18th Century kilt and sporran

[Dale Seago]

It's most likely hanging barrel down, clipped to a separate strap that doesn't show up in the painting.

I agree. Scottish pistols in that era typically had a belt clip for attachment to a strap or belt, rather than being carried holstered. There are some good example pics from a modern reproduction here:

http://www.middlesexvillagetrading.c...tmurdoch.shtml

A close-up:

[Dall_Piobaire]

Just curious, and remember it is really hard for me to see in these portraits, would/could it not be the way he held it to cock it...