This isn't a 100% sure list, but Rockstar posted them on their website, and took them down right after. Literally like 4 minutes after they posted them, odd... Anyway, here they are:

Call it a Comeback!

Come back from 2-0 deficit and win a Hold Your Own game.

Who needs Deadeye?

Kill 3 or more players in a standoff or showdown.

Stick and Move

Get 3 kills with knives or throwing knives in a single competitive match.

Double Bagger

Double capture 3 times in a single Gold Rush map.

Headhunter

Kill 5 players via headshot in a single Shootout or Gang Shootout.

Legendary

Reach level 50 and pass into Legend.

Hail Mary

Get a kill greater than 35 yards with a Tomahawk.

Axe Master

Complete all Tomahawk challenges in Single Player.

Original Gunslinger

Get 25 Deadeye kills with Red.

Reeeeal Good

Get 25 Dynamite kills with Pig Josh.

Some of those achievements look pretty tough and others look not so bad. Hopefully I will get the Legends and Killers pack cause the Tomahawk looks badass but I have my eyes set on getting madden which is released the same day.

Legendary

Reach level 50 and pass into Legend.

How's that one different than

How the West Was Won

Reach the top rank for multiplayer experience.

Both are level 50. Only difference is, this new one gives it to you if you decide to legend? What if you've maxed out in legends already like I have?

Both are level 50. Only difference is, this new one gives it to you if you decide to legend? What if you've maxed out in legends already like I have?

A community manager for Rockstar said that the achievement will automatically unlock when you go into a MP session, if you are already in Legend. Once again, I don't have a source, because Rockstar keeps pulling them down. So, if I'm lying you can beat me with a baseball bat.

Edit* PS3 Trophies.org just posted these, so yea. Linky.