Everything you can equip. The Holster hotkey is "H" and can hide any Handgun.
The Springfieldrifle and its possible modifikations.
The Remington rollingblockrifle and its possible modifikations.
The Springfield rifled Musket, Model 3 Revolver, Colt Single Action Revolver, Pump Action Shotgun, Colt Navy Revolver and sawed off double barrel. Those have no modifikations.(yet)
The blue Mailbos to sell canned goods or any ingots, the catalog and its paced down version to buy stuff and the money to do so.
The 1866 Winchester and possible modifikations.
Revolverbullet and its possible modifikations and storage item.
The Volcanicpistol and its possible modifikation.
Everything you can Eat, Drink or Smoke.
The .22 Varmint pump action rifle and its possible modifikations.
The Sharp weapons.
Doublebarrelshotgun and its possible modifikations.
All other bullets and if possible their storage item.
The Spencer Carbine and its possible modifikations.