Since you can replicate items including common items from Xanathar's guide, this includes the Ruby Gem of the War Mage which can turn any weapon into an Arcane focus which takes care of the Material component (that don't have a cost) and Warcaster takes care of Somantic. This could mean you could hold a Crossbow (light or Heavy) whilst still casting spells but you lose out on the +1 to spell casting from Enhanced Arcane Focus (and the ignoring half cover) and the extra D8 from Arcane Firearm.

Is that worth the trade for an Artillerist spec since you lose out on an additional D8 on things like cantrips etc.

Not even a little bit.

Replicating one of the common magic items from XGtE is still using up one of your infusions for the day, and any infused item can already be used as a spellcasting focus.

If you really want to use a crossbow as a spellcasting focus, just infuse it with Repeating Shot.

If you're playing an Artillerist, Enhanced Arcane Focus is still your best choice.

Somatic components aren't a concern with two-handed weapons. You can just take one of your hands off the weapon as needed. And in addition to not providing any benefits like derek already mentioned, using a Ruby of the War Mage means wasting an attunement slot.

I'd say no. That's not worth the trade.

Ruby of the war mage can make a weapon into a spellcasting focus but it requires attunement to do so. Maybe if you weren't using an infusion to get this ruby, but if you are as an Artificer you have much better options.

For an artificer any infusion can act as a spellcasting focus so an infusion like Enhanced Weapon, Repeating Shot, or Radiant Weapon on a crossbow will allow you to cast spells with it in addition to granting you a +1 to weapon attack and damage rolls. Further you can pick other XGTE common magic items that can be applied to a weapon as an infusion and thus turn your weapon into a spellcasting focus that way.

Further if you're just worried about Somatic components then you don't need the Warcaster feat at all. Artificers already don't need to worry about somatic components due to ALL of their spells requiring a spellcasting focus (and thus a material component)

https://www.dndbeyond.com/sources/basic-rules/spellcasting#Components

A spellcaster must have a hand free to access a spell's material components -- or to hold a spellcasting focus -- but it can be the same hand that he or she uses to perform somatic components.

So your infused weapon counts as a spellcasting focus and thus is allowed to be in your hands when you perform the somatic components of a spell.

Now using an enhanced crossbow (light since Artillerists aren't proficient with martial weapons) might make sense at lower levels.
1d8 + 1 + DEX magical piercing damage on a hit is pretty good until level 5.

But Artillerists don't get extra attack so from level 5 onward an Artillerist with enhanced Arcane Focus Firearm casting Firebolt will always get better results than a light crossbow.

Enhanced Arcane Focus Arcane Firearm casting Firebolt (Level 5 Artillerist):
2d10 + 1 + 1d8 fire damage on a hit (1 potential hit per turn)

Now if you go with Battle Smith it'd be a different story since you then you can use heavy crossbows with INT and get extra attack.

Repeating Shot Heavy Crossbow (Level 5 Battle Smith):
1d10 + 1 + INT magical piercing damage on a hit (2 potential hits per turn)

Thanks for the advice guys, it's much appreciated!

Not even remotely.

Quote from Unclevertitle >>

I'd say no. That's not worth the trade.

Ruby of the war mage can make a weapon into a spellcasting focus but it requires attunement to do so. Maybe if you weren't using an infusion to get this ruby, but if you are as an Artificer you have much better options.

For an artificer any infusion can act as a spellcasting focus so an infusion like Enhanced Weapon, Repeating Shot, or Radiant Weapon on a crossbow will allow you to cast spells with it in addition to granting you a +1 to weapon attack and damage rolls. Further you can pick other XGTE common magic items that can be applied to a weapon as an infusion and thus turn your weapon into a spellcasting focus that way.

Further if you're just worried about Somatic components then you don't need the Warcaster feat at all. Artificers already don't need to worry about somatic components due to ALL of their spells requiring a spellcasting focus (and thus a material component)

https://www.dndbeyond.com/sources/basic-rules/spellcasting#Components

A spellcaster must have a hand free to access a spell's material components -- or to hold a spellcasting focus -- but it can be the same hand that he or she uses to perform somatic components.

So your infused weapon counts as a spellcasting focus and thus is allowed to be in your hands when you perform the somatic components of a spell.

Now using an enhanced crossbow (light since Artillerists aren't proficient with martial weapons) might make sense at lower levels.
1d8 + 1 + DEX magical piercing damage on a hit is pretty good until level 5.

But Artillerists don't get extra attack so from level 5 onward an Artillerist with enhanced Arcane Focus Firearm casting Firebolt will always get better results than a light crossbow.

Enhanced Arcane Focus Arcane Firearm casting Firebolt (Level 5 Artillerist):
2d10 + 1 + 1d8 fire damage on a hit (1 potential hit per turn)

Now if you go with Battle Smith it'd be a different story since you then you can use heavy crossbows with INT and get extra attack.

Repeating Shot Heavy Crossbow (Level 5 Battle Smith):
1d10 + 1 + INT magical piercing damage on a hit (2 potential hits per turn)

 At level 5 that Firebolt is a 2D10 cantrip 

So 2D10 + 1D8 damage for the Artillerist 

Quote from Jacqsynn >>

 At level 5 that Firebolt is a 2D10 cantrip 

So 2D10 + 1D8 damage for the Artillerist 

 Oh right. Forgot Enhanced Arcane Focus doesn't add a +1 damage, just a +1 to attack.

You could as an Artificer..... just craft Ruby Gem of the War Mage using the magic crafting rules from XGtE and at 10th level it is "quarter of the normal time, and it costs you half as much of the usual gold."  :)

Why does an Artillerist have a crossbow?

I'd say not worth it. Ruby may be necessary only if casting spells other than artificer's. Other than that I think the class specializations focus are pretty neat on their own, maybe even op at times. 

Quote from DrMechano >>

Since you can replicate items including common items from Xanathar's guide, this includes the Ruby Gem of the War Mage which can turn any weapon into an Arcane focus which takes care of the Material component (that don't have a cost) and Warcaster takes care of Somantic. This could mean you could hold a Crossbow (light or Heavy) whilst still casting spells but you lose out on the +1 to spell casting from Enhanced Arcane Focus (and the ignoring half cover) and the extra D8 from Arcane Firearm.

Is that worth the trade for an Artillerist spec since you lose out on an additional D8 on things like cantrips etc.

Anyone can cast while holding a two-handed weapon - the only downside is needing to use your free item interaction on re-readying the weapon in order to attack with it.

The only *general* situation in which the need for a "free" casting arm comes up is shields, since they take an entire action to equip and unequip, and reaction spellcasting, where you don't have a free item interaction handy.  The cantrips Booming Blade and Green Flame Blade are examples of spells where casting with a shield equipped can be an issue.

But for your described case, just wield your crossbow and use your focus.  You won't have any issues.