Is this OP strategy one of my fighters came up with actually feasible

One of my fighters determined that if they took an anti matter rifle with two echo knights and the gunner feat with an artificer in the party they could deal 144d8 24d10 48d6 damage with one move.

Here's how antimatter rifle does 6d8 times 4 for the extra fighter attacks times that by 2 for action surge and times 3 for the 2 echo knights at level 20. for 144d8. then artificer casts elemental bane for 2d6 of fire damage, and enchants the gun with fire damage so the 24 attacks give 24d10 fire damage and elemental bane allows an addition 48d6 fire damage.

a little scatterbrained first time posting or DMing

Unleash Incarnation (the Echo Knight feature that allows making an extra attack from the Echo's position) is limited to melee attacks, so you can't use that ability with ranged attacks from an Antimatter Rifle. 

It's not clear to me what you mean when you say that the Artificer "enchants the gun with fire damage". What spell or ability are you using to do that, exactly?

Elemental weapon is a 3rd-level spell that allows an artificer to infuse a weapon with acid, cold, fire, lightning, or thunder damage, adding +1 to attack rolls and an extra 1d4 damage of the chosen type. so it should be 24d4 not 24d10 my bad.

They were also attempting to try it with a great sword and it slipped my mind that echoes can't make ranged attacks so all damage would just be divided by 3.

Quote from DarkBoiDruid >>

Elemental weapon is a 3rd-level spell that allows an artificer to infuse a weapon with acid, cold, fire, lightning, or thunder damage, adding +1 to attack rolls and an extra 1d4 damage of the chosen type. so it should be 24d4 not 24d10 my bad.

Elemental Bane and Elemental Weapon are both Concentration spells, so you would need two casters to be maintaining them at the same time (they don't have to be Artificers; both spells are also on multiple other lists) but in theory that is possible.

However, Elemental Bane will not add 48d6 damage in this scenario — since it only applies once per turn per target — unless each attack were directed at a different enemy. Even so, even with a 9th level spell slot you can only put it on six targets at a time.

Gunner doesn’t remove the Reload tag, meaning it has only two shots before you need to use a action or bonus action to reload.

If your using the anti-matter rifle, you would use an attack action to do two attacks, reload using your bonus action, continue the last two attacks. You can’t shoot again because the reload needs an action before you can use it again, with an action surge only giving one.

the total damage with Elemental Bane would be: 4(6d8 + 5) + 2d6 = 135 damage, impressive but not obscene.

Your math is way off base. Your echoes will only ever give you one more attack per attack action. They are not duplicating your turn.

 Wagnarokkr is half right - Echo Knights' Unleash Incarnation feature is triggered simply by taking the Attack action, but the extra attack granted by the echo MUST be a melee attack. Also bears mentioning that, as Maruntoryx pointed out, Unleash Incarnation only grants one total additional attack per Attack action, NOT a full round of attacks PER ECHO per Attack action. ClassicCharismaCaster is ALSO correct that you would need to reload after two shots, but that could TECHNICALLY be overcome by swapping weapons mid action. That said, you would then need a bunch of antimatter rifles and an ARMY of spellcasters to enchant every weapon, IN ADDITION TO whatever melee weapon the player was having their echo Unleash with.

Best-case scenario:

Your player has six casters cast Elemental Weapon on five anti-matter Rifles and a Greatsword (let's be generous and say they're 7th-level casts, since we're fantasizing), while another caster casts Haste on your player. Your player positions an Echo in melee range of three enemies with the Greatsword at hand. You have another two spellcasters cast 9th-level Elemental Bane on twelve separate enemies (three of which are the ones bordering your player's Echo). Your player takes an Action and fires their first two shots at two different enemies before dropping the rifle and using their free equip-during-an-Attack-action action to equip another rifle and fire it two more times at two more targets. Their Echo strikes a fifth target with the greatsword. Then your player uses their free Utilize action to equip another rifle and Action Surge. They fire two shots at two new targets, use the free equip action to equip the fourth rifle, fire it twice more, and their Echo attacks a tenth target. Then, they use their free equip action on the final Attack action granted by Haste to equip their fifth and final rifle, fire it once, and have their Echo attack one last target.

At the end of the day, your player will have fired nine shots at nine separate targets, and their echo will have taken three additional attacks at three additional targets. Assuming all attacks hit (and all saves are failed), your player will have spread out 54d8 Necrotic + 6d6 Slashing + 24d6 + 36d4 Fire damage among twelve targets, so long as they have the assistance of nine spellcasters and five antimatter rifles. Of course, nine spellcasters capable of casting AT LEAST 3rd-level spells could lob nine Fireballs, dealing 72d6 damage on their own (spellcasters capable of casting 7th- and 9th-level spells could obviously dwarf those numbers), but it'd be a helluva round.

EDIT for grammar and to add: the targets would all have to be within 30 feet of each other for Elemental Bane to target them all, so your player would 100% be better served having the spellcasters lob Fireballs at them than doing all this, but, again, we're fantasizing.