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Tuning Benjamin Kratos .25 Eating o-rings

anyone else having problems with their Kratos? I just bought this rifle a month ago. just finished my first box of NSA slugs. I've replaced the O-rings 3 times ( all the spare Orings came with the rifle)... its only the o rings around the barrel on both sides of the port. slugs keep getting stuck in the muzzle of the barrel and air hisses out of the breach. that's how I know another O ring took a poop..... can't find sizes to match.

Slugs shouldn’t be eating the O rings in 20 shots though




Feb 5, 2021
FL, United States
    a symptom or causing it ... its a pellet gun way i see it, use non standard stuff and theres high probability of problems that need sorting ..but first thing id do is polish up the oring grooves and knock sharp edges off of them and around the barrel hole with 600, they look cut .. dont go crazy, just wipe things down carefully a few swipes concentrating on edges ..
    Outer o-rings that look like that are from the transfer port cutting into them, usually from the barrel being removed. The fix for that has always been sanding the edges on the port (often tricky) Inner o-rings being ripped or torn cause the hiss during the shot. Air comes back out of the chamber at the moment the shot is fired and cause a drop in velocity. Are these inner (1) or outer (2) o-rings?
    ill use 600 grit on them. that's not a problem. finding Orings locally has become an issue. Home Depot and ace hardware only care standard sizes. not metric. if y'all could guide me on sizes, id be much appreciated. I'm new to the PCP world. I've been reading the slugs transfer energy better at 60 yards vs pellets. that's why I'm shooting slugs. My city limits has me limited as to what I can shoot. I'm an AR guy. cops wouldn't like me plinking head shot with a 223 so pellet gun it is, since its considered a Childs toy. 
    The crosman schematics call for a size 12.6mm OD X 1.5mm CS (1.5mm CS X 9.6mm ID in more standard format). I do not believe it is correct though. Spares I ordered with a 25 cal Kratos barrel measured close to 1.5mm CS X 8mm ID. They are a stretch fit, and the ones I removed from barrel measured 1.5mm CS X 8.5mm ID.

    IMO: The O-rings are getting sliced on the installation and you don't notice the leak until the slug jams in the muzzle. The location of the slices on the barrel should give you a good idea where the sharp edge is. Deburr & clean. Lubricate O-rings well with silicone oil and when installing, rotate the barrel back and forth (about a 1/4 turn) while you are pushing it in.

    PS: The 25 barrel I received has a very tight choke. Not surprised that slugs are jamming on it.
    Yes I was surprised on how well slugs were doing. I’ve never shot a pellet out of this gun yet. As I posted earlier, I just finished my first box of slugs.. I ordered some 26.39 grain JSB to try. Maybe the tolerances are just a tad to tight to perform with slugs. I was also shooting NSA 33 grain slugs. Maybe a lighter slug won’t blow out the seals. I’ll test once I find the rings needed. Home Depot, lowes and Ace only carry standard here in south Florida. 


    I think metric is hard to find in most local hardware stores. None around me either. Many SAE sizes are hard to come by, too. 

    Below you will find pictures of JSB Kings that were pushed through my 25 Kratos barrel. It took a bit of effort to push the soft lead pellet through the choke. You can see that the pellet head has been resized. The 22 barrel I checked, wasn't as tight. Did you slug the barrel and check for size?


    If it's happening to both orings it is the top front barrel retaining screw hole. It's the only one that intercepts both orings as the transfer port and rear retaining screw hole are behind the furthest oring. The ammo has zero affect on the outer orings. The other issue is are slugs getting stuck while shooting and not exiting? Because that's a different problem all together. 


    Jul 8, 2018
    CA, United States
      Tip for next time-

      If ever you you buy a gun that comes with an o ring kit, or if you get lucky and find a replacement kit for that gun somewhere, make that the master set and pull dimensions from each and every o ring(specifically the I.D and the thickness), make your list, and order replacement o rings from the many o ring vendors out there. I favor oringsandmore.com.

      then you’ll never be in this predicament. When you get your first order you’ll see approximately 25-50 o rings of each size, as that’s how they are sold. You’ll have enough o rings to re seal that guns lifetime 
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