Air Gun Home :: View topic

Mentolio · Veteran Member

I'm fairly certain I read somewhere on these forums (though I can't for the life of me find that post now) that if a PCP power curve starts at it's highest velocity with the first shot, and then decreases with each additional shot, that something is "wrong." I started playing around with my Marauder because I noticed an accuracy issue, and found when I Chrony it the velocity is highest with the first shot, and is "all downhill" from there. Is this normal for this model?

My MRod started life as a .25 caliber. My home state then decided not to allow any caliber larger than .22 for hunting. I replaced the whole "top end" (barrel, breech, bolt, transfer port sleeve) with .22 equipment, but did not replace the valve with the .22 cal version (different part numbers, but no other info). The rifle seemed to shoot and group well with several different pellets, tho I only shot lightweights at the time (14.3-18gr pellets). When I noticed it grouping poorly with pellets it used to like, and the aforementioned steady decrease in velocity from shot one; I started playing around with the valve, hammer tension, and hammer travel.
After dialing all of the above down quite a bit, it groups better, but still has the steady decrease in power. No "arc," just a downward "ramp." Should I be worried? It's not as accurate as it used to be, or should be, but I don't know if this could still be a "tuning" issue, or a problem with the "hardware" (.25 valve I'm using). Could the .25 valve just be a bad fit for the .22 combo, or is that ridiculous? Also of note: I doubt I have more than 3 or 4 hundred rounds through it (.25 and .22 combined). Could this just be breaking-in?
Any advice would be greatly appreciated. My search attempts seem to only yield the same results, no matter how I word the query, so I apologize if this kind of question is nothing more than an annoyance.

rsterne · Moderator

You need to back off on the hammer spring preload until the first shot is about 3-4% slower than the fastest shot.... The goal is to tune for a bell-curve that starts 3-4% below the peak, and then stop shooting and refill at 4% below the peak as well.... That will give you minimal vertical POI change out to about 50 yards.... If you shoot a lot at longer range, you wan to tighten up the % variation (ES) to 3% for 75 yards, and 2% for 100 yards....

Bob