To help finding the right size when you need to replace o-rings on the Walther LG300, LG300 Dominator and LG400 air rifles, below are the sizes of these o-rings, their position in the drawings that Walther publishes and their article numbers for ordering from Walther.
Position numbers refer to the position in the LG400 exploded view drawing at the bottom of this page. LG300 o-rings are the same size in the same position. The drawing can also be found on the Walther website ( http://www.carl-walther.de/fileadmin/uploads/media/partsdrawings/LG400_System_2777398_g.pdf ). Article numbers are the numbers Walther uses when you need to order parts. Schiesssport Billharz can help you out when you want to order original Walther parts.
O-ring on the pressure cylinder where it seals on the regulator filling probe is 7x2mm (not in any drawing)
O-ring on the pellet probe to barrel seal is 4x1mm (pos. 85) [article no. 2472309]
O-rings (2x) on the barrel that seals around the barrel in front and rear of the transfer port is 9×1,5mm (pos. 77) LG300 ONLY [art.no. 2472295]
O-ring on the plunger of the regulator is 6x2mm (pos. 88) [art.no. 2490021]
O-ring around the brass disc of the reg is 13x2mm (pos. 89) [art.no. 2654181]
O-ring that seals the housing of the reg is 28x2mm. LG300 regulator ONLY. [art.no. 2600269]
These three o-rings in the regulator are made from a green shiny material. Harder material than the other o-rings on the rifle. My guess is that these are polyurethane o-rings.
O-ring around the barrel shroud in the front barrel clamp is 16x1mm (pos. 86) [art.no. 2661900]
O-ring that seals the valve seal 10x2mm (pos. 87) [art.no.2761823]
O-ring that is on the rear of the pellet probe (absorber side) 4×1,5mm, 70 shore, EPDM. (pos. 66) [art.no. 2787938]