Possible Clues:

• Breaking of the law, say, concerning gun part
• Brother at ease with alpine country from behind gun
• Brecht replaces troop leader with engineer found at back of gun
• Musket part
• Place to load a bullet
• Disagreement, we hear, in one form of birth
• Part of a gun
• (Half of) lower garment
• Back of barrel runs into tree
• Stern king concealed in a tree