How many extra pellets are fired from a Two Shot Prefix Shotgun?

I was looking at the legendary weapon effects wikia page and came to the two shot entry. It says that it "fires an additional projectile." I remembered that I recently found a two shot combat shotgun. Does its prefix mean that it fires one more pellet or twice the pellets when compared to a normal combat shotgun?

Solution 1:

Well, let's put the speculations aside and test it experimentally!

If we compare two identical stock double-barrel shotgun (With a long barrel) in our inventory, we can see that the predicted damage for the shotgun with Two-Shot is already twice the damage of the standard shotgun:

If we test it point-blank on a gentle Deathclaw, we can observe the real damage difference between the two shots, which is just over 2x:

The slight difference can be explained by the pellets spread. I tried to reduce it to the maximum by using the Long Barrel mod and having my character placed at the exact same spot, doing a Body shot in VATS

So, in conclusion, Two-Shot will fire twice the amount of pellets from a shotgun doing correctly twice the damage.

Solution 2:

Does its prefix mean that it fires one more pellet or twice the pellets when compared to a normal combat shotgun?

Per your link to the Wikia:

The Two shot modifier makes the weapon fire an additional projectile while still using only 1 ammo and at slightly reduced accuracy.

This means that 2 rounds will be expended. Here are some examples that will help answer your question:

• If you have 10 pellets from a shotgun cartridge, 20 will be fired instead.

• If you have 1 slug fired from a shotgun, 2 will be fired for 1 trigger pull.

• If the two shot was on a hunting rifle, instead of 1 bullet leaving the barrel, 2 will be sent downrange.