This could have any number of explainations. Such as: 1) There were two
people in Tybalt's car, it could have been the other guy's gun. Or 2) Tybalt
has at least two guns on him (if you recall the gas station scene).
When Romeo kills Tybalt he uses Tybalts gun. Because once he has shot Tybalt he looks at the gun in horror because used Tybalt's gun to kill Tybalt. and also the gas station scene you see a quick glimpse of his gun!
Romeo most likely uses Tybalt's other gun. The guns are the same, they both have the Virgin Mary on them. When Abra takes all the bullets but one out of Tybalt's gun, he uses the one that's on the hip holster. He takes Tybalt's other gun from his shoulder holster and puts it in his own waist band before putting the gun with one bullet in Tybalt's shoulder holster. Tybalt's gun probably flies out of Abra's waist band during the car crash and that's the one Romeo uses to kill Tybalt.
This is not a mistake! The reason they only had one bullet in each of their guns is because they were kind of telling the opponent that that one bullet was destined for them.
It is also Tybalt's other gun that Romeo uses! Tybalt drops it and Romeo picks it up to shoot Tybalt, so, yeah!