The 123gn projectiles are to short unless you seat them as far out as possible then they could still be to short. Have used many but gave them away as a bad idea. The 303 is not a varmit calibre/rifle i tried. 150gn are minimum weight just, 170/215 best. Cordite worn barrels need longer projectiles. Unless you use a chronny your guess to velocity maybe way out the 123gn is light and the pressure will be lower depending on powder used.