Going off message how? Honest question I've never seen him speak. I find it hard to picture going off base more than that tool LaPierre did.
I tried to do some googling but the Piers Morgan thing dominates the hits.
In the past, in addition to the Veteran's bill misrepresentation, he called for pro-gun Dems who voted for Obamacare to be opposed because that was "socialism." Now you can agree or disagree with that, but Obamacare had no direct tie to gun rights (he stated it would include mandatory doctor notification as was being fought in Florida, as I recall).
Basically he has used GOA to push "Republican" issues in addition to gun rights. The reason NRA works is it stays on a bi-partisan narrow focus message and largely avoids more general political fights. Pratt has not chosen that path, which reduces GOA's potential political appeal.
When that is combined with his "no compromises" stance, which makes the perfect the enemy of the good and ignores how all the actual advances in gun rights have been accomplished over the past decades (incrementally). He provides in some cases a stalking horse the anti's can and do use to misrepresent all gun owners as their stereotype of white, male Republicans.