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.

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

  • Bob Says:

    So while you’re talking to the head of RWS (or any other of the manufacturers) why not ask them why they don’t put a min/max pellet weight reccommendation in their manuals?

    Seems like that would be a good idea for them to save on warranty work and frustrated consumers.

  • Tenuc Says:

    Hi BB,

    I thought that it was the tightness of the pellet in the breach that had the major effect on springer timing, rather than its weight?

    All the Best

    Tenuc

  • Crimson Sky Says:

    This is good stuff. Understanding how these mechanisms work (and operate at their best) helps us to become better shooters. You really have to be intimate with the airgun, meaning that you must know your specific model’s quirks, limitations and potential. Reaching that almost Zen-like meditative state while target shooting most surely requires this…and is the part for me at least that is the most fun! Thanks B.B. for the great info.

  • B.B. Pelletier Says:

    Bob,

    That would be great, but it assumes the heads of the manufacturers know anything about the guns they make. Most often, they do not. Witness the launch of the Gamo Viper Express.

    B.B.

  • B.B. Pelletier Says:

    Tenuc,

    Tightness does play a part. How much I’m not sure, because sizing a pellet (making it less tight at the breech) only affects velocity a little.

    B.B.

  • Anonymous Says:

    B.B.,

    While a pellet that is too light can result in lower velocities, can a too light pellet also damage a springer in a fashion similar to dry firing? That is, can a too light pellet fail to slow the piston sufficiently?

  • Anonymous Says:

    Hey BB, Great blog lots of info!
    Could you tell me what the psi is in the Air reservoirs at max no. of pumps of pneumatic air guns such as the Benjamin Sheridan 392s or the old Crossman 1400s ?

    Any Info would be great!
    Thanks
    Solucien@neo.rr.com

  • Jim Baxter Says:

    Question, slightly off topic. I have an old Sheridan and the pin on the bolt that catches the cocking piston has worn such that it slips off the intended ridge on the piston. How do I get the worn pin out of the bolt shaft? Thanks, Jim Baxter

  • Tenuc Says:

    Hi BB,

    Is it true that heavy pellets produce higher energy in PCP and CO2 guns, while light pellets give higher energy in springers, provided both have good fit to the barrel?

    ATB,

    Tencu

  • B.B. Pelletier Says:

    Pellets that are too light?

    Tomorrow.

    B.B.

  • B.B. Pelletier Says:

    Max pressure in a precharged pneumatic rifle,

    W.H.B. Smith did a test to determine this in his book, Gas, Air and Spring guns of the World. He came up with 1,200 psi, as I recall.

    Dennis Quackenbush did the same and got 1,000 psi.

    Of course any test gauge you attrach to the gun increases the reservoir volume and must be accounted for.

    B.B.

  • B.B. Pelletier Says:

    Jim,

    That’s a question for an airgunsmithy. Try Rick Willnecker. Contact him at airgunshop@aol.com or call 717-382-1481.

    B.B.

  • B.B. Pelletier Says:

    Tencu,

    Generally speaking, it’s true.

    B.B.

  • Josh Says:

    A little off topic…

    While we’re talking about ideal pellet weights, I’ve got a question about ideal pellet speed. I remember you mentioning that 700-800 fps is the ideal spead in your optinion. Why is that?

    -Josh

  • B.B. Pelletier Says:

    Josh,

    I would increase the top end to 900, but the low end can stay the same.

    The reason for those velocities is taken from the sport of field target, which is the most demanding accuracy game I know of for airguns. And the British shooters are the best shooters, as a group. They shoot pellets from 700 to 800 f.p.s. because of their 12 foot-pound law. The best American shooters shoot from 825 to 900.

    These shooters can adjust the strike of their pellet by a half-pellet diameter at 20 yards. No other shooter comes close to that precision. For that reason, the extreme long-range accuracy of the top field target shooters, I choose the velocity spready I did as the best.

    B.B.

