This is my first time submitting in tabs, so I'm sorry if you don't like it...
Should be played the way Aaron does it, in Drop D tuning.
There are only 4 chords, G, Cmadd9, D and Em but the last two stings you keep on the 3rd frets. 
The chords should look like this.

        E A D G B E
     G: 3 2 0 0 3 3
Cmadd9: 0 3 2 0 3 3 
     D: 0 0 0 2 3 2
    Em: 0 2 2 0 3 3

G                        Cmadd9
It sets above the mantel on a couple rusty nails
G                
It's not worth a lot of money
Cmadd9
But it damn sure ain't for sale
D                                Cmadd9
The good lord only knows all the stories it could tell
                  G    ( I add a Cmadd9 but its optional)
My Granddaddy's gun

G                                     Cmadd9
He bought it new out of the sears and robuck catalog
G                                  Cmadd9
And it shot a many shells over the back of an old bird dog
D                                 Cmadd9
And it backed a burglar down when grandma took the safety off
               G
Granddaddy's gun.

[Chorus:]
Cmadd9                  G
It's just an old double barrel twelve
D                           Em
The stock is cracked and it kick's like hell
Cmadd9                   G               D
It wouldn't mean what it mean's to me to no one
Cmadd9                                  G
Cause I can still hear his voice when I put it to my shoulder
D                             Em              
A gun's like a woman son it's all how you hold her
Cmadd9                   G                D     
He taught me a whole lot more than how to hunt
Cmadd9                     G                D                                
And one of these days I'll pass it on to my grandson
                  G         (I add a Cmadd9 here) optional
My Granddaddy's Gun

G                                 Cmadd9
That old man gave it to me on the day I turned thirteen
G                                  Cmadd9
With a half shot box of shells and a kit to keep it clean
D                                    Cmadd9                   
I keep a picture in the case of that sweet old man and me
                   G
And Granddaddy's gun

[Chorus:]
Cmadd9                  G
It's just an old double barrel twelve
D                           Em
The stock is cracked and it kick's like hell
Cmadd9                   G               D
It wouldn't mean what it mean's to me to no one
Cmadd9                                  G
Cause I can still hear his voice when I put it to my shoulder
D                                 Em              
A gun's like a woman son it's all how you hold her
Cmadd9                   G                D     
He taught me a whole lot more than how to hunt
Cmadd9                     G                D                 
And one of these days I'll pass it on to my grandson
                  G      (I like to add a Cmadd9 and another G as the bridge)
My Granddaddy's Gun

Cmadd9                               G         
There's a long beard hanging on the livin' room wall
D                              Cmadd9       G   D
That I got with a box call and Granddaddy's gun
Cmadd9                              G                              
There's a Shot up sign out there on forty nine
D                          Em                Cmadd9       G   D
That me and billy joe shot up one night With granddaddy's gun

G                        Cmadd9   
It sets above the mantel on a couple rusty nails
G                                    Cmadd9                    
It ain't worth a lot of money but it damn sure ain't for sale.

(I add a D then G and let it ring out)