Piston bounce: When is a pellet “JUST RIGHT”? – Part 2

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.

18 thoughts on “Piston bounce: When is a pellet “JUST RIGHT”? – Part 2”

  1. BB, in yesterdays comments you stated the optimum speed for pellets was 800-900fps for accuracy.

    I was thinking about getting the IZH61 as an inexpensive yet dedicated target gun. It has a lower velocity although your own review states that it is very accurate.

    My real question is do you think that it has the energy for silhouette shooting? My real concern is knocking down that ram at 40 yards.


  3. Hi BB,

    I’m pretty new to all of this, and your blog is a fantastic place to learn, thank you! I have recently acquired a Crosman SSP 250, and also an IZH61. I want to add a scope to these. I need to understand the basics. How do I make the choice, what do I need? Would a red dot sight be better? Have you written about this? Or has anyone else on the net? I will be shooting 10M targets in the backyard.

    Thanks. Mazz.

  4. BAM 40,

    I’ve thought about reviewing this rifle. It’s a copy of the TX200, which puts it in a very precarious spot. The TX is the best spring rifle in the world, right now, and the BAM will have to be extremely good to compete with it.

    That said, the BAM 40 looks very good on the outside. But it is the inside where the Chinese have failed to impress me over the years.

    On the other hand, we might discover a new rifle in the Gamo CF-X class. I didn’t have high hopes for that one and I was pleasantly surprised. If a Chinese air rifle could get up to that level it would be worth consideration.


  5. Mazz,

    The SSP250 requires an internount that clamps to the barrel. It will provide a dovetail base for any 11mm scope rings or sights with integral dovetails to clamp to.

    The 61 will be much harder to scope, because it doesn’t have a good way of providing a base. I hope some of our readers have experience with this one and can offer assistance.


  6. Thanks B.B. for the great information. I’ve just had a great weekend of shooting with the Diana 34, and boy are my arms sore! The rifle proved to be too much for the B-Square 17101 scope mount–it was a miserable mechanical failure, just as I had expected. The iron sights were a much more pleasant experience. The built in scope stop on the mount even began to dig a channel onto the front of the rail after a few dozen rounds.

    Diablo sport wadcutters and crow magnum pellets proved to be the most accurate out of the selection I used thus far. cheers!~

  7. B.B.,

    You recommend using a chronograph to determine muzzle energy but I wonder if you could use penetration, say into a transparent bar of soap, as a gauge of relative muzzle energy for different pellets or are there too many other variables involved.


  8. Diana 34 and “The built in scope stop on the mount even began to dig a channel onto the front of the rail after a few dozen rounds.”

    I’ve been there (with a Diana 36) and I bet there are too many others to count who have also. Frustrating isn’t it?

    I like B.B.’s solution of hanging a stop pin off the front of the rail. Great idea.

  9. Hi BB, Thanks for the answer… I was after something more basic on the appropriate specs to look for in a scope for shooting close targets – 10M. I’ve been googling it and am I right that a 4×32 fixed, with adjustments down to below 10M is appropriate? Can you point to an example that you would suggest from Pyramidair? Thanks – Mazz

  10. Hi BB. I have a question about the Smith and Wesson pistols. I ordered the 8″ version of the Blued revolver. When I recieved it, the finish was hardly what I expected. It was not very shiny, and was actually rather splotchy, dull, clouded, and frankly it was ugly. It was no where near the mirror polished, deeply-blued finish I had expected (or that the pictures showed for that matter, especially for $230). It was bad enough that I decided to return it for an exchange or a refund. My question is, are all the S&W pistols’ finish like this? Or is mine just a fluke? If I could expect a better finish, I would simply exchange it for a better one. If the finish on all the pistols is not up to par (par being the picture on Umarex’s website), I would simply ask for a refund, or possibly exchange it for a nickle plated version. Any thoughts or sugguestions would be greatly appreciated. Thanks BB.

    PS. If I do ask for the refund, which pistol do you think would be a better choice, one of the Benjamin Sheridan pistols (.22), or a custom Crosman pistol (you can do it on Crosman.com) with a 10.1″ .22 barrel, long steel breech and bolt, LPA mim rear sight, and a silver muzzle for the front sight (I think this would end up being similar to the 2300S). Would both be suitable for hunting small game such as squirrels and small birds (they would both be .22). Thanks again.

  11. Hi S&W 8″,

    My 6″ was exactly like you describe. I rubbed over it hard with a coarse cloth with a few drops some light mineral oil on it (3in1) and it came up a beautiful, deep shinny blue. On mine I think Umarex had put some waxy stuff on to protect the finnish and this made it look dull and blotchy. Try this before you send it back. These are excellent CO2 guns and I love mine.



  12. Mazz,

    The two best scopes for you are the Bug Buster I and II. The I is a 4x and the II is a 6X. Both focus down to 3 yards, the closest in the business.

    Look at the Leapers scopes. Both are compact (mini) scopes, so you must use two-piece rings.


  13. I have the SW revolver also. Great fit and finish. I use Breakfree CLP on a lint free cloth to wipe all my airguns and firearms. It does wonders on real bluing, but is probably not necessary on the Umarex finish. By necessary, I mean rust protection, it does make the finish look great!! Smells pretty good too. (Skip the product they sell as gun wipes– very wet and messy, and expensive)

    The finish on my SW is the same as my other Umarex products, they are all excellent. Follow BB’s advice, clean it first, then ask for an exchange if it is really defective.


    PS. Enjoy that pistol– it’s a great shooter!

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