Properties of siege engines

Both Siege Engines and Naval Siege Engines on Proficiency say, "Siege engines are exotic weapons." Further, Siege Engines on Direct-Fire Siege Engines says that a "direct-fire weapon uses a normal attack roll," and Naval Siege Engines on Direct-Fire Siege Engines says that a "direct-fire weapon uses a normal ranged attack roll." (A gunner instead makes a targeting check to attack using an indirect-fire siege engine.) So, because a direct-fire siege engine is a ranged projectile weapon and attacks made with such a siege engine require an attack roll, any feat that mandates one or both elements (and, obviously, isn't excluded by an individual siege engine's other elements) should work normally with a direct-fire siege engine.

Keep in mind, though, that this GM would rule that a gunner does not technically wield a siege engine, but if another GM were to rule that a gunner wields a siege engine to fire it, even more feats are available.

Some things that work and others that don't

• The weapon special ability distance "can only be placed on a ranged weapon," but that means it doubles the range increment of a siege engine just fine.
• The benefit of the combat feat Far Shot says, "You only suffer a –1 penalty per full range increment between you and your target when using a ranged weapon" (emphasis mine), so a gunner that has the feat can apply its benefit to a siege engine.

• Nothing I found says that making an attack with a direct-fire siege engine is any different from making an attack with any other weapon (that is, nothing says a creature must take a standard action to make an attack with a siege engine). So, while it's complicated, requires another feat, and takes a big crew, even the combat feat Rapid Shot can be used with a siege engine!

  A siege engine crew that's led by a creature that possesses the feat Master Siege Engineer (gained as a bonus feat at level 8 by a gunslinger with the archetype siege gunner) prepares a siege engine to be fired after its crew takes a number of move actions equal to the siege engine's Load entry plus its Aim entry. Thus some members of a large enough crew can take a standard action to take the action ready, picking the move action Load the siege engine and the trigger after an attack's made with the siege engine, and the other crew members can also take the action ready, picking the move action Aim the siege engine and the trigger after the siege engine's loaded again.

  For example, a standard bombard has entries of Crew 2, Aim 1, and Load 3. A standard bombard crew of 9 (eight advanced-to-3-Hit-Dice Small homunculi and led by one gunner that possesses the feat Master Siege Engineer) can be allocated so that 2 of the crew each take take a move action to load the standard bombard and 6 of the crew each take a move action to aim the standard bombard (1 to aim plus 1 because of the siege engine's entry Crew 2 and 4 more—1 more to aim and the remaining 3 to offset the gunner's attack roll penalty for a Small creature aiming a Huge siege engine).

  The gunner takes a full-round action, declares he's using the benefit of the feat Rapid Shot, and makes an attack with an already-loaded siege engine. The freshly unloaded siege engine triggers some of the crew's ready actions and those crew members reload it. Then the freshly reloaded siege engine triggers some of the other crew's ready actions and those crew members aim it. Then the gunner makes his second attack with the siege engine via the feat Rapid Shot during his same full attack action! (This is even easier if the direct-fire siege engine needn't be aimed at a new target.)

  As there's no apparent maximum size to a siege engine's crew, with a large enough crew a high-level gunner possessing the feat Master Siege Engineer can organize the siege engine's crew to such a degree that the gunner can make iterative attacks with a siege engine!

• The combat trait Roving Range "[i]ncrease[s] the range increment of ranged weapons… you wield by 5 feet" (emphasis mine), so its benefit typically doesn't apply to siege weapons.

• The weapon special ability shadowshooting "can only be added to ranged projectile weapons," which makes it immediately applicable to siege engines. A successful Will saving throw (probably DC 16) means the target takes the weapon's minimum damage, but not needing to load a siege engine is such a boon that the possibility of minimum damage may be worth the risk. (This weapon special ability makes iterative attacks become the norm with a direct-fire siege engine that's already been aimed at a target! Further, unattended mundane objects always fail their saving throws.)
• The combat feat Sword and Pistol mandates the firearm be wielded to realize the feat's benefit, unsurprisingly making the feat ineligible for use with a siege engine.
• The warpriest minor air blessing Zephyr's Gift causes "one [touched] ranged weapon… for 1 minute [to] take no penalties due to range," so it works with siege engines. (Also see the feat War Blessing.)

Also, both Siege Engines and Naval Siege Engines say, "Siege engines do not gain the benefit of critical feats the crew or the crew leader may have."

More things that work

• The combat feat Improved Critical lacks the feat type critical. This makes, for example, the feat Improved Critical (standard bombard) a valid choice. (It's pretty cool having a 19–20 threat range on a weapon like the standard bombard that deals at least 28d6 points of damage on a critical hit!)
• One of the benefits of the racial feat Surge of Success doesn't care under what circumstances the human critically hit his foe, only that the human made a critical hit. Although it's not a particularly good feat, this works fine with siege engines.

