Successfully complete story mode to unlock Magruder's seven barrel Nock gun.
Successfully complete story mode and all side missions (100% completion) to unlock Reed's armored horse and Magruder's Cannon Nock gun. This horse cannot be killed and if ever lost, it will return to the mine.
.69 Ferguson Rifle
Successfully complete the Pony Express missions.
Go to Dodge City. Go to the lower part of the bridge. Follow the path on the other side to reach a beach cave. Look inside for a life power-up which will reappear over time after you leave the area. Note: The cave also contains ammunition and money.
Go to the Indian Hunter's hut, which you can find on your map. Once there, go to the building to the left of the hut, with the stairs going up the side. Go up the first set of stairs, and continue going straight. You should see a spout-type object coming out of the side of the building. Jump onto that spout, and walk toward the wall of the building. If done correctly, you should go through the wall, and into the inside of the house. There is only a small ledge for you to drop onto inside, so move slowly. Once inside, look to your left and you should see three whiskey bottles on the ground. You can pick these up as many times as desired and they will never go away.
For unlimited health, go to Hecht Ranch. Go to the shack next to the windmill. There you will find three whiskey bottles to restore your health.
Sneak up on him and shoot him once. Then, run off and get behind a rock on top of risen ground in the middle of the arena. When he is circling the rock your are behind, you should also circle as well. He will eventually stop and yell at you to come out. He should pace back and forth slowly while you hide behind your rock. When he stops and is not facing you, shoot him in the head. Perform this until the point that he becomes too smart to fall for it again. By then his health should be low enough so that you can run out and keep shooting while strafing. After you have gotten his health down all the way there will be a intermission sequence. Look at your compass at the bottom of the screen. He is the red dot. Find him and he will run at you. Run, but keep firing at him with your pistols and he will blow up.
Never let Reed out of your sight. If he is by himself and you cannot see him, he gains life.
Easy poker game wins
When you are playing Texas Hold 'em inside the Alhambra, you can easily win with or without a good hand. Raise the bid to the maximum. This should make at least one to three players fold. You can then cheat to make your hand better, and if it is a worse card, forget about the cheat card and go back to your original hand. Afterwards, keep checking your hands until one of you wins. It should be you most of the time. Do not worry if you lose; the tournaments are free.
You can make more money on the bounty hunter side missions by capturing bounties alive. When you find a wanted poster that offers more money if you capture them alive and are having trouble doing so, use the following trick. Kill all gang members or bodyguards (if any). If the bounty is on a horse, kill the horse. Get close and melee attack. This will stun the fugitive and present an opportunity to grab them using LB. Then, press X to subdue the fugitive. This awards you more money for each bounty. If you press LB accidentally and release him, you can try to melee him and grab him again. However, if you do this too many times or if he has been shot, you may kill him and get less money.
Hold Dash while repeatedly jumping your horse to travel about double the speed.
This is a list of all the achievements in Gun, and how to get them:
Across The Badlands Complete (25 points) - Progress through the 'Across The Badlands' level on any difficulty.
Ambush the Train Complete - Progress through the Ambush the Train level on any difficulty.
Attack the Fort Complete (20 points) - Progress through the 'Attack the Fort' level on any difficulty.
Battle at the Steamboat Complete - Progress through the Attack the Fort level on any difficulty.
Bounty Hunter (40 points) - Find and complete all of the Bounty Hunter side missions.
Card Shark (40 points) - Get 100% completion in Poker Tournaments
Defend the Hideout Complete (20 points) - Progress through the 'Defend the Hideout' level on any difficulty.
Easy Difficulty Complete (10 points) - Complete the game on the Easy difficulty.
Escape From Jail Complete (15 points) - Progress through the 'Escape From Jail' level on any difficulty.
Escape the Ambush Complete - Progress through the Escape the Ambush level on any difficulty.
Every upgrade collected - Collect every upgrade in the game.
Every weapon collected - Collect every weapon in the game.
Hard Difficulty Complete - Complete the game on the Hard difficulty.
Hollister's Fort Complete - Progress through the Hollister's Fort level on any difficulty.
Honest Tom Complete - Progress through the Honest Tom level on any difficulty.
Insane Difficulty Complete - Complete the game on the Insane difficulty.
Keeper of the Peace - Find and complete the Keep the Peace side mission.
Law and Order Complete - Progress through the Law and Order level on any difficulty.
Magruder's Mine Complete (70 points) - Progress through the 'Magruder's Mine' level on any difficulty.
Normal Difficulty Complete - Complete the game on the Normal difficulty.
Professional Postman - Find and complete all of the Pony Express side missions.
Professional Prospector - Mine every gold ingot in the West.
Quick Killer Complete - Progress through the Quick Killer level on any difficulty.
Save Soapy Complete (20 points) - Progress through the 'Save Soapy' level on any difficulty.
Stage Coach Complete (20 points) - Progress through the 'Stage Coach' level on any difficulty.
Steamboat Massacre Complete (15 points) - Progress through the 'Steamboat Massacre' level on any difficulty.
Take Down Hoodoo Complete - Progress through the Take Down Hoodoo level on any difficulty.
The Greatest Hunter (40 points) - Find and complete all of the Hunting side missions.
The Hunt Complete - Progress through The Hunt level on any difficulty.
The Red Hand Gang Complete (20 points) - Progress through the 'The Red Hand Gang' level on any difficulty.
The Unrivaled Horseman (40 points) - Find and complete all of the Horse Challenges.
New Game +
Complete the game and return to the starting area (Blackfoot Territory). From the LEFT of the HIGHER boulder leading up to a ledge and gap, jump and spur your horse so it lands on top, then onto the ledge. Cross the next two areas. Stand between the only two rocks in the water. Press and hold LB+LT+RB+RT+A for a few seconds until you hear a strange sound. Go to Options (press [Start]),for a new selection - GUN. Choose any Mission or Cutscene. You will retain ALL weapons, items, and skills.