Roleplaying question- Thumper, Sniper and Long Tom, any idea what caliber weapon system these would be regarded as?
I have always considered (and fluffed in my fanfics and scenarios) the Thumper to be a heavy 120mm or equivalent Mortar, the Sniper is a 155mm or equivalent Artillery gun, and the Long Tom more akin to the old german railway guns or coastal defense artillery.
I was originally mostly interested in towed infantry artillery akin to the Kurita Thumper platoon and Capellan Missile Artillery in TRO 3085. But now I'm seeing stuff on Gun Trailers in a TRO I don't own yet. So now I'm torn, Pros and Cons between the two?
An infantry platoon is more vulnerable to area of effect damage but likely cheaper in cost both in c-bills and BV while being able to move albeit very slowly. A gun trailer needs a tractor and is effectively a vehicle although one unable to move under its own power.
Are there rules for Gun Trailers in a rules book somewhere, aside from TRO 3145 Mercenaries? The biggest questions that comes to mind is deployment speed (setting it up) and to what degree a Gun Trailer slows down the vehicle hauling it.
Both of those rules are in Total Warfare and TechManual. It may have changed but I have always used the trailer as added weight to its tractor when computing cruise MP (for example a 20 ton vehicle with a 20 ton trailer assuming they use the same motive type is a 40 ton vehicle. So a 200 rated engine would move them 5/8 instead of 10/15 assuming its tracked). A trailer is a vehicle without an engine but following all the same rules for vehicle construction.
Both the Thumper and Arrow 4 artillery platoons weigh 40 tons. I'm looking for VTOLs that could make them air-mobile, at least strategically, any recommendations?
No standard VTOL can carry a 40 ton load as they max out at 30 tons themselves, there are some super-heavy VTOLs that can but they are rare, expensive, and better used elsewhere. If you want artillery to move make it self-propelled or tow it.
Strategically you can always deliver the infantry and artillery piece separately and just say they come together for that battle (drop the platoon and ammo off with a Karnov then make a second trip to pick up an underslung artillery piece being the easiest)
Finally, is there any consensus on what constitutes an Artillery "Battery" in Battletech? In the old 3050s-era Field Manuals I see the terms Artillery Company and Artillery Thrown thrown around a lot in infantry units or as part of armor brigades. Anything resembling canon standardized organization on towed artillery organization would be helpful.
There is no canon consensus on artillery organization for any faction that I know of and I have looked. The fiction is all about Battlemechs (Davion, Kurita, Wolf, and Jade Falcon Battlemechs in particular) after all. Armor is a distant third (with Elementals and Battle Armored Infantry being second), and everything else is more distant from there.
My fiction is always a 6 piece battery as that makes the most sense in my head-canon. The closest you get is in the CCAF which is the most prolific user of both infantry and artillery in the Inner Sphere. Their augmented lance (company, etc...) system is the basis that I typically default to.