FX Impact settings .177

I had the same problem with my .25 Impact.
i has previously owned a FX bobcat .30 and was just amazing accuracy but when I got the impact It just won’t group at all and was frustrating but one day got bored and tried Air Arms pellets in my impact and I was getting amazing groups unexpectedly so I started wondering and got some more pellet tins online and it turns out it likes the MKIIs so just because it’s JSB doesn’t mean it will Group so try different kinds of the JSB and see what happens and if that doesn’t work try clearing the barrel when it ships it comes with some stuff in the barrel that needs to be cleaned out
 
My impact in .177 likes just one pellet, at just one speed. Like Billy Crystal learned in 'City Slickers' , it's up to you what that one is? 
for me, it's around 840 fps with jsb 10.3's. it gives good accuracy after a good jb paste scrubbing, but the .177 Pulsar, 1720T w a lothar barrel, and a CZ200 I have are more accurate.
I'm on the lowest hammer setting and regulating the speed through the valve setting.
I've a piece of plastic eraser jammed into the slot that the pushes the hammer adjustment (black piece) consistently against the hammer adjustment wheel.
Single loading helps as well, but gosh, that's a pain, even with a loader.
Cheek weld and forearm tension is important, as well as follow through- you know all that.
It did like the 16 grain JSB's but the magazine was sticky with them and @ longer distances, they fell apart ..




 
I'd encourage you to go .22 if that is an option. The .22 barrel is the most accurate rifle I've shot, bar none. It shocked me with how well it does, once adjusted.
18 grain JSB's, 4 shots all touching at 100 yards.
It was a remarkably calm day in the Sierra's, but I was on a camp chair, shooting off of a Primos tripod/shooting stick thing.

I was squirrel hunting for a week, and would just pick a small part of the animal, and do surgery with the gun.

I snuck up on a Douglas squirrel, facing the other way, at about 30 yards. He was looking away from me, laid out on a branch with his reared pointing back at me, at lamost the same level I was at, as I was on a slope, facing down.
That impact pushed a .22 hemorrhoid pill into the winking, pink starfish of his rear end, through the length of his body, and out his forehead.
Please excuse my graphic description, but I swear that I could have put finger nail polish on a pellet and do a complete manicure on the local squirrels.

These aren't big squirrels either....