Maximum FPE Range for Discovery

[carbonblack]

Hi, I am relatively new to airguns and airgun modification. I recently purchased a MK-177, .22 Discovery and .22 2289 and have started to make some modifications to the .22's to better suit my needs.

I have modified my .22 Discovery with a power adjuster w/ the stock hammer spring w/ 2-3 coils cut and .17" ID transfer port.  Currently I am able to achieve a FPE range from 15-24 FPE with 14.4g CPHP by adjusting the spring tension, as verified by measuring FPS with my Chrony.

Is it possible to achieve have an even wider adjustable FPE range, say 12-30 FPE through the adjustment knob, and if so with what type of modifications?

I was thinking or replacing the shortened stock hammer spring with dual spring setup, consisting of a lighter spring at full length and a shorter spring with smaller diameter, concentrically oriented to the taller spring.  The ideas was that for low FPE adjustments, the power adjustment knob would be adjusted such that only the taller spring is compressed, and for high FPE adjustments, both springs would be compressed to maximize hammer acceleration.  I would think a progressively wound spring may able to achieve the same type of effect, but I was not able to identify a suitable part.

Has anyone tried this approach, or is there a product out there that would allow for this range of FPE/FPS?  I would like to have this capability for my Discovery for backyard pesting duties.  I have a 7.5" TKO and would like to have the ability to turn down the power to around 10-12 FPE for birds and to minimize sound, but still be able to set my airgun to 30 FPE for raccoons when the occasion presents itself.

Thanks for any input!

[Rdsail]

your range might be too big. the reason is that if you do the mods to have 30fpe (valve ports) when you tune it down your extreme spread is going to be large so your accuracy will suffer because of the vertical change in POI.

if you tune for 12fpe it is going to be hard to reach 30 fpe.

as far as the dual spring. I would avoid that because it will generate a lot of hammer bounce which will waste a lot of air.

a better range might be 20-30fpe.

[rsterne]

While it might be possible to achieve such a wide range of FPE using hammer spring adjustments, by the very nature of doing that you will have to change your fill pressure along with the preload changes.... In other words, your 30 FPE gun might be only a few shots at 2000 psi, at 25 FPE you may get a normal bell-curve starting at 2000 psi and ending at 1200, by the time you get down to 20 FPE you will only be able to fill to maybe 1600 and stop at 900ish, and IF you were able to get down to 12 FPE you would probably only be filling to 1000 psi and refilling at 500.... This of course means that your shot count will not increase as you might expect with the lower power, because your usable pressure range decreases....

In order to cover such a wide range of FPE levels and maintain a 2000 psi fill, you would have to figure out a way to "strangle" the transfer port, like they do in an MRod.... not a particularly easy thing to do on a Disco.... There is one other alternative you might consider, and that is to convert the gun over to a Challenger/PRod hammer with adjustable stroke and fit the bstaley O-ring buffers.... That allows you to detune the gun a lot, the problem is going the other way, to get the gun up to 30 FPE.... Unfortunately, there is no easy solution without stripping the gun to retune it by changing the diameter of the transfer port.... Here is what you can do with a STOCK Disco by just changing the transfer port size.... The range of power is 6.5 to 23 FPE....



That gun had a 2260 tube, which is only half the volume of a Disco reservoir, so double the shot counts with a Disco tube.... That testing was done with a stock Disco hammer and hammer spring, with NO preload adjustment, only the transfer port was changed.... If you modded the Disco valve to increase the peak power up to your desired 30 FPE (certainly possible) then you could extend the range even further.... However, it's a PITA to strip the gun to change out the transfer port to change the tune....

Bob

[carbonblack]

Thank you for your responses Rdsail and rsterner.  I did try a light hammer spring with increased length, hoping to achieve a lower FPE range without sacrificing the high range.  It resulted in a 12 - 18 FPE adjustable range with a 2000 PSI fill. 

I have since put back the cut, OEM hammer spring and will more than likely look into the Challenger hammer with adjustable stroke and different TP sizes to see if I can keep near the 22-24 FPE for the upper range adjustment and 12 FPE for lower adjustment, without too much loss in accuracy.

I should note that I was able to humanely dispatch a possum and a raccoon this pass weekend with the 18 FPE setup, so a reduced upper range would be acceptable.

[gnef]

Mind if I ask where you got the light hammer spring that you were able to get a 12-18 FPE range with? I have been thinking about a power adjuster for my Disco, but would want to lower the power a bit, rather than increase the power.

[carbonblack]

Mind if I ask where you got the light hammer spring that you were able to get a 12-18 FPE range with? I have been thinking about a power adjuster for my Disco, but would want to lower the power a bit, rather than increase the power.

It was one the lightest Crosman hammer springs available as a spare part directly from Crosman.  I will see if I can dig up the part #.

[rsterne]

Lightest Crosman spring is from a 22XX, next is from a 13XX/2289, both are lighter than a Disco spring.... The same problem remains, if you retune by using a lighter hammer strike, you must fill to a lower pressure to stay within a reasonable ES.... If you're just using your low power setting for plinking at short range, it won't matter of course that the (starting) velocity is a long way below the (new, lower) peak velocity....

Bob