Replacing Hatsan Hercules bully breech oring seal

Yesterday the breech o ring on my bully popped out... I noticed when I went to shoot, I got a nice blast of air in the face!

Has anybody had the honor of replacing one of these O-rings? I tried to do it but the breech is very tight and does not give much space to work. I tried to put the O ring around a pellet in order to have it ride in on the projectile and seat it but that did not seem to work. I'm worried I may have to take it apart. 

Anyone have any luck with this?
 
What caliber? Most Hatsan barrels up to 30 cal you can just unscrew the end cap of the barrel and replace the o-ring.
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