Speech + Charisma = Good at Persuasion
Speech + Strength/Endurance/a particularly big gun in your hands = good at intimidation.
The speech skill should be reflecting the character's eloquence and ability to discuss the topic clearly and convincingly. Realistically, the gun nut shouldn't need the speech skill to discuss anything about guns, unless of course it came to a debate where someone need convincing or if he's trying to help someone understand something particularly complex, in which case a decent speech skill would be a requirement.
I think a good real life example of this is a teacher i had at school. The guy had a PhD in maths, so he was undoubtedly pretty knowledgeable but he could not for the life of him explain how to actually do anything. So, in this example obviously one skill is through the roof, but rendered useless by the inability to actually speak properly!