My best advice is contact Eberlestock. I recently looked at a ton of their packs and this is probably one of the few I didn't fondle. I ended up with an Operator. I believe all of their buttcovers are adjustable length wise. I didn't see any that weren't. Even the smaller scabbard that they sell seperately has an optional butt cover you can buy and it extends. I really think with the widemouth you should be fine. Reading on their site I think it's designed with what you have in mind. The scabbard is pretty wide on the Operator. It doesn't taper until the very bottom. The butt cover is going to be the same width as the scabbard. Hope this helps.