Mirror'd Cappy227's tune

[jimmyk]

I've never been a believer in having to re-invent the wheel so I tried Cappy's tune for his 22 Mrod which was 3.25 on the hammer spring 6 in on the stroke and 3.5 out on the velocity screw. I started at 2850 instead of 2800 and shot down to 1850 which was 40 shots. The curve went as follows..Looks like the best curve is going to be a 30 shot between 2800 and 1950 or shots 5 thru 36. That's 32 fps spread over 30 or 52 fps over 40, still not bad. I tried filling to 3050 but that was a waste of air untill I shot down to 2750 -2800. Haven't tried it for accuracy yet. What would be my best move to tighten the ends up a little more, more spring?? Almost forgot this was shooting Crosman pointed 14.3 in the 175 count can from Big 5. Waiting on some JSB's 18.1 and H&N 21.1's. Thanks Cappy227, hope you didn't mind me coping.
Jimmy

856@2850
864
880
866@2800
876
866@ 2720
875
874
881@2600
880
881
889
891
895@ 2550
893
897
893
892
899@2400
902
908
904
903
898@2300
905
903
897
897
893@2150
896
894
885
884
879@1980
876@1950est
871
865
859
855@ 1850

[bstaley]

Assuming you don't mind filling to 3000 (or even higher) Increase the spring tension to raise your sweet spot. This will probably raise your velocity some to, so then you might want to turn the velocity screw in by small amounts (1/8 - 1/4 turn at a time) to bring the velocity back down and flatten out the curve.

With the heavier pellets your velocity will drop some anyway and you'll get a few more shots too.

[jimmyk]

I kind of like the 2700 to 2800 range. I'll get a few more fills out of my smaller tank. When you say "raise the sweet spot" are you refering to the fill pressure range or the velocity curve? If I leave it as is and shoot the heavier pellets are you saying I'd get a few more shots or a few more shots after a little more adjustment? I'm still trying to figure out this whole relationship between spring tension,stroke and pressure.
JImmy

[bstaley]

People use the term differently, when I say sweet spot, I'm referring to the portion of the velocity curve, surrounding the peak of the curve, that contains the shots within the range of velocity that you are willing to accept. So currently, based on what you posted, you consider your current sweet spot to be between 2800 and 1950.

Spring tension and stroke length determine the amount of energy and momentum that the hammer has when it strikes the valve stem. More energy and the peak of the curve moves up in pressure, less energy and the peak moves down. So if you want to raise your starting pressure, increase spring tension or lengthen the stoke, and to lower it, decrease them. You can achieve the same peak with a short stroke and a high spring tension, a long stroke with a lower spring tension, or with many combinations in between. I generally find a shorter stroke and higher spring tension to result in more efficient strings (less wasted air).

Once you get the peak of the curve where you want it, use the metering screw (velocity adjustment) to control the velocity and flatten out (squish if you will) the top of the curve. The more you flatten the curve ( and lower the velocity), the more shots you will get within a reasonable velocity spread.......Higher velocity=fewer usable shots, lower velocity=more. Also as you lower velocity, your sweet spot will expand....just think of it as chopping off the top of the curve.....the more you chop off, the wider the flat spot is. Even though velocity curves are generally plotted as velocity vs. shot number, pressure is fairly linear with shot count so you can think of it as velocity vs. pressure if that helps you visualize it better.

As for pellet weight, heavier pellets are generally more efficient in a pcp so even though you get a reduced velocity (at a given tune) you actually get more energy (FPE) and will usually get a few more usable shots as well. Basically this is because the heavier pellet resists the flow of air more and reduces the amount of air that can be released during the short period of time the valve is open. This is especially true for higher power/velocity tunes.

So yes, with your current tune, you will probably get a few more usable shots with heavier pellets with more FPE, but reduced velocity.

[jimmyk]

Thanks again for the help. Your explanations helps a lot. My main goal is to plink and hunt with 30 good hard hitting and accurate shots. Looking at my shots I had a low number six shot and then I had a peak high 908 that also seemed a little out of place. If I boilerhoused those two numbers to something a little more consistent, my vel spread drops to 29 over 30 shots. That's charging to 2700 and shooting down to 1950. I don't know how accurate the 890 average is going to be, but like you said I may have to drop my velocity setting a little for accuracy which will flatten things out a little also. Everyone have a good weekend.
JimmyK

[ward360]

I came from powder burner thought, I tuned my pcp like that, When I shoot I don't want to have to worry about What pressure my gun it at, And I have found that in the "sweet spot" the gun's are very efficient.

I have settled with a 20 shot string with a 10 fps swing and high efficiency, I fill more Often but I don't fill as much.

With a 30 yard range you can probably get away with a bigger swing and not notice it. Shoot some paper at your max range and see how much your POI changes, then decide it you can live with it.

[cappy227]

Nice! Glad to see they're building the rifles so that you can get from point A to point B with other samples if you try!

My recc based on your shot string? Fill to 2,600PSI and shoot down to 1,950 - that really keeps a tight POI, and has been working for me. Installing AoA's hammer de-bounce device (HDD) for $30 should stretch that spread, too, to 30 shots!