It's been a couple years since this trick was discovered, but there have been no records of it and it was only passed around by words of mouth. Since Mass Effect 3 is already over 5 years old, I would like talk about it now.
--- How to do it ---
1: Fully charge Venom
2: Melee (light and heavy both works, but light melee is harder to do on characters with fast ones like default humans)
3: Press missile button a tiny pause after pressing melee
4: Release the trigger right after pressing missile button
This is done in a sequence. You can not do it too fast or too slow. Because each character's light melee animations are different, they also have different timings for this input sequence, but heavy melee timings are mostly the same on characters I have tried.
You must be at least 1 second of distance away from your enemies for this to work. If you are close enough for your melee attack to hit an enemy, the missile will probably not happen.
---Video demonstration---
---Pros---
Unlike the more common Destroyer infinite missiles, this only has 3 requirements, a Venom shotgun, a character with a real melee attack (I.E. doesn't work on Volus unless you have Batarian gauntlet), and you have at least 1 missile.
So you can take cover, you can use the ammo box, you do not need at least 3 people, and you do not need to waste missiles before doing this.
---Cons---
Each shot requires you to wait for melee animation to be over, and you also have to switch weapon back to Venom every time, so you can't carpet bomb the stage like Destroyer can
---Notes---
Definitely put on ammo expansion mod on your Venom so you don't have to run to ammo box as frequently
Ideally the best characters to make use of this move are the Geth because Geth's heavy melee comes out very fast and cancellable, and heavy melee input sequence is easier to buffer than light melee. Juggernaut can do it even with an empty heavy melee.
Even though you can do this with Volus by equipping Batarian gauntlet, you are most likely going to die if there are still enemies around you after you fire your shot because you have almost no health and Batarian gauntlet animation is a lot longer than other characters.
--- How to do it ---
1: Fully charge Venom
2: Melee (light and heavy both works, but light melee is harder to do on characters with fast ones like default humans)
3: Press missile button a tiny pause after pressing melee
4: Release the trigger right after pressing missile button
This is done in a sequence. You can not do it too fast or too slow. Because each character's light melee animations are different, they also have different timings for this input sequence, but heavy melee timings are mostly the same on characters I have tried.
You must be at least 1 second of distance away from your enemies for this to work. If you are close enough for your melee attack to hit an enemy, the missile will probably not happen.
---Video demonstration---
---Pros---
Unlike the more common Destroyer infinite missiles, this only has 3 requirements, a Venom shotgun, a character with a real melee attack (I.E. doesn't work on Volus unless you have Batarian gauntlet), and you have at least 1 missile.
So you can take cover, you can use the ammo box, you do not need at least 3 people, and you do not need to waste missiles before doing this.
---Cons---
Each shot requires you to wait for melee animation to be over, and you also have to switch weapon back to Venom every time, so you can't carpet bomb the stage like Destroyer can
---Notes---
Definitely put on ammo expansion mod on your Venom so you don't have to run to ammo box as frequently
Ideally the best characters to make use of this move are the Geth because Geth's heavy melee comes out very fast and cancellable, and heavy melee input sequence is easier to buffer than light melee. Juggernaut can do it even with an empty heavy melee.
Even though you can do this with Volus by equipping Batarian gauntlet, you are most likely going to die if there are still enemies around you after you fire your shot because you have almost no health and Batarian gauntlet animation is a lot longer than other characters.