Benjamin Kratos Velocity Change

[100acres]

I need some help with my Benjamin Krato 22 cal. I haven't shot the gun since last December. I had the velocity set to 870 fps with my chronograph. The velocity has stayed at 870 since I adjusted the gun when new a year and half ago until now. The power adjuster is set all the way open. I noticed the gun report sounded a little weak and the poi was almost 1" low. I pulled out my chronograph and sure enough the gun was shooting at 811 fps. I gave the hammer spring 3/4 of a turn and checked the velocity and it was back around 870 fps. As soon as I installed the stock and checked the velocity it was down to 845. If I remove the stock the velocity goes back up to around 870. I'm not sure what's going on here anyone have any ideas?

I have a JTS Airacuda Max in .22 cal. Before Travis (thanks again Travis) sent me the stock spacer I had a similar issue where if a vent hole was blocked in front of the trigger the velocity would drop when in the stock. On my Kratos i don't feel any air escaping from any of the holes under the action when I fire the gun. I don't think this is the same issue but I could be wrong. Thanks for any suggestions.

 

[Airgun-Revisions]

Remove the rear hammer spring cap and use a q-tip to lightly lube the hammer bore.

 

[Jaybird4328]

I have the same problem,stock screw out I gain 20-30fps.It has to be the hammer not venting.

 

[100acres]

I have the same problem,stock screw out I gain 20-30fps.It has to be the hammer not venting.

 

[100acres]

Thanks guy your suggestion makes sense. I'll try putting some light lube in the hammer bore and see what the results are

 

[Dr. Kralenstein]

Or the stock is pinching down on the trigger assembly some how and causing the sear to drag. I've had this issue on crosman 2240 and marauders before so it's possible.
Try installing it in the stock but not tightening the stock screw and check velocity. If it's 870, then tighten the screw a bit and test. If it drops, maybe try a spacer washer under the action.

 

[100acres]

I played around with the gun yesterday. I discovered a small hole under the action in front of the trigger. This appeared to be a vent hole for the hammer. If I put my finger over this hole and fired the gun the velocity would drop. A interesting note is I really didn't feel much air escaping from this hole when i fired the gun. I ended up cutting a grove inside my stock in the same area as the vent hole in the action to let the hammer vent better. This seemed to fix the problem. I'm not sure when this issue exactly began but velocity seems to be staying around 870 fps.

 

[Jaybird4328]

I got another reciever bolt and filed thread side down to relieve some pressure,it helped.If you shine a light down the reciever hole you will see the end of the valve stem and the end of the hammer.When I have the time I will see if I can get a spare hammer and vent it.