Piston bounce: When is a pellet “JUST RIGHT?” – Part 1
by B.B. Pelletier
Yesterday’s post about picking the right pellets brought a question I hadn’t anticipated. When I tried to formulate my answer, I realized it might make a very good blog. Here we go.
Question from Crimson Sky
I’ve read that using pellets that are beyond a gun’s power rating (e.g. a Beeman Kodiak heavy pellet in a Diana 34) can be detrimental. What truth is in this, if any? Just trying to avoid these beginner pitfalls. =) cheers.
An anonymous answer
I think the potential issue in using pellets that are “too heavy” for a given gun is that the internal pressure in the gun needs to get to a higher level before the heavier pellet shoots. If the gun isn’t designed for this higher pressure, there could be damage over time.
A Diana 34 is a powerful springer. I can’t imagine any reason not to use heavy pellets in this rifle.
That was a good answer
That would have been my answer, as well; but, thinking more about it, I decided to expand the answer. You seem to want to know more, and I’d like to tell you what I do know about this.
RWS USA used to recommend heavy pellets for their big springers
Tim Challener, the technical man at RWS USA, told many people that using pellets that were too light was a primary reason Diana springers broke in new guns. Everyone was wild about shooting faster than 1,000 f.p.s. in the 1990s and breaking springs were a real problem for the powerful Dianas.
The chat forums are loaded with talk about the performance of heavy vs. light pellets
I normally don’t read the airgun chat forums except for comic relief, but there are a few people who really do know what they’re talking about. Guys like Russ Best will tell you straight out the way things are, so I’ve learned to pay attention whenever he or a few other writers make their comments.
One topic I’ve seen in the past few years concerns heavy pellets and piston bounce, so I’d like to address that phenomenon today. When a spring-piston airgun fires, the piston is propelled forward by the mainspring. It compresses the air in the chamber in front of it. As it nears the end of the compression chamber, the air has been squashed down into the tiny volume of the transfer port and a few hundredths of an inch of remaining space in the compression chamber. At this point, the air pressure, which was ambient when the piston started, has risen to over 1,000 psi.
The pellet sitting in the breech of the barrel holds this pressure back for an instant. While it doesn’t stay still very long, it’s long enough for the air pressure to rise very high.
Depending on the weight of the piston and the strength of the mainspring, there comes a point at which the air pressure is so high that it stops the moving piston before it slams into the front of the compression chamber. That is the reason a well-tuned spring gun can last so long and also why dry-firing can destroy it so quickly.
Then the pellet takes off!
The pellet cannot contain the pressure that’s built up behind it so suddenly, so it takes off. As it moves, the volume behind it grows rapidly and the pressure of the air drops off. When the pellet finally exits the muzzle, the pressure has decreased to only a small amount above the ambient, which is why spring guns are so quiet compared to pneumatics and CO2 guns.
And here is the big deal
If you’ve read and understood how this activity takes place, you should begin to understand that the timing of the pellet starting to move is important to the efficiency of the air rifle or pistol. If it takes off too soon, for instance, the pressure never reaches its potential peak, and the velocity/energy of that pellet is lower than it should be. If it takes off too late and the piston cannot continue to move against the compressed air it’s created, it bounces backwards. Hence, the term “piston bounce.”
A piston that rebounds prematurely lowers the available pressure and also sends an impulse back into the mainspring that had almost reached its most relaxed state. The impulse is usually very low unless there is a detonation to compound it. If that happens, the piston can slam back far enough to even re-cock the gun in rare instances. But, the rebounding piston has lowered the available air pressure and that has an affect on the muzzle velocity of that pellet.
I’ll continue this discussion tomorrow.
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