So, by default there is not way to straight up make homebrew ammunition. But I think we can figure it out.

So far making a wondrous item creates the arrow and giving it charges gives it a stacking counter.

However, I have yet to figure out how to apply a modifier that only gives bonuses to ranged weapon attacks only.
Doing Bonus > Magic > +1 applies the hit/dmg bonus to all attacks. So, that's a no go.

Doing Bonus > Ranged Weapon Attacks  > +1 gives a bonus to ranged +hit but not to damage.

Adding Modifier Damage > Shortbow and Damage > Longbow apply the +1 damage mod to said bows.

As is it's great for standard bows but does not work for crossbows or firearms.

Any ideas?

I made homebrew set of broad heads that are a modified Glyph. Instead of moving 10' from the point of origin, once they accelerate past 300 feet per second they activate.  When they stop they detonate. They are actually pretty powerful and I will probably regret it. I will have them in my treasure hoard on Wednesday.  As they will detonate regardless of where they stop I have made a chart to indicate where it goes. 

Glyph Broadheads

Broadhead is armed with acceleration and activates when velocity reaches zero. Meaning when it leaves your bow it is armed, and when it hits something it goes boom. All broadheads to normal damage plus benefits listed below.

1 Broadhead of fireball.  When you shoot this broadhead it detonates a low-level fireball 2d4 damage. If it is embedded in a body, it will be a compressed blast.  DC Con 20 or instant death. On a miss it will detonate in a 5-foot radius from the point of impact. Missing by more than 10 will be a random direction and distance.

1 Broadhead of Levitate.  On successful hit up to 250 pounds of target will raise 15 feet in the air and drop back down taking 2d6 damage. Any player engaged gets an attack of opportunity. If you miss and hit something that weighs less than 250 pounds it will levitate. 

1 Broadhead of Silence. On impact a Silence of 15’ radius is generated that lasts 1d4 rounds. If hits a monster or player, it will move with them.

1 Broadhead of Acid Ball. When you shoot this arrow into a creature it will release a magic ball of acid inside the target that will eat them out from the inside. DC 16 Dex save (to rip the arrow out in time).  4d6 on failed save, half on save. Internal acid damage. On critical hit instant death from acid. On failed save DC 14 Con to stay conscious from the pain. Up to large creatures. On a miss it will spray acid in a cone 10 feet from impact. 

1 Broadhead of Opportunity.  This broadhead makes a shrill screaming noise when it is fired.  All creatures with in 15 feet of its path roll DC 14 Wisdom or be distracted. If the shooter yells “Opportunity” before releasing any player currently engaged with that creature gets an attack of Opportunity if they roll a DC8 vs Wisdom.  If they fail, they miss the attack of opportunity. If the create makes its roll but he player does not, the creature gets an attack of opportunity/

The Glyph will go off regardless of a hit or miss. On a miss use the chart below to determine where the arrow goes. 

Miss by 1 to 5. Roll 1d6.  

If you miss by more than 5 roll a d10

4 to 6 is 5, 10, or 15 behind

Miss by 7 or more roll 1d10

1 Missile falls beyond target by 1d4 times 5 feet.  10 feet to the left of center.

2 Missile falls beyond target by 1d4 times 5 feet.  10 feet to the right of center.

3 Missile falls beyond target by 1d4 times 5 feet.  5 feet to the left of center.

4 Missile falls beyond target by 1d4 times 5 feet.  5 feet to the right of center.

5 Missile falls beyond target by 1d4 times 5 feet.  In line with the target.

6 Missile falls short or target by 1d4 times 5 feet.  10 feet to the left of center.

7 Missile falls short or target by 1d4 times 5 feet.  10 feet to the right of center.

8 Missile falls short or target by 1d4 times 5 feet.  5 feet to the left of center.

9 Missile falls short or target by 1d4 times 5 feet.  5 feet to the right of center.

10 Missile falls short or target by 1d4 times 5 feet.  In line with the target.

On a critical fumble roll from the Fumble effects chart for missile weapons.