Adapter with rubber O-rings to help seal the barrel. Comes in different sizes to fit various inner diameter speargun barrels. Available in 25.5 mm (Aimrite), 26mm (MannySub, Pathos, Cressi, Picasso, Hammer Head, Meandros), 26.5mm (Rob Allen, JBL, Mako). Double check the barrrel inner diameter measurement of your particular gun prior to ordering.