Mont. Code § 72-16-610
Neither the personal representative nor other person required to pay the tax is under any duty to institute any action to recover from any person interested in the estate the amount of the tax apportioned to the person until the expiration of the 3 months next following final determination of the tax. A personal representative or other person required to pay the tax who institutes the action within a reasonable time after the 3-month period is not subject to any liability or surcharge because any portion of the tax apportioned to any person interested in the estate was collectible at a time following the death of the decedent but thereafter became uncollectible. If the personal representative or other person required to pay the tax cannot collect from any person interested in the estate the amount of the tax apportioned to the person, the amount not recoverable shall be equitably apportioned among the other persons interested in the estate who are subject to apportionment.
§ 72-16-610, MCA