Impact MK2 25cal with JSB exact king heavy MKII: what fps am i looking for?

I think you need to get hold of Hawke Chairgun or Strelok Pro app for smart phone, When you have used the FX chrono you can input the data and the apps will give POI drop.
I have a .25 impact mkII 700mm superior barrel and have found that the JSB 34 gr heavies like to fly at around 870 fps max for good grouping but have gone over to FX hybrid slugs (960 fps) which fly a lot flatter for longer ranges, I am also using Wildman 30 gr slugs (934 fps) which seem to have good grouping and fly flat, I intend to try these at a higher speed in the future.
You could use adjustable mounts - I use Eaglevision - rather that set the scope at its optical centre set it low, use the mounts to zero then you should still have elevation clicks left for longer ranges, later impacts are said to be sent form the factory with a 20 MOA pic rail to aid long range set up, how old is your gun?

Thanks for the answer John.
So i would be looking for 870 FPS then? Or a different number because of my 600mm barrel vs your 700mm?

Since you also stated the FPS number for the slugs my return question still is: how do you know that you want 960fps for them but 870 for the JSB's?
Or is it als simple (but teadious) as starting high, shoot, lower the speed and shoot again until you get good groupings?

About the 20MOA rail: i bought mine July 22nd from Dicks.nl so i would imagine it is produced somewhere in June.

get some adjustable mounts also and then you can zero it at 50Y with the top turret almost bottomed out.

I have a 600mm barrel in .25 and with Hybrids at 950 I go from 50Y to 150y and its one turret rotation, (Helix) I have some Mk2 Heavies to compare, but they will probably be at least 2 turrets rotation, due thier extra weight and BC - but I doubt they will group like Hybrids at 150Y

Check out teds holdover - he did a test on these pellets recently and in the Superior Standard they grouped the worst - but still good enough for hunting medium game.

With the adjustable mounts you can "cant" the front of the scope down compared to the rear - this gives you the full turret range.

Smid, you really have to experiment, have a look at Matt Dubber's you tube vids (Air Arms Hunting SA). He has a series of videos called AB101 which explain in detail why it is a bad idea to fire pellets too fast, barrel twist rates, lengths and harmonic tuning to name but a few concepts. This should clear matters up for you.

I found that above 870 fps the JSB 34 gr heavies lost their ability to group well . The FX hydrids are reputed to perform best from around 930 to 970 fps and I have found this to be correct, It just happens that the settings of my Impact suit both the JSB 34 gr heavies at 870 fps which also gives me 963 fps for the FX hybrids without touching any settings, this also gives me 934 fps for the 30 gr Wildman slugs, I am sure these slugs may perform better at even higher speeds but for the moment this gun setting gives good results for all three types of ammo (power wheel on max, reg at 120 bar, valve adjuster at about 3.5 marks open). The gun also shoots 26 gr Hades and 25 gr JSB kings at 900 fps at power wheel 4.
I am waiting for the FX slug power kit then the tuning will start all over again. I should mention that at the moment I have the Huma dual port transfer port and matching probe which gave an increase in fps to the levels I mentioned above.
The longer the barrel the more time the air has to impart momentum to the slug/pellet and also for the valve to be closed before the ammo leaves the muzzle, thus also reducing turbulent air from behind the pellet/slug as it leaves the muzzle/moderator.
This is the kind of thing explained really well in Matt's videos - spend the afternoon watching all the AB101 series, his channel is on airgun101.com - enjoy, your gun is about the same age as mine so you probably have the 20 MOA pic rail.
Watch Airbuks two you tube vids on Impact power tuning and harmonic tuning as well.
good luck

Second vote for Matt Dubber's vids on pellet aerodynamics and pellet precession, very clear and informative especially on the issue of muzzle velocities and how more is very much less when you approach the speed of sound with a diabolo.

Hi guys .......it’s very interesting all the setting that we have to do in order to get the right velocity, grouping at such distance with the right pellet/slug. I’ve been trying to do the same thing with my gun however in other forums some people recommend to change the liner if you want to shot slugs and that’s were I’m getting confuse , so did you guys change your liners to shot both pellets and slugs ? It’s my understanding that you cannot use pellet liner to shot slugs ( I might be wrong) please correct me , right now my set up is reg at 130 bar, Max on the wheel and the 3rd line valve adj, the speed is 930ish, and other thing that I noticed is after 6 or 7 shots the speed drops and the next shot goes back or goes a little higher. So what kind of liner are you guys using ?

My Impact was supplied with the Superior liner, waiting for my Superior heavy liner to be delivered, tried slug Liner A but for the pellets and slugs I listed there was no improvement, might have been better with heavier slugs.
See the you tube vids from Ted's Holdover on airgun101.com, he has tested various liners with various pellets and slugs for the Impact with some results that seem to tally with the accepted common knowledge.
Pellet liners/barrels in general don't seem to be good for slugs in guns equiped with certain barrels that are designed predominantly for pellets, for example the LW barrel on my AA US xtra FAC does not do so well with FX Hybrids or Wildman 30 gr slugs but is a tack driver with 26 gr and 34 gr JSB pellets, but the Impact gives me the higher power to overcome the increased friction of the slug contact with the barrel - horses for courses.
The Superior liner seems to do well with pellets of slugs - well done FX, but if you want to shoot heavier slugs it would appear that slug liner A or the Superior heavy liner is the way to go.

Check out this video here from 68_Whiskey. I have my .30 cal set up this way.