Slamming or bouncy

[Neil54]

Dear all

I did some power testing with light and heavy pellets on a couple of spring rifles. Both produced more power with lighter pellets.
What should I be aiming for? Pun intended.

Equal power with both heavy and light.
Close on power but tending toward higher with light.

Or am I completely barking up the wrong tree.

[TonyL]

I know others may tune with a more focused approach to this, Neil, but as long as I am happy with the power output overall and as long as the gun is safely under the limit with its most efficient pellet and I'm happy with the manners, it's job done for me.

[TonyL]

To qualify my previous post a little, so much depends on how much time you have and how much importance you place on the power output being similar with a wide range of pellets and differing weights. For those willing to experiment and devote the time, we're playing with factors like transfer port diameter, piston weight, piston momentum, spring rate / force / preload, piston seal fit etc. Is a rifle that is "balanced" to shoot different weight pellets at similar energy levels any better than one that doesn't? To my mind, no. There can only ever be one truly 100% perfect individual set-up between rifle and pellet and, to my mind, we accept the compromise when it comes to differing performance with different pellets. As I'm sure you do, I'd always put far more emphasis on accuracy.

[Shed tuner]

pellets are very different to one another.. so many variables it may not be telling you anything. lead mix, hardness, skirt thickness, size, profile, starting pressures all vary and affect power.

However, JSB 7.3, 7.9 and 8.44 are about as similar as you can possibly get, except for the weight variable.
In larger bored .22s, RWS Hobby and Superdome are pretty similar too (we don't care about shape). JSB 13.43s and 15.9s grains are similarish, but the lighter pelles have much thinner skirts.

So in short, unless the pellets are very similar in other aspects, there's too much variablity to draw any useful conclusions.
Also, using pellets way out of the "envolope" for a 12 Fp springer, i.e. 10+ grain .177s or 18+ grain .22 is not useful. Stick with 7-9 and 12-16 respectively.

OTOH, when they are similar, I find the data very insightful...

[Neil54]

With the lighter pellets, the power is too close to 12 for comfort. So instead of lopping off the spring I could increase piston weight or open the TP slightly.
But which is it?
According to the information I read my rifle is a little bouncy.

[TonyL]

With the lighter pellets, the power is too close to 12 for comfort. So instead of lopping off the spring I could increase piston weight or open the TP slightly.
But which is it?
According to the information I read my rifle is a little bouncy.

Mmm, depending on other factors also, Neil, increasing piston weight or opening the TP might cause it to produce even more power.. Unless the TP is already at its upper size limit, in which case creating that bit more lost volume might help drop the power.

I'd be tempted to source a different, softer spring, or to shorten the existing one a little (although the latter approach will make the spring stiffer). Either way will also result in easier cocking. If you already have minimal preload, I'd definitely go longer / softer spring, as too little preload will likely result in increased piston bounce.

[TonyL]

What rifle(s) are we concerned with here, Neil?

Only asking as, if it is a model with a large swept volume it may be that you may need to reduce the stroke if you wanted to retain a "quick" as opposed to "lazy" feel to the action. For example, guns like the Weihrauch HW80 and Diana 52 are notoriously tricky to keep safely below if retaining standard bore and stroke and preferring a "quick" feel.. Extreme examples, admitted.....

[Neil54]

The two rifles I happened to try are Kral devil's, same chassis as the Gamo Hunter, meteor mk6 and smk 19.
25mm bore.
I have a .177 with only 4mm preload, so this one I think i will short stroke.
The other is a .22 and has had nothing done yet. Still twangy.

My original question was should I try to get even power with light and heavy pellets.

It actually shoots quite well.

[maximus]

I personally wouldn't worry too much if it shoots nice with the lighter pellets. I have also left a gun cocked for a few hrs (obviously with no pellet in) to knock the power back a bit. Easier than chopping springs and doesn't increase the stiffness.

[TonyL]

I personally wouldn't worry too much if it shoots nice with the lighter pellets. I have also left a gun cocked for a few hrs (obviously with no pellet in) to knock the power back a bit. Easier than chopping springs and doesn't increase the stiffness.

The two rifles I happened to try are Kral devil's, same chassis as the Gamo Hunter, meteor mk6 and smk 19.
25mm bore.
I have a .177 with only 4mm preload, so this one I think i will short stroke.
The other is a .22 and has had nothing done yet. Still twangy.

My original question was should I try to get even power with light and heavy pellets.

It actually shoots quite well.

Fully agree with Max re not getting too hung up about balancing the power output between light and heavy pellets, as said before.

Max - good thinking re leaving the gun cocked for a long period to weaken it a bit.. However, with only 4mm preload to start with, conditions are rife for surge. So a longer, softer spring or short stroking it look to be the avenues to explore.

Neil, thank you for letting us know the make and model. I suspected (but didn't know, having no first hand experience) that these were based on the ubiquitous Gamo platform. One which I very much like. The Krals do look to offer very good value. What were your findings regarding internal build quality and finish?

[Shed tuner]

With the lighter pellets, the power is too close to 12 for comfort. So instead of lopping off the spring I could increase piston weight or open the TP slightly.
But which is it?
According to the information I read my rifle is a little bouncy.

Notwithstanding my caveats earlier in this thread, yes. Opening out the port or increasing the piston weight will both tend to move the rifle towards favouring heavier pellets from it's current bias for light. So, BTW, will fitting a longer, softer mainspring, which is what I'd actually be more tempted to do.

In order to decide, what are the current specs - bore and stroke, calibre, TP dia and length, piston weight, spring rate ideally but wire guage and dia if not, and amount of preload for each please.

[Neil54]

I also like the Gamo platform. I have 8 at the moment.
I bought the Kral because it has a walnut stock. Then I decided to get a .22 and leave it open sights. The .177 I made an adjustable cheek riser as I plan to scope it.

I have stripped three of these rifles so far. Two being standard with metal top hat, 25 to 30mm precompression.
The third was slightly over 12 and on stripping there was hardly any precompression. The spring had been hacked off, not finished, and no metal top hat.

Overall parts are the better made of any I have seen. Piston appears turned and not rolled. Trigger sears are the best I have seen, Gamo being the worst ( looks like gamos stamps have worn out).
Poor alignment of some holes. Poor finish on individual parts like burrs. Tiny bits of metal swarf on the piston seal. Barrel shims are plastic and squashed. The trigger assembly has no adjustment screw or little plastic block at the rear. It's easier to reassemble if you put the trigger in first.

So I am still playing with these rifles. It is my intension to change the spring for a number 6 but in the meantime I may just have a fiddle.