PCP Dwell vs. Caliber

I just had one of those "AHA!" moments while writing another post.... It's probably one of those things that lots of guys know, or at least allow for (I did), when tuning a PCP, but until moments ago, I hadn't really organized my actions into words, so here goes....

When you change the caliber, you need to make a corresponding change in the dwell (via hammer strike).... This is necessary to compensate for the larger (or smaller) bore volume.... It's a simple concept, but one I had somehow glossed over.... Here are my thoughts on the matter.... Let's take the three common calibers, and a barrel length of 24".... The bore volumes are:

.177 - 9.7 cc
.22 - 14.5 cc
.25 - 19.3 cc

For the moment, let's assume we start with a .22 cal PCP that is tuned for a beautiful, classic bell-curve starting at 2500 psi and down to 1500, all shots with a 4% ES, and peaking at 2000 psi.... What does that mean?.... Well, it means that the combination of valve dwell and pressure is balanced to produce velocities with that 4% range over that range of pressures.... At 2500 psi, the pressure is higher, but the valve is not opening for very long, so the volume of (higher pressure) air is smaller.... As the pressure drops, the valve dwell increases, and the valve releases more air but at lower pressure.... Eventually, the volume peaks, so does the velocity, and then as the pressure drops, the velocity declines with it....

Now let's assume we take off the .22 barrel, and substitute a .177 barrel.... It only has 2/3rds the volume, and yet our valve and hammer are balanced for the larger bore volume of the .22 cal.... Now that small amount of 2500 psi air is more than needed to make the velocity stay 4% below the peak it will be at 2000 psi.... and MUCH more than is required to balance the velocity at 1500 psi.... In fact, it may be enough volume so that the first shot (at 2500 psi) is the fastest in the string.... In order to rebalance the shot string over the same range of pressures, we need to REDUCE the hammer strike....

The reverse occurs if we install a .25 cal barrel.... Now the bore volume is 1/3rd larger than the .22 cal barrel, and the (relatively) small amount of 2500 psi air isn't enough to push the pellet to within 4% of what it will be at 2000 psi.... In fact, with the larger caliber, we would expect the peak of the velocity curve to move to a lower pressure, possibly down into (for example) the 1700-1800 psi range.... To rebalance the shot string over the original range of 2500 down to 1500 psi, we will need to INCREASE the hammer strike....

The same thing will happen in a regulated gun.... Most of you realize that I am a proponent of balancing the hammer strike against the regulator setpoint so that the gun is operating on the "knee" of the velocity curve.... which is basically the same thing as always operating at the top of the sweet spot (velocity peak) in an unregulated gun.... If you don't change the hammer strike and go to a smaller caliber, you will be moving up onto the "plateau" of the velocity curve, and therefore wasting air.... so reducing the hammer strike won't lose velocity, but will increase shot count.... When going to a larger caliber, you will be operating on the downslope of the curve, and increasing the hammer strike will move you closer to the "knee" and hence increase the velocity.... both of the above without changing the regulator setpoint....

I hope I've made the above argument clearly and understandably.... Sometimes things are obvious, and adjusted for, without knowing (or articulating) WHY they occur.... For me, until this morning.... that was the case....

Bob