm NOT saying I'm backing this theory even 75%, because it sounds far fetched even when I read it. Its just a possibility. Hey, we can all make random guesses once in a while
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All I can say is, the probability is higher with a group, because of faster kills (and POSSIBLY my little idea.)