  • dm20 Says:

    something to think about- a semi-auto springer! loud, i would think, but it would be great to see one in action

  • dm20 Says:

    sorry, forgot to add, i’ve backed off on replacing the spring. i just dont have the confidence to dive into such a mechanical wonder. then, is it okay to shoot it with the mainspring still broken? i’m really enjoying the lighter cocking effort and slightly improved trigger. i dont mind if the power has degraded. with about 50 shots per session, about 3 times a week, how long do you think it will be until it degrades so badly it stops penetrating carpet? i still want SOME power, of course.

  • tatohead Says:

    Hi BB,
    This is a little off topic, but hopefully you can help me. I’m looking for a fiber optic front sight for my RX-2, to go along with a Williams peep I just purchased. I’ve been informed that Weihrauch has TRUGLO sights for the following rifles, Fiber optic sights HW 25, HW 30, HW 50, HW 57. Do you know if any of these have a front sight that mounts the same as the one on the RX-2?

  • tatohead Says:

    P.S. And if one does fit, where can I purchase it? I can’t find any info on these TRUGLU sights other than from the Weihrauch website. Which unfortunatly has very little info about them. Also, do you know of any other brand of fiber optic sight that would be a direct fit on an RX-2?

  • B.B. Pelletier Says:

    tatohead,

    Your question is too specific for me. Why not call Pyramyd Air and ask them? They help customers with questions like yours every day.

    B.B.

  • Anonymous Says:

    BB

    re:IZH61

    Pardon for interjecting. The IZH61 continues to be great!!! Year later even after adding about 3 dozen to the collection. Still one of the most fun to shoot!!
    Wish they had a 22.

    maintenance- Just shoot its Russian!!!!

    pellets- Almost used bolts by accident, Fires almost anything well. Must be its heritage.

    Anyone who misses out when discontinued will kick themselves.

    Thanks,

    Ray

  • B.B. Pelletier Says:

    Ray,

    Thanks for that feedback. As an owner, your observations carry a lot of weight with those who are shopping.

    B.B.

  • Anonymous Says:

    A very interesting blog post, but now you have me worried about some of my pellets. Do you think that RWS Hobby’s at 7gr are safe for an RWS 46, which is rated at 950 fps?

  • B.B. Pelletier Says:

    Yes, I think they are. I wouldn’t use them in a 48/52/54 or a 360 Magnum, but in your rifle I think they work. That’s about as light as you should go.

    B.B.

  • kodiak Says:

    Hello every one, I’m new to the Airgun world and have a couple of questions if anyone would be interested in giving me some pointers. So here I go; I have purchased a Winchester 800x w/o a scope. To be used for varmit elimination, ie: chipmunks, squirels and an occational raccoon.
    1) What would be the best Pellet/Manufacture for eliminating these critters?
    2) What cleaning materials are prefered for an AirRifle with a break barrel that won’t damage the seals, mechinisms or rifeling?

  • B.B. Pelletier Says:

    Kodiak,

    Your rifle will probably perform best with JSB .177 Diabolo Exact 8.4-grain pellets.

    Because you have a .177 and the velocity is not very high, I would advise you not to shoot at game larger than a cottontail rabbit. Limit your range to the distance at which you can shoot five shots into a quarter-sized group.

    B.B.

Leave a Reply


+ 7 = 15

Top-notch springer
Air Arms TX200 air rifle

When it comes to spring-piston air rifles, the Air Arms TX200 Mk III is a favorite of many airgunners, including airgun writer Tom Gaylord. His favorite caliber is .177. While the gun will initially impress you with its beauty and superior craftsmanship, you'll be even more impressed with the incredible accuracy! Tom claims this is "the most accurate spring gun below $3,000." Beech or walnut, left-hand or right-hand stock. Isn't it time you got yours?

All the fun, none of the hassles!
Uzi CO2 BB submachine gun

You've seen tons of movies with guys spraying bullets from their Uzi submachine guns and probably thought it would be a blast. Except for the cost of ammo! You can have all that fun with this Uzi BB submachine gun at just pennies a round. Throw shots downrange for hours on end with all the fun, none of the firearm hassles and a fraction of the cost.

Archives