Stock and velocity change mystery

[UDABEGGER]

Airacuda Max is a very accurate PCP for the money but I have made a discovery that will probable make me go mad or madder . You must remove the stock in order to adjust hammer spring . I do this and use a crony to get the velocity just right .Example 875 fps and install back into stock and depending on how tight the stock screw is I end up 20 to 40 fps slower .Just setting it into stock makes 10 or 15 fps diff . I was very happy not knowing this . Any thoughts can only be helpful . Or not !

 

[elh0102]

I can't imagine how the stock screw tension would have that effect. But, if it does so consistently, I guess you just have to make it part of your tuning process. The only thing that comes to mind, I would check to be sure that the barreled action fits the stock inletting without any binding or tension. I'm not sure it could have the result you describe, but I'd check for a proper fit.

 

[EquivalentDelta]

What is your SD and ES on a 20 shot string?

Sounds more like a statistics problem hiding behind a mechanical one.

 

[nervoustrig]

Tightening the stock screw can deform the tube and cause the hammer to bind. There is a spacer available to address this issue. Contact the manufacturer to get it.

Also, the stock needs a groove to allow the air cushion to evacuate...the air trapped by the hammer flying forward. A search should reveal some earlier threads on the topic.

 

[UDABEGGER]

I have the spacer in and tried without with the same results . SPENT most of an hour doing this with 20 plus shots fired . the out of stock # changed by about 2 or 3 fps . The stock to barrel action sounds possible . thanks

 

[UDABEGGER]

Sanding on stock did help a little . If I leave the spacer out and just snug stock bolt I can stay within 10 fps. The max has never stopped shooting 1/2 inch repeatable groups at 50 yds but still drives me nuts arr .

 

[JimD]

I also think your stock is distorting the tube with the hammer inside resulting in less force against the valve so less velocity. I don't know how these parts are shaped but I suspect that adding a spacer against one of the stock attachment screws will help if not eliminate this issue. My first choice would be a washer to go around the screw and between the action and the stock. If the surface is curved, you might have to make the washer out of aluminum sheet material. If the surfaces permit, you might even put a washer or two at each end to float the action away from the stock then try pulling a washer at one end and then to other. If you see improvement but not total elimination of the decrease you might have to make thinner washers to fine tune the pressure on the action.