Some siege engines are specifically firearms (although none mention the amount of effort—light, 1-handed, or 2-handed—necessary to employ them), so such siege engines benefit from effects that affect any, all, or generally firearms.

Even more things that work

• Many gunslinger deeds function with firearms generally. For example, the gunslinger deeds deadeye and dead shot appear to work fine with siege engine firearms.
• It's pretty shady, but the combat and style feat Empty Quiver Style says, "While using this style, you can make melee attacks with the chosen weapon [i.e. based on this feat's prerequisite feat Weapon Focus] as if it were a heavy mace." The feat chain's later feats amplify the cheese in relation to siege engine firearms to such a degree the GM will likely just say No, but it's worth an ask.

By contrast, a ballista "[r]esembl[es] a massive crossbow" and "is essentially a [big] heavy crossbow fixed in place" yet, technically, is not a crossbow but, instead, a ballista. A GM that nonetheless rules a ballista is just a another (albeit big) crossbow opens the door for a ballista gunner to benefit from all kinds of feats (the feat Crossbow Mastery being the most valuable).

Siege engine tips and tricks

The rules for siege engines are extremely complicated. Here're some things I happened upon that might make your life as a siege gunner easier.

• Bring the siege engine's crew with you. If the GM can be convinced (perhaps via beer and pizza) that adding 1 Hit Die to a homunculus increases the homunculus's size from Tiny to Small (see Monster Advancement on Step 1: Plan the Monster) and allows the creator to put the homunculus's skill ranks from that extra 1 Hit Die into Knowledge (engineering), such a 4,050 gp homunculus makes an awesome crew member. Commission the creation of as many as you can reasonably afford. (The future master need only contribute some blood during the homunculus's creation to be its master, no feats or special abilities necessary.)

• Remember ammunition. A GM that rules that a ballista is just a scaled-up crossbow means a gunslinger can, for example, spend 36,800 gp on a +1 endless ammunition light ballista. While this GM wouldn't have the weapon special ability change the siege engine's Load or Aim entries, your GM may disagree. Either way, such a siege engine never wants for ammunition. Even a light ballista bolt costs 10 gp and weighs 10 lbs.

  If an extremely generous GM can be persuaded (perhaps with more beer and pizza) to allow a beneficial bandolier to store ammunition for any firearm, that should include siege engines that are also firearms! This player would not expect, like, a bombard to be affected by the bandolier's reload ability as the bombard's not wielded, but a 2-lb., 1,000-gp magic item that stores three tons of standard bombard ammunition is still beautiful. A standard bombard ball, for example, costs 35 gp and weighs 30 lbs.

• Make a portable, no-assembly-required siege engine. An initially mundane assembled siege engine can "be shrunk and expanded an indefinite number of times" with the spells shrink item and permanency. However, "only by the original caster" of the spell shrink item can issue such commands. Thus a gunslinger must find a way to generate the initial shrink item spell effect, maybe by making a Use Magic Device skill check to use a wand of shrink item (DC 20) or by borrowing, for example, an ioun stone (vibrant purple prism).

  A strict GM may mandate the gunslinger must generate both the shrink item effect and the permanency effect, in which case the gunslinger may be better off just shoving the siege engine (ahem) whole cloth into a portable hole.

Be aware that by using siege engines you're playing a minigame that only you alone at the table might understand. Be sure the GM wants you to play that minigame before committing to it!

Yes, there is a weapon enhancement that is capable of increasing the crit range of a bludgeoning weapon.

It is a slightly more indirect way, though. As you know, the Improved Critical feat can achieve this effect. And it just so happens that there is a weapon enhancement that gives you a bonus combat feat.

The weapon enhancement I'm talking about is Training, from the Inner Sea Intrigue book. Its price is the equivalent of a +1 bonus, just like Keen.

Here's the full text:

Price +1 bonus; CL 3rd; Aura faint transmutation

DESCRIPTION

Popular among those who seek to impersonate skilled warriors, a training weapon grants one combat feat to the wielder as long as the weapon is drawn and in hand. The feat is chosen when this special ability is placed on the weapon. That feat cannot be used as a prerequisite for any other feats and functions for the wielder only if she meets its prerequisites. Once chosen, the feat stored in the weapon cannot be changed.

CONSTRUCTION REQUIREMENTS

Cost +1 bonus Craft Magic Arms and Armor; Spells magic weapon; Special creator must have the chosen combat feat and its prerequisites

The only downside of this method is that you still need to meet the feat's prerequisites, which in this case is proficiency with the weapon and +8 BAB.

The upside however is that if you are dual-wielding identical weapons, only one of them needs to have the enhancement.