When you roll a 20 with this magic weapon, the target takes an extra 7 Piercing damage.
Note: According to the SRD, it is an extra 2d6 Piercing damage, although this is incorrect.
A weapon that can be used to make a ranged attack has a range shown in parentheses after the ammunition or thrown property. The range lists two numbers. The first is the weapon's normal range in feet, and the second indicates the weapon's maximum range. When attacking a target beyond normal range, you have disadvantage on the attack roll. You can't attack a target beyond the weapon's long range.
You can use a weapon that has the ammunition property to make a ranged attack only if you have ammunition to fire from the weapon. Each time you attack with the weapon, you expend one piece of ammunition. Drawing the ammunition from a quiver, case, or other container is part of the attack. Loading a one-handed weapon requires a free hand. The ammunition of a firearm is destroyed upon use.
If you use a weapon that has the ammunition property to make a melee attack, you treat the weapon as an improvised weapon. A sling must be loaded to deal any damage when used in this way.
A limited number of shots can be made with a weapon that has the reload property. A character must then reload it using an action or a bonus action (the character's choice).