Newbie scriptor needing help with cannon

Hi, Iv been tyring to make a cannon (just a cylinder really) to fire straight out the hollow point but problem is its rotated, when facing east this is its rotation X : 0.00 , Y : 84.40 , Z : 180.00 and this is the script im using.

default
{
    link_message(integer send, integer num, string str, key id)
    {
        if(str == "fire")
        {
        vector b = llGetPos(); 
        rotation a = llGetRot(); 
        vector c = llRot2Fwd(a); 
        b += c; c *= 20; 
        llRezObject("Bullet", b, c, a, 0); 
        }
    }
}

With this script its just been firing straight up :/ so any help would be great.
Many Thanks in advance

Keno