I was at the range yesterday (Cow Mountain Rifle Range, Ukiah, CA) trying to see what pellet my QB78 liked best. Since it's not a managed range, I wasn't able to set targets, but there were targets on the range to shoot at. Temps were close to 100, and wind was very mild.
Rifle: QB78, .177 cal, XP mod kit from Archer.
I picked up some JSB Exact RS pellets (7.33 gr) to try, as well as Crosman Premier Heavy (10.5 gr). I have already ruled out wadcutters for this range (50 yards), they fly all over the place. The pointed pellets too go all over, not as bad as the wadcutters though.
I tried the JSB's first, and my point of impact was around 1 foot lower than point of aim, and kind of wandering around. I kept trying, thinking maybe I was running out of CO2, however I was not. Even still, I ran the gun down & installed some fresh cart's and tried again, with the same results.
I went to the Crosman Premiers, and Point of Impact was right at Point of Aim. Huh? Again, I shot several more, and the Premiers were right on target.
My question comes from the JSB's. I have searched, and it seems a lot of people mention that their gun likes "JSB Exacts", but looking at say Pyramydair's website, I see 3 weights, the lightest (and the ones I was using) being the Exact RS.
What weight are people referencing when they say they like these pellets?
I'm tempted to get the Crosman Premier lights, however I see they only come in a box of 1250. That's a bit much to see if my gun likes them. Or, should I just stick with the Crosman Premier Heavies...
PS so far my 1377 likes these JSB Exact RS pellets... but I've only used them at low power (3 pumps) and close range (20 feet).
You mention that the temperature was about 100 degrees F. In my testing, I've found that valve lock on these guns takes place at about 96 degrees F. Above this temperature, muzzle velocity falls dramatically - and with it the point of impact, of course.
At a guess, it may be that your QB78 was suffering from valve lock when shooting the JSBs. Running down two sets of Powerlets may have cooled the gun enough for it to have stabilised below 96 degrees - even if the ambient temperature remained higher - and the Premiers were thus shooting at full muzzle velocity and to the correct point of impact. At this temperature, your QB78 was probably shooting the Premiers at close to 800fps with the XP Tune Kit installed.
This could at least explain the elevation change in the point of impact.
Archer Airguns Inc.