Piston bounce:
When is a pellet “JUST RIGHT”? – Part 1

by B.B. Pelletier

Boy, did we ever hear from a lot of readers on this topic! I knew there were questions, but I had no idea so many people wanted to know this stuff.

We ended after the discussion of the pellet starting to move too early and too late. The ideal is for the pellet to start moving when the piston is almost fully forward, so the air cushion stops the piston and the pellet is pushed as hard as possible. The remaining bit of air that will be compressed as the piston settles to the end of the compression chamber only serves to sustain the air pressure an instant longer once the pellet starts moving.

Many shooters do not appreciate that a spring-piston airgun does an incredible amount on work of just the smallest bit of compressed air. That’s why all acceleration is over so quickly, and the pellet just coasts down the barrel the rest of the way. So, a shorter barrel on a springer isn’t a velocity disadvantage until the barrel gets below 10 inches, or so. The exact optimum length differs with every powerplant and with each caliber, too. However, that doesn’t mean that a longer barrel is bad. Shortening a springer barrel is also counterproductive, because the friction in that length of barrel where the pellet is not accelerating is so minimal that it’s nearly meaningless.

Can a pellet be too light?
Of course! The proof is felt cleaning pellets that are much too light for most spring rifles. It takes five of them to adequately cushion a Beeman R1, and I wish you good luck trying to load that many. Any less and you get a dry-fire detonation. Super lightweight pellets can also be harmful in powerful springers. I would avoid them altogether.

This is the reason I do not advocate the use of synthetic lightweight pellets in spring airguns. They’re too light to adequately cushion the piston, and their synthetic skirt material is too easily engraved by the rifling. All of this adds up to pellets that move too soon.

A real reason to buy a chronograph
Finding a pellet that performs well in a spring gun means you need to know the muzzle energy each pellet is developing. For that you need a chronograph. I’ve tested spring guns that delivered 16.5 foot-pounds with one 7.5-grain pellet and 18.75 foot-pounds with a different 7.5-grain pellet. The only way to know about huge discrepancies like this is to know what kind of energy each pellet is developing.

What about tight-fitting pellets?
Robert Beeman used to liken a pellet to a champaign cork that pops out violently when the resistance of the cork is overcome. I have tested tight-fitting pellets by seating them flush with the breech and also seating deep into the rifling. The velocity varies, but not by much – maybe 20 f.p.s. or so. Remember, too, once a tight-fitting pellet is inside the bore, it isn’t a tight-fitting pellet any longer. After the rifling has engraved the pellet and the bore has sized it, the friction drops to very little. Tight-fitting pellets do affect the velocity in pneumatics and CO2 guns to the extent that sometimes a too-tight pellet that won’t move can create a backpressure wave that holds the firing valve open and exhausts all the air or gas.

So, Crimson Sky, that would be my answer to your question about whether or not to shoot heavy pellets in your Diana 34. As you can see, there is a lot to consider.