Why are first shots different?
by B.B. Pelletier
This post came from a comment left by K. Rihanek, who was surprised that the first shot from his spring gun was so different than the rest of the shots. In fact, this is a phenomenon that applies across the board, and not just to airguns. But, airguns are all that I will address.
Starting with springers
The first shot from a spring gun usually goes faster than the ones that follow. In fact, I can't remember a time when I saw anything else, though I have heard that in extreme cold, springers do need to warm up before reaching their normal velocity. Anyway, a faster-than-expected first shot was what our reader had noticed.
Some people think that when left to sit, air rifles will weep oil at the piston seal. Either that or the first shot will not inflate the piston seal fast enough, resulting in a slight detonation that boosts velocity. After that, the seal is flexible once more and velocity will remain within the expected range. But, put the gun aside for a length of time (12 hours or longer), and you're back with the first shot phenomenon again. I believe in the second explanation (stiff seals) over the first (weeping oil), because even guns that are not greased in any way will display this phenomenon.
There's not much that can be done to prevent this, but hunters and field target competitors will always shoot several shots to get their guns up and running before they go to work.
Pneumatics are not immune to the first shot phenomenon, either. Precharged pneumatics are the worst, with regulated guns being the absolute worst of all. They always need a first shot to "Wake up the regulator." Guns with adjustable power, such as the AirForce rifles also need a first shot after a power adjustment before they will shoot as expected. In my experience, they always shoot the first shot after an adjustment on the side from which the adjustment was made. In other words, if they were adjusted from faster to slower, the first shot will be faster. If adjusted up in velocity, the first shot after adjustment will be slower.
Even multi-pumps and single-strokes show a difference with shot No. 1. It seems they also need the exercise of going through the motions of firing before they can settle down and shoot normally. I think that they are no different than the spring guns, in that their seals need to be exercised, however, with these kind of guns the first shot after adjustment is usually slower than expected.
If a gun was going to be immune from the first shot phenomenon, you would think it would be a CO2 gun. But, again, they do display it. In their case, the first shot is usually slower, though the temperature has something to do with it, as well. If it is very cold, the first shot might be very fast, and the follow-on shots might all be slower, because the gun is cooling and cannot recover.
This is a tip I wanted to pass along to all of you. I always shoot several "wake-up" shots before expecting the gun to perform as expected. I thought everyone did the same, but from the reader's observation, I'm now guessing they don't.