Tuning Energy vs. pellet weight. Possible air volume regulation

These threads will most likely answer your question.

chromvis said:

I measured muzzle energy for pellets with different weight. It is not surprise that lighter pellets show lower energy due to faster acceleration and leaving barrel much before full gas expansion. My question: Does anyone know of a rifle that allow adjustment of either plenum volume or hummer power for used pellet weight? This may allow for highr number of shots for lighter pellets.
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Many of our PCP’s are highly adjustable for various weights and types of projectiles. What gun are you shooting?

Many PCPs have a way to adjust hammer spring tension on the fly. Doing so will change valve lift and dwell to be more suited to lighter or heavier pellets.

I am not aware of any such on-the-fly adjustment for plenum volume. Granted, I can’t think of much good reason to have this feature since a smaller plenum can only harm efficiency, whereas adjusting hammer strike can improve it and is simultaneously easier to implement.

A good example is the implementation on FX rifles

...a numbered wheel with an eccentric cam on the reverse that applies more or less hammer spring preload when rotated.

You can time the valve event in several ways, most being spring rate oriented. The aha moment for me was using plenum pressure to assist on valve closure even tho I have a valve adjuster on my impact. It made the cycle more consistent using more pressure. Spring rate, hammer mass/travel, plenum pressure transfer port and projectile weight all pay in unison. The art is finding the best balance of them all at once.

On some guns it is fairly easy to change the valve return spring. A stiffer spring would close the valve quicker potentially making the gun quieter and use less air. On nearly all PCPs the hammer spring can be adjusted but it also needs to be balanced against the regulator setting. Lowering the regulator will support less hammer spring force but it will also mean the valve closes slower since the air pressure is the primary force closing the valve. It's all interrelated. But normally a lighter pellet needs less air to get up to speed so a lighter regulator setting is typically indicated. You could try doing the speed adjustment with just the hammer spring but my guns do not seem to like that much. The first shot velocity will be low.

Ive run into a bit of a quandary in this dept myself. I do have 700mm of barrel which masks valve timing errors a little easier, but i found my gun for any given setting just shoots fpe almost regardless of projectile weight so long as it's subsonic, and sometimes supersonic if I can measure it.

I have return springs for my guns and have just never messed with them. I suppose leaning on pressure is a bit of a lazy way to do it, but it's been working for me...... Which doesn't mean it's the proper way. I should add that I DO mess with hammer mass quite often owning a little lathe and all. Sort of a circuitous way about it but it's the language I understand best.

I tune my .177 DreamTac to run 10.4gr pellets at 920 FPS at ~95 bar. Then I pull out my light pellets and tweak the transfer port down and HS down to run 7.8gr pellets at ~650 FPS. I usually find a few settings that are good for a number of lighter pellets I use at short range. If I recall, I was getting somewhere around 120 shots per fill on my lightest setting, while running full hog 10.4gr pellets yielded around 60 shots.