How do arcane pool, mage bullets and similar abilities interact with specific magic weapons?

Question

Specific magic weapons have their own unique properties and generally cannot be improved further with item creation feats. Specific magic weapons all come with an enhancement bonus and sometimes a few standard special abilities, of which the equivalent cost can be calculated.

A few classes have the ability to temporarily add extra enhancement bonuses and special abilities to a weapon (magic or mundane), the magus's arcane pool and the spellslinger's mage bullets are two examples. Both follow a common rule : a weapon enhanced this way cannot have a total enhancement bonus above +5 and cannot recieve special abilities that would increase their equivalent enhancement bonus above +10, the same restriction applied to magic weapon creation.

But is it possible for a magus or a spellslinger to enhance a specific magic weapon with their respective class features ? If so, how does it work ?

Answer 1

Specific magic weapons have their own unique properties and generally cannot be improved further with item creation feats.

This is false; they can be. It is often difficult to determine exactly what the cost of doing so should be, but that’s a problem for the player and GM to work out together. The point is that the rules do let you do this.

See Upgrading Magic Items:

For specific magic armor and weapons, the price for the base item may be hard to determine, as some abilities may have been priced as plus-based properties and some as gp-based properties. Without knowing which is which, how to increase the price (using the plus-based table or flat gp addition) can’t be determined. If this happens and nobody can agree on a fair price, it’s best to not upgrade the item, or ask the GM for permission to pseudo-upgrade the item by swapping it for a different item with a price that can be calculated with the normal rules.

As for abilities like arcane pool, they do not mention any limit at +10. However, hiding in a parenthetical in the magic item rules is a single line stating

A single weapon cannot have a modified bonus (enhancement bonus plus special ability bonus equivalents, including those from character abilities and spells) higher than +10.

The “including those from character abilities and spells” here is a terrible rule. You should ignore it. Everyone should ignore it. Any time it comes up and is enforced, these abilities are badly nerfed—at the highest levels, when you already have a +10-equivalent weapon, these abilities become entirely worthless.

But even if you do use this rule, it still doesn’t act any differently for a specific magic weapon than it does for a regular one: you can still use it, you just have to come to an agreement with the DM about which parts of the weapon are due to enhancement-equivalent effects (and thus count towards the limit) and which come from flat costs (and thus do not count towards the limit). And since you do not have to actually price things, just be mindful of the limit, for the majority of the game you won’t even have to consider it, because your weapon will clearly be well under the +10 limit even if everything about it is considered enhancement-equivalent.

Answer 2

These abilities stack with the specific weapon's enhancements.

Enhancements from arcane pool and mage bullets stack with the weapon's existing magical enhancements as long as the ability specifically says they stack, as arcane pool below.

These bonuses can be added to the weapon, stacking with existing weapon enhancement to a maximum of +5.

Whether or not the weapon is a specific magic weeapon is irrelevant. Even the bladebound magus's black blade can be enhanced using arcane pool (see this FAQ).

As KRyan points out, no class ability can enhance a weapon beyond a maximum +10 worth of enhancement bonuses and special abilities. Mythic Adventures imply this ceiling can be raised using mythic rules.

Specific magic weapons can be modified.

The question carries a misconception that you cannot use magic item creation feats to modify specific magic weapons. This is not the case.

You can use magic item creation feats as well as class abilities to modify specific magic items. The magic item creation rules in the core rulebook has a section on adding abilities to existing magic items. You can do so by paying the difference in price.

Sometimes, lack of funds or time make it impossible for a magic item crafter to create the desired item from scratch. Fortunately, it is possible to enhance or build upon an existing magic item. Only time, gold, and the various prerequisites required of the new ability to be added to the magic item restrict the type of additional powers one can place.

The only magic weapons that cannot be modified are artifacts or special weapons provided by class features that have pre-determined magical enhancements. The bladebound magus's black blade is the most obvious example. You cannot use magic item creation to modify a black blade because the class feature determines its enhancement bonuses and magical abilities. However, even such weapons can be enhanced temporarily by features like arcane pool.