Greetings! To start things off, i'd like to note that this subclass is meant to act as an in-between Subclass, taking inspiration from primarily the Artificer's own Artillerist and the Fighter's Arcane Archer subclass while also drawing some of the thinking from the newer UA Armorer Artificer subclass - in that it provides less of linear upgrade path and instead has a few choices to build your character along so you can fill different roles, sort of like mini-subclasses. The concept for the Rifleman is to have the player build a weapon and modify it to suit their own needs by providing a lot of possible upgrades, but only allowing a few to be chosen at a time - upgrades can be built and swapped out once per long rest working much like the Artificer's Cantrip rules and Infusions. I'll be removing flavor text for anything major here for the following list for brevity

So, Starting with Level 3 we have 3 Major concepts/ list of items to pick from. First off: Much like how the Artillerist can pick from a list of 3 unique turrets, the player can choose to craft one of 3 Archetypes of weapon, all of which are fed from removable Magazines [detailed below]:

Level 3

I Need A Weapon

1. Rifle: 1d10,  special(lance), ammunition (range 150/600), loading[magazine]
2. Pistol: 1d8, versatile(2d4), ammunition (range 50/100), loading[magazine], light
3. Blunderbuss: 1d6, versatile(2d6), ammunition (range 20/60), loading[magazine]

Fire for Effect

•  You may use Half of your [INT modifier] (round up) and Half of your [DEX mod] (round down) instead of [DEX] for your attack and damage rolls.
• You gain proficiency with your crafted weapons, which count as crossbows for determining compatibility with feats.
• Magazines can ony be filled with rounds of the same type. Round Capacity is determined by 1+[ your INT modifier]. Magazines weigh 1Ib each
• Mundane magazines are crafted at a rate of 2x [INT mod] and 1x [INT mod] for Specialist magazines per long rest, and 1x [INT mod] mundane magazines per short rest.

One for every Occasion

Specialist Ammo types. Swapping magazines (specialist or mundane) takes one action. You may craft and carry up to two choices of specialist ammo in any combination. Add these effects when attacking a target with your crafted weapon when loaded.

• Hollow Point: Increase damage by 1d4 + [INT mod] 
• Sabot: Double your crafted weapon's effective range. Attack an additional 2 creatures that are in a line emanating from you no more than 15 ft total behind the nearest target. Apply [+INT mod], [no mod], [-INT mod] for attack rolls against the first, second, and third targets respectively, for the purpose of overcoming armor.
• Multishot: Reduce your crafted weapon's maximum range to it's effective range. Add an additional 1d4 to your attack and damage rolls. Creatures damaged by these rounds are slowed by 10ft.
• Flashbang: attacks from a weapon firing these rounds have a flat 1d4 radiant/thunder damage. Creatures hit by these rounds must make a constitution save at the start of every turn or be blinded/deafened until the start of it's turn where it may take an action to remove the effect.
• Beanbag: add 1d4 bludgeoning damage. The Target's health cannot drop below 0 if hit by these rounds.
• Angelis: Range of crafted weapons becomes (30/60) and heals a target by 1d4 health points.

Level 5

Extra Attack: Starting at 5th level, you can attack twice, rather than once, whenever you take the attack action on your turn.

The weapon is for defense, but the mind is for victory

Starting at 5th level, you may craft and add one attachment to your crafted weapon from the following list upon taking a long rest.

• Bayonet Lug: you may attach any one-handed melee weapon to your crafted weapon, with which you may make an attack as an action or bonus action if an enemy is within reach.
• Twin Magazine: you may ignore the loading[magazine] property every other reload.
• Night Sight: A creature looking down these sights gains Darkvision if they do not already have it  and may make a DC10 check to see in faint color. Creatures with Darkvision gain Superior Darkvision.
• Underbarrel Arcane Launcher: add an extra +1 attack or damage to any spell cast through the attached weapon. It is considered an arcane focus, even if it is not magical.
• Stabilizer: A critical hit may occur on a 19 or 20 on the first attack made on your turn.
• Controlled Burst: You may reroll an attack made with your crafted weapon at the cost of one extra round, if available.
• Quick Sling: Switching to or from a weapon with this attachment takes no time. You may draw and fire your crafted weapon in one smooth motion.

Level 9

All aboard, we ain't a-goin' fishin'

Pass the Ammunition: You craft double the amount of ammuntion you would normally make, and may make [1d4] specialist magazines on one type on a short rest. Mundane and Specialist Magazine capacity is increased by +1. You may also craft a third type of specialty ammunition

Point Towards Enemy

Heavy Weapons: You may craft and carry an extra weapon from the list as follows on a long rest at a location that allow for advanced forging techniques, or you may spend 1500 gold. The amount of ammunition for heavy weapons that may be crafted per long rest is equal to half of your [INT mod] rounded up. The amount carried is equal to your [STR mod].

