Fx Crown spiralling over 50 m

what pressure is your regulator working at?

what hammer spring tension level?

When the pressure of the regulator is very low and the tension of the hammer is too high, the valve releases a large amount of air and while a good energy is produced, that amount of air may destabilize the pellet at the exit of the barrel and makes your rifle very inefficient because it spends a lot of air ..... otherwise, increase the pressure of the regulator and reduce a little the tension of the hammer, the valve will release less amount of air but at higher pressure, a very good energy is achieved and also more efficiency because the valve is going to close faster... (sorry about my english, i hope this helps you)

What pellet are you using? Brand and Weight?

How many shots through the barrel since polishing?

Are the shots consistent speed or do they vary in the same magazine?

Have you checked the magazine to see if it's clipping pellets while being inserted into the breech?

Do you check for damaged skirts before using the pellet?

Spiraling usually happens to me with adirty barrel but yours is clean. I would shoot by hand loading. No magazine and see what happens

Take a picture of your crown, not the rifle but barrel crown, and post it.

Does this happen with all JSB pellets, or just the jsb heavy king mk2?

Try to increase the velocity as much as you can. If you can hit between 285-290mps.

CA

clean barrel

single load the rifle

check the moderator to avoid pellet clipping it

I would suggest trying the MK1 heavies. Sounds like your speed is low enough and not causing instability (see below). The MKII pellets have been known to do this in some barrels and the solution sometimes is simply changing to the slightly longer MK1 JSB Heavies. Then again, if you increase your speed up toward 880fps (268m/s) and test in incremental steps with the MKII/MKI pellets you might find a speed that the pellet is still stable beyond 50M. Keep experimenting, you will get there. Your on the right track, you have already defined many of the parameters that you need to get to a solution. Good luck.

Post edit: It dawned on me that your problem might also be related to head size. JSB is really good about being consistent from tin to tin of keeping the head sizes consistent to within .02mm. BUT, with that said they can vary. 

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If you have a pelletgage, I would size them and keep track of the good vs the bad for your gun. There are several videos on youtube that discuss this in regards to different guns. I was not convinced that it mattered till I got one and experimented with different sized pellets and different guns....IT DOES MATTER, is about all I want to say right now, but give that some consideration in your testing. again, good luck.

My Impact with the STX barrel did the same thing shooting MKII’s @ 830-820 FPS. I slowed it down to 800-810 FPS and the spiraling went away. I’d like to get them in the 880-900 FPS range, so I may try some of these suggestions. My original ST barrel shot good at 830-840 FPS. Thinking about changing back.

I had some spiraling with my Crown 22 yesterday......first time I've seen it. It was with JSB 15.89 pellets out of the tin, shooting 926fps at 60*F.......but outside temp was 10*F. It's been over 500 pellets since I've cleaned it, so maybe a dirty barrel is partly the cause. But I increased velocity with the Power Wheel (Hammer spring) to about 950fps and the pellets became stable again.

I had the same experience with my new Crown with the Air Arms 16gr pellets at 920 FPS. At 950 it stopped and it is unbelievably accurate at this speed with those pellets. 880 FPS was good too but not as accurate as the higher speed. And very consistent. Regs settled nicely after tuning. Single digit ESs.

Nice Kurt!!! 

Interesting that spiraling is being noted by multiple people in the low 900's but then stability is found around 950~ fps, whats even odder is that accuracy is good at 880 and 950 but not 920... That is a brain tickler....do note though shooting pellets at these velocities will likely dirty the barrel quicker, but if it provides the best groupings then I would just clean after every tin or so.

I remember having to clean the X-liner in the WC I had more often....like three to four hundred pellets. I was shooting 25.4gr 25 caliber pellets at 950.....

.....you could be onto something! I think I'll put a cleaning on the Crown liner tonight.