Does adding a buttpad to a service gun change my class?

[BigJ]

Hey guys. Sorry for the newb question. Just trying to get my head wrapped around what I can and can't do to a service bolt gun to stay in the service (not Match) class(es).

I have a slip on pad for my otherwise as issued K31. The LOP of the K31 is just too short for me without it. Is that an acceptable modification, or does using it exclude me from the service/as issued classes in an NRA/CMP High Power competition?

Thanks in advance.