There are three types of Big gunz in the Ork Dex.
For X points per gun you get a Kannon.
For X more you get a Lobbas.
The ammo runts are a seperate cost as listed on P.103 of the ork Dex.
Now to answer what I think is your question:
An Ammo runt will let you re-roll one to hit roll for a shooting attack.
As per P.30 of the BRB this allows you to re-roll the 2d6 and the scatter Dice for the Lobba.
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