.22 cal.pellet sizes

[easyridemm]

I have a brand new mk 2 impact. With a 600mm barrel in .22 caliber it shoots the 18 grain JSB pellets at 920fps with the hammer spring on min. setting. When I changed to the JSB monster 25grn. pellets the best the gun will do is 840fps. with the hammer spring on max. Anyone have any idea why the severe drop in velocity? My regulator pressure is set at 90 bar.

 

[Rj]

Run the reg up to 115- 120. Should get you around 950.

 

[luxurycat]

That is normal. The Monsters do not have the same skirt and 22% more weight.

 

[AirNGasman]

Just a wild guess; but the 9.7% drop in velocity might have something to do with the 40% gain in projectile weight.

 

[Glem.Chally]

Yeah not a huge surprise here.

 

[ackuric]

No surprise as others mentioned, and as mentioned earlier, bump that pressure up if you want a higher velocity.

 

[oledawg]

If it groups well why does the FPS matter? 

 

[ctshooter]

Anyone have any idea why the severe drop in velocity? My regulator pressure is set at 90 bar.

It's because you are using a heavier pellet. It is better to rate a gun using FPE (foot-pounds energy) as this will help you translate and calculate the approx power you will get with other projectiles. 

Check out the fpe calculator here: https://www.pyramydair.com/airgun-resources/widgets/convert.php?Units&u=14

Your 18.1gr pellet going 920fps is approx 34 fpe. Now put a 25.4gr pellet in for the weight and put in different fps until you get to what the first fpe number was. For a 25.4gr pellet it is right at 780 fps. So usually a heavier pellet will be able to achieve a slightly higher fpe with the same air output compared with a lighter pellet. The speed you are stating puts you up around 39-40 fpe so you are actually gaining more power, and efficiency, per shot even though the pellet is traveling slower. Usually the difference is closer to ~2fpe, but the Power Plenum would help a bit there.