An easy way to do it is to inherit all the values and defines you need from the Xwhatever its called
e.g
Something like:
(Note these aren't the actual class names, i can't remember em, but look thru the config.cpp and you should be able to work the correct ones out)
class Default{};
class Guns : Default {};
class Xwhatever : Guns {};
class MyNewGun : Xwhatever
{
Then only change the stuff you want down here
If you don't specify new .paa's, it should just use the original ones automatically
};
~~~~~~~~~~~~~~~~
Note the class Guns : Default {}
This means inherit values for Guns from Default
So if you don't specify new values in Guns, then OFP uses the values from higher up, in this case from Default
Sorry i couldn't be of more help with the actual class names, if your still stuck i'll have a look round for them
Hope this helps!!
Marksman