The 340 will be a dream to pocket carry and it won't play peek-a-boo, the 640 is doable but if someone is behind you on an angle they'll see the butt. Make sure your pockets are deep enough.

My 642 I had was 6.31 the Ruger LCR was 6.5 and right at the edge. The 640 I believe is 6.6 the 340 the same as the 642.

Now, let's talk which one you will enjoy shooting. And if you don't like shooting it you will not practice with it, and a snubby is very hard to master.
The 340 is a work of art in my opinion but at 13.3 oz. you'll feel even the standard .38 loads and get ready if you try to go magnum.

Tough choice, I know the 640 will feel better to shoot, but there is just something about that 340 that always captivates me, I especially like how they have covered the ejector rob unlike the 642 that looks naked out in the open. And makes me think it's vulnerable.

Good luck with your decision.

p.s. don't be fooled by the 640 because it's heavier magnum loads out of that thing won't be fun either after awhile. But unlike the 340 you should be able to do more than 2 or 3 before you say enough of this.

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Mulebuk Sam and the Governor