False muzzles are prohibited because they are a feature of high-end, purpose made target rifles, usually bench guns. Chunk guns are of a traditional design, just with longer and heavier barrels. Target guns with false muzzles, adjustable butt stocks, thumb-holes, padded cheeks and butts, forearm grips, etc...., are a whole other level above chunk guns.
The key to chunk guns is "traditional" design. A chunk gun looks like any other longrifle, just a bit bigger. There are what the guys in the mountains could cobble together.
Sometimes a purpose made chunk gun will have a built in cant block where the front ramrod thimble would normally be. Otherwise, it looks like any other rifle.