In the 5e session I am playing a level 7 ranger was given a shortbow. The bow works as follows:
Any arrow shot from the bow turns into a force arrow, adding 1d4 force damage to the attack. This arrow knocks any target struck by it back 10 feet. The arrow has a maximum range of 30 and dissipates if fired beyond that range.
The arrow still does its normal 1d6 + stats damage in addition to the force damage. So it's 1d6 + stats piercing + 1d4 force.
So here's my question(s)… is it a magical bow? Does the 1d6 + stats overcome damage reduction? Wouldn't the fact that it's "enchanted" to turn the arrow into force automatically make it a magical item since it has a magic effect?
In my mind it should overcome because it's clearly a magical weapon and per the DMG any arrow shot from a magical bow overcomes magical resistance. My DM told me it does not, only the force arrow (the 1d4 portion of the roll) does.
Unless I am mistaken this is a completely homebrew item so he may have his reasons behind it. Obviously I will go with what I am told because I do not know this reasons but I am curious what other's opinions are because this is odd to me.
A weapon is magical or it isn't
A weapon does not do "magical" damage - it does piercing, or bludgeoning, or slashing, or force, or cold, or ..., or a combination of these.
If the weapon is magical than all of the damage it deals comes from a magical weapon, if it isn't then all of the damage it deals comes from a nonmagical weapon.
If a weapon has some form of magic (like dealing additional force damage) then it is a magical weapon - even if it has no bonus to hit or damage. Even a weapon that can, say, detect secret doors 1/day and nothing else is a magical weapon.
Most creatures have this:
The weapon should do full damage against this because the weapon is magical.
Some creatures (e.g. raging Barbarians) have this:
The weapon should do half damage (except the 1d4 force damage - unless you are fighting a Helmed Horror - the only MM creature immune to force) against this because it doesn't matter that the weapon is magical.