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Cutting the stock is fine. Cutting the barrel is fine. It's a combination of the barrel length and overall length of the shotgun that determines the legality - the barrel can't be less than 18" and the overall length can't be less than 26" in the US. I have a friend that is an NRA-certified firearm instructor and he actually recommends a 20 gauge shotgun shortened to the legal limit of 26" as the ultimate home defense weapon.
In Montana, home defense is the law. If you feel threatened by an intruder in your home, you are within your right to shoot.