Darker guns (442) conceal better. Lighter guns (642) have easier to see sights in low light.

Might want to hold out for one of the new Ultra Carry Centennials that are about to hit the market as they have a really good front sight (more important that you might think at close range).

While I have pocket carried a 637 on occasion, I don't consider the exposed hammer models to be the ideal solution for that style of carry.

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Centennial Every Day