Barrel arrangement is the position of the barrels, relative to one another, on multi-barrel guns.

related:

• over and under (O/U, O&U)
• side by side (SxS)

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.