I have a triangular bag that is mounted at the junction of the seat tube and top tube, it has a pad on the bottom. It is intended to be used as a pad for your shoulder when you have to carry your Mnt bike thru terrain you cant ride thru.
Its just big enough for a M-60, and is accessed very easily, you dont have to muck around trying to get to a bag mounted under the saddle. It is also somewhat protected in a crash. I did have to add some closed cell foam so the pistol wont knock aganst the frame when the trail is rough.
I like the "Wilderness Safepacker" holster for hiking (and shopping, and driving, and sleeping...) but its not so great for running.