Arm Roast after 5 hours still tough
If it's too tough, keep cooking it. Some people use "tender" to describe a pot roast that is tender like a good steak, others want it to fall apart with no knife required. I regularly cook pot roast 8-12 hours. As log as you have it covered for most of the time (like in a crock pot or in the over covered with foil), it will  keep getting more tender until it falls apart under its own weight (which sounds like it may be what you want).