Heirloom weapon trait

[Kirbdog]

Can you pick a firearm pistol with the heirloom trait?

[Lelomenia]

The current language of Heirloom Weapon restricts it a simple or martial weapons; pistols count as exotic, so no.

[Kirbdog]

Guns: While still expensive and tricky to wield, early firearms are readily available. Instead of requiring the Exotic Weapon Proficiencyfeat, all firearms are martial weapons.

[Weables]

if your GM is using those rules, then yes. but keep in mind those arent the default rules for the setting

[Taja the Barbarian]

Kirbdog wrote:

Guns: While still expensive and tricky to wield, early firearms are readily available. Instead of requiring the Exotic Weapon Proficiencyfeat, all firearms are martial weapons.

Each campaign can have a different level of Firearm development: The one you quoted ('Commonplace Guns') is NOT the default level for Pathfinder ('Emerging Guns').

Emerging Guns wrote:

Firearms become more common. They are mass-produced by small guilds, lone gunsmiths, dwarven clans, or maybe even a nation or two—the secret is slipping out, and the occasional rare adventurer uses guns. The baseline gunslinger rules and the prices for ammunition given in this chapter are for this type of campaign. Early firearms are available, but are relatively rare. Adventurers who want to use guns must take the Gunsmithing feat just to make them feasible weapons. Advanced firearms may exist, but only as rare and wondrous items—the stuff of high-level treasure troves.

You'll need to ask your GM which Firearm development level the campaign is using.