1. S.A.W. :  1d10, special(lance), ammunition, ammunition(200, 1000): This heavy weapon has a bespoke mundane magazine equal to 20x[Magazine] that cannot be fitted to your normal crafted weapon, but can accept normal specialist magazines. As an action or bonus action, mark a 30ft sphereical area you can see clearly. When a creature enters this area, you may make a reactionary attack to force this creature to make a [DEX] saving throw. The creature takes 3d8(p) damage on a failed throw, and half that on a saved throw. A reaction attack takes 20 rounds from your magazine or until it is empty.
2. Coilgun: A projectile is launched in a line 100ft long and 5ft wide in a direction you choose. Each creature in the path must make a dexterity saving throw. A creature takes 8d6 thunder damage on a dailed save and takes half as much damage on a succesful one. Passes through solid objects and ignores cover.
3. M.G.L: This weapon hold six projectiles per magazine[heavy]. Before you fire this weapon you must aim it, which takes one action or bonus action. After you aim this weapon, you may fire one or two porjectiles as an action or bonus action. The projectiles have a maximum range of 200ft and explode when they strike a creature or solid surface. Each creature within 5 ft of where the point where projectile explodes must make a dexterity saving throw. A creature takes 2d8 fire damage on a failed throw and half as much damage on a successful one.
4. Rocket Launcher: A self-contained launch tube containing a single reloadable self-propelled warhead. When the warhead is launched and either impacts with a creature or solid surface, the rocket detonates, resulting in an explosion with a radius of 20 ft. Each creature caught within the blast must make a dexterity saving throw. A creature takes 8d6 fire/thunder damage on a failed save or half as much damage on a success. The fire spreads around corners and ignites flammable objects in the area.

Level 15

Heavy Load: Whenever you craft ammunition, you now craft twice the amount you normally would and can craft a fourth type of rounds. Specialty Round effectiveness is increased to 2d4. You may make another attack as a bonus action.

There's No Such Thing as an Unsalted E4

With all of your experience gained in the field, you feel like you've found how to make the best use of your abilities and know-how. Pick a Specialization from the following list:

1. Combat Medic: Gain Medical proficiency if you don't already have it. You always craft an extra magazine of Angelis rounds when you craft ammunition, and doesn't count against your carried ammunition penalty. You learn Spare The Dying and Healing Word with a seperate amount of spell slots equal to [INT modifier] which do not count against your normal spells or spell slots. Gain the Healer Feat
2. Dreadnought: Gain Heavy Armor proficiency if you do not already have it and gain the Heavy Armor Master feat if you do not already have it. You can Use two handed Weapons and a Shield. While you do so your speed is reduced by 10 ft and lose your proficiency bonus on attack rolls.
3. Grenadier: When using your heavy weapon you may add 1d8 to the damage rolls. You may also carry and craft one extra set of Heavy Ammunition on a short or long rest. You may also craft and attach an Underbarrel Arcane Launcher to any of your crafted weapons if it doesn't already have one, and spell attack/damage bonuses are increased to +3.
4. Combat Engineer: You gain The following spells: Galder's Tower, Magic Circle, Cordon of Arrows, Mold Earth. These spells count as artificer spells for you, but they don’t count against the number of artificer spells you prepare.You may craft a number of your level 3 weapons equal to 1d4 per long rest, following the same rules as crafting them normally, however they must be attuned and count against your infusions.
5. Commando: Gain the Medium Armor Master, Mobile, and Skulker feats. You may craft and mount an extra attachment to either your primary crafted weapon or heavy crafted weapon. Learn Misty Step and True Strike

Rifleman Spells

Level 3: Shield, Guiding Bolt

Level 5: Blur, Acid Arrow

Level 9: Sleet Storm, Lightning Arrow

Level 13: Fire Shield, Black Tentacles

Level 17: Wall of Force, Flame Strike

Final Thoughts

So that's it. That's my WIP Rifleman class. Obviously i'm asking for thoughts, opinions, suggestions, etc... The goal here isn't to make an OP class, but rather a sort of in-between for the Arcane Archer and Artillerist: Less nuker and more medium-duration build that isn't particulary strong, but is well-balanced. The heavy weapons are meant to essentilly be re-flavored Level 3 spells that are able to be used a bit more often as a replacement for the very-long-lasting Arcane Turrets for the Artillerist and the Ultra-powerful Arcane Archer arrow. Personal thoughts are: Is everything too confusing? Is there too much to keep track of? Are the specializations all balanced around each-other, or too powerful in general, especially compared to other classes? Are the Heavy weapons too confusing, especially with the Heavy ammunition rules? What kind of tweaks would you like to see before you allowed it into you games?

So, on a re-read, yeah there's a lot to digest, so here's the breakdown per-level.

LEVEL 3: primary weapon, ammo magazines, use INT to attk

LEVEL 5: extra attack, weapon attachment

LEVEL 9: Heavy weapon, craft more ammo

LEVEL 15: subclass specialization, improved ammo damage, bonus action attack

CHANGELOG:

• Re-worded Flashbang rounds - now change damage profile for any attack to a flat 1d4 and a easier-to-remove dc10 blindness check once per turn and removed on a creature's turn as an action.
• removed 15ft cone property from blunderbuss, too strong w/it
• reworked Fire For Effect INT for attack , now uses 1/2 INT (round up) + 1/2 DEX (round down) as a sort of 'point tax' to bring more in line with a typical ranged class.