[Captain Ahab Jr]

Ah ha, a thread I can contribute to!

Can you post up a picture of the stone wall?

I just use a 5: mix of local sand and cement, using a small builders trowel to spread/squeeze it into to the joints making the mortar flush with the stone.

The secret is leave it for a few hours until it dries out a bit, not too much but slightly crumbly so you can wire brush off the excess to clean back the the stone without it being so wet it just smears over the stone surface

Wire brush diagonally across the joints so you don't remove too much, then followed up with a stiff brush and then finally a softer brush so the mortar is under flush to the the stone.

If you want a more defined mortar edge I the head and point of nail to scrape it back, spending on how wide each joint is.

If you can wait until next week I'll be doing exactly this to the front of my house and would be happy to post up some destruction's with pictures of each stage.

No builder I know uses the mortar guns as sand/cement is so cheap it's quicker/cheaper to trowel it on and brush off the excess.

The cartridges on the the guns will take longer to fill with mortar than it would to fill the joints between the stone so they won't save you any time.