Armor is a tricky thing when it comes to BT vehicles Dread Moores. It crits pretty easily especially when you compare vehicles to Mechs. Actually it is another good reason to have a turret in my mind cause you retain some type of mobility with your weaponry if you blow a tread or jam a turret. But enough spin on that I just might trade ammo for armor on the Farslayer because ideal it isn't suppose to be in range of too much.
Either way works honestly. I think it'll really just come down to personal preference on this point (more armor or more ammo).
I'm curious though what you might recommend for the Hellfire (Sniper) and Landsweeper (Thumper) designs.
This is where I would start looking at existing artillery tanks, since there are more tanks using Thumpers and Snipers. If we start there, you have the Thor as one example. A 5/8 tank with a Thumper, 2 tons of ammo, and enough armor to take an AC/20 anywhere and a Heavy Gauss in the important locations. (It also has a turret, but not for the artillery piece). My own experiences talking here, but I've found that 5/8 to be very important with direct fire. It lets you get that magic +6 or +7 to hit modifier, while returning fire with only a +1 or +2 (not including terrain or other concerns with either number). Ideally, you're firing at between 15 to 17 hexes (long range for the majority of weaponry) and you're getting 5 or 7 hexes of movement at least. So on direct fire, you're getting 15 points of damage to a hex with say a +2 for your shot. That's more than enough to give a very bad day to many non-assault combat vehicles, most medium (or lower) Mechs, and wipe out virtually all infantry and battle armor available at this time (presuming open hexes for infantry and no damage divisor). The enemy is shooting back on that +6 or +7 and you have respectable armor. Keep that Thor protected with some infantry or smaller vehicles, and you can do this for quite a while, all at a relatively low cost and BV. So, if you're looking at Thumper vehicles, I would use that Thor as a baseline. If you increase weight, I'd still strongly recommend keeping 5/8 as the speed. That really is a sweet spot for your "light" artillery vehicle. If you have tonnage to spare, look at getting the Thumper in a turret and increasing armor/ammo
without losing ground speed.
Snipers? Well, let's take a look at the Marskman then. 4/6, very reliable armor on the front, and weak armor elsewhere. However, since it doesn't have a turret, it's not as likely to see those side/rear shots if it is well protected and kept at the ideal range. Again, you're looking to keep the enemy shooting at you on a +5 or +6, while you're returning fire on a +1 or +2. That means 15 to 17 hexes again, so you can return accurate 20 point shots. Yes, that 20 points will be scattered around in groups of five. Don't worry about that. Guided artillery (Copperheads/Arrow IV guided) get a lot of love from the player base in Battletech, but it often makes players blind to the power of direct-fire artillery. It really is quite spectacular when used well, even if it in
NO WAY resembles real world artillery. 20 points of damage (in the case of the Sniper) is being dealt to your opponents at one shot, for low BV, with a lower rate of succesful return fire. That 20 points of damage can also affect multiple targets in one hex, multiple hexes, and will do uniform damage to all infantry and battle armor. All of this without the need for a TAGger. Battletech and reality parted ways back in the early 1980's. If you're looking for changes on the Marksman, it's pretty much the same hear. Increase weight (or save weight) but keep the speed at 4/6 and put free tonnage into armor/ammo/defensive systems. Anti-missile systems and Guardian are never an awful idea here (particularly if you use the Ghost Targets rule).
There's three other key points to remember about direct-fire artillery:
1. Never ever get closer than 12 hexes. In fact, if you've let the enemy get closer than 15 hexes, you've already exposed yourself to a lot of unnecessary danger. If you can't reach them from 15 hexes (due to terrain, smoke, whatever) than just pop indirect shots and hope for the best. As soon as you lose that long range modifier for opponents shooting at you, you're in serious trouble.
2. Don't ever forget to load some cluster artillery rounds, if you have ammo tonnage to spare and access to this munition type. This stuff is near broken. Sure, you lose some damage. But you gain the ability to use the "Shots from Above" hit locations table. It's downright scary against Mechs.
3. Never underestimate the power of improved Gunnery skills when it comes to direct fire artillery. While it seems like the BV investment isn't often worth it for some units, the low BV on many artillery units will help absorb some of the cost. Most importantly though, that improved Gunnery pays off immensely when it comes to indirect shots (and even direct fire shots). Even if you only successfully land one or two shells (let alone one or two salvos combined from several different artillery vehicles), those 15 or 20 or 25 point shots can turn the tide of the battle in your favor if you manage to land them in heavy traffic.
Edit: Something I just remembered, and should mention as well. My experience with artillery has often found the small guns to be the most effective in terms of BV versus effectiveness. That Thor really is a beauty of a direct fire vehicle. Enough armor to soak one or two rounds of heavy fire, but it gets that great +2/+3 movement modifier. You can cram three of these plus a protective unit in for 2.5 to 3K BV, and you now have three Gauss Rifles with a slightly shorter range (17 hexes for direct fire) that hit multiple targets and multiple hexes in a single shot and effectively ignore all range and movement modifiers. That means for the BV of many Clan heavies (and even some mediums) you have three super-duper targeting computer powered Gauss Rifles that are going to be a pain to hit back.