FX Royale 400 Troubles

Assuming it is the oring in the breech...
that the loading probe slips into and seals the air behind the pellet preventing the air from escaping out the breech,
I'm gonna say YES you will need to replace that oring......a blown breech oring should result in random lower pellet velocities maybe even a little air blown back in your face....noticeably loss of accuracy...

In general if a PCP has an oring, that oring is vital to the proper functioning of the gun....if it is worn, nicked, age hardened, missing, leaking or 
melting...it most likely needs replacing...

Call AoA 480-461-1113
or FX USA 866-639-0772