Q: Why many of the new turrets have such narrow firing arcs, together with minimal ranges?
A: Like the vanilla flamethrower turret, these turrets have some special projectiles, which either deal area damage or piercing damage. They will surely deal a lot of friendly fire to friendly units/structures, if without limitations and turret fire got disrupted because of enemies getting too close. This is especially hazardous since some ammo (like explosive rockets and high caliber cannon shells) deal an insane amount of damage. With implementation of firing arcs and/or minimal ranges, such collateral damage should be kept to a minimum if they are placed well.

Q: Can we place walls in front of the new turrets protecting them?
A: A game mechanic "collision box" of the projectile is responsible for the premature explosions in the wall. It should be OK for sniper turrets and rocket turrets, since their projectiles are of "instant" type.
For autocannon turrets, you should have option "Collision box Autocannon explosive shell" turned OFF under options settings. For cannon turrets, you should have option "Collision box Cannon explosive shell" turned OFF under options settings, if you want to feed explosive shells into them.
Incendiary cannon shells have no collision box by default, so are safe to use.
The armour-piercing types of cannon shells, however, should not be used. The main power of these ammo relies on penetration damage, so a wall placed ahead will become the first target of these shells... If you want to apply these armour-piercing shells, the cannon turrets should be placed only at the first line of fire, without any walls ahead.

Q: Why the tank machine gun has damage bonus back (which is removed from base game version 0.17)?
A: Yes, the tank machine gun no longer has the +100% damage bonus in 0.17.4. I feel this nerf is really bad, because players need to spend their focus to use the weapon, but end up worse than gun turret. Does it mean encouraging people to use turret creep to attack instead? It is a really bad setting! So this mod re-introduced +50% damage bonus to tank machine gun.

Q: Why the sniper turret share the same damage bonus from "Gun turret damage" modifier, with gun turret?
A: Yes, I feel this modifier is really out of balance (at vanilla version 0.16.51 to 0.17.4), as shown in the above table...
From the table, we can see the "Gun turret damage" tech confers a very fast growth in damage to the gun turret. And each infinite tech level will give another +70% over the base, which is IMO really too much, and makes things very difficult to balance.
Sadly, removing this modifier may upset a lot of vanilla players, and needing increasing lots of parameters in "physical projectile damge" techs. This may create problem to existing saves. So I decided to "give up", and let sniper turret and minigun turret complying to that...

Technical Details

Ammo Turrets

Minigun turret has a slightly longer range than the original gun turret (22 vs 18), and quadruple RoF (40/s vs 10/s, base). It has -20% damage malus for balance.
Sniper turret has more than double range over the gun turret (48 vs 18), much lower RoF (0.5/s vs 10/s, base).
Other turrets receive the same range, rate of fire, damage bonus, together with their upgrades as their corresponding guns.

Fluid Turrets

Vanilla flamethrower has two main types:

• Flammable liquid form: catching entities on fire for sustained damage, leave flames on ground. (Used by handheld flamethrower, vanilla flamethrower turret.)
• Propane gas form: damaging entities directly, no remaining flames. (Used by tank flamethrower.)

The vanilla flamethrower is renamed as "Flamethrower turret - Flammable liquid". (Stats unchanged.), since this mod introduced the propane gas form for turret as well.
"Flamethrower turret - Propane gas" has a relative longer range (18) than tank flamethrower, but still much less than the original flammable liquid form. Best used as last line of defense, for its high damage. Two types can be used together to cover both the long range and close range.
In contrast with original turret, it can and has the best damage bonus when using petroleum gas as fuel, as listed below:

• Petroleum gas: +10% (Not usable on flammable liquid turrets.)
• Light oil: ±0%
• Heavy oil: -10%
• Crude oil: -20%

Full List of New (or Changed) Items

Turrets

• 2 bullet:
  • Minigun turret
  • Sniper turret
• 1 rocket:
  • Rocket turret
• 3 cannon:
  • Cannon turret (75 mm)
  • Cannon turret (88 mm)
  • Cannon turret (128 mm)
• 1 autocannon:
  • Autocannon turret (20 mm)
• (1 new, 1 changed) flamethrower:
  • Flamethrower turret - Flammable liquid [stats UNCHANGED from vanilla flamethrower turret, only rename]
  • Flamethrower turret - Propane gas

Technologies

• 7 turret-related:
  • Minigun turret
  • Sniper turret
  • Rocket turret
  • Cannon turret (75 mm)
  • Cannon turret (88 mm)
  • Cannon turret (128 mm)
  • Autocannon turret (20 mm)