Red Dead Redemption (UP)

there are five guns in the game that count as rare they are

Mauser Pistol-available at the Blackwater gunsmith
Lemat Revolver-available at the escalera gunsmith
Carcano Rifle-available at the Blackwater gunsmith
Evans Repeater-available at the Blackwater gunsmith
Semi-Auto Shotgun-available at the escalera gunsmith

As stated by 'Heaven In Vain' there are 5 Rare weapons, each one will be required for the 100% achievement, but only one for 'Exquisite Taste'

These guns can only be found in specific towns however, the two towns are Escalera and BlackWater
The Guns are;
- Lemat Revolver - Escalera Gunsmith.
- Semi-Auto shotgun - Escalera Gunsmith.
- Mauser Pistol - Blackwater Gunsmith.
- Evans Repeater - Blackwater Gunsmith.
- Carcano Rifle -Blackwater Gunsmith.

The blackwater gunsmith is only avaliable in Act III, where as the Escalera Gunsmith is avaliable in Act II. Obviously, if you are wanting the achievement as soon as possible, it is better to buy either the Lemat Revolver or the Semi-Auto shotgun and the Escalera Gunsmith.
The guns that are required cost alot of money, so its worth investing into a gun which will either help in the campaign, or be able to earn you some money in the further stages of the game. I heard that the 'Bearly Legal' achievement was made easier with the Semi-Auto shotgun due to the fact that killing bears with this gun is much easier than using a repeater, or revolver.
I chose the Semi-Auto shot gun to buy first for this reason and the fact that killing animals further in the game will be easier, allowing me to skin them and gain items which I will then be able to spend on other things, such as the other rare weapons.

Another thing to note is that the Guns prices will differ acccording to your honour, A high honour will decrease the prices of the guns, as the staff respect you I suppose. =P. Remember to buy the bandana form the tailors in Thieve's Landing to prevent chang in your honour.

Hope this Helped =D

*The Semi-Auto shotgun was avaliable near the end of act II for me at $500, But has been seen in Act III With full honour for $300.

A rare weapon is any one of these weapons:
-Carcano Rifle
-Evans Repeater
-LeMat Revolver
-Mauser Pistol
-Semi auto shotgun
The only two gunsmiths who sell these weapons are located in either Escalera, or Blackwater. Therefore, the earilest you can get this achievement is when you get to Escalera.

Cheers!