I feel FX barrels are a bit hit and miss.
When i got my Maverick sniper, it shot just fine if you stuck to the SUB12 setup it came in ( my country FAC country ) ( shot 13.43 gr monster redesign pellets at 575 FPS on hammer #7 which is full power, 90 BAR regulator setting from factory )
So trying to turn up the speed, well then it only shot well with one pellet.
I then changed to 700 mm Barrel ( STD one ) and dual hole transfer port and that parried with a +100 mm barrel tube extension and a home made carbon fiber shroud, and suddenly i was shooting just fine, both pellets and lighter slugs, all the way out to 130 M with a .177 rifle.
I then got the 700 MM heavy barrel, it too shoot fine but not anything heavier so was still stuck shooting 13 grain slugs, but it would also shoot pellets just fine.
But really head 2 head with the STD barrel and i would be hard pressed to tell which one was better.
There is a difference though, if you set up rifle with STD barrel, and then change to the heavy barrel, your speeds will be lower.
I have then lately restored the Maverick back to its original form with a new STD barrel, and wouldn’t you know this barrel will absolutely NOT shoot any weight slug, but it do seem to shoot several models of pellets just fine.
I will absolutely NOT get a heavy barrel in 600 mm, i am done with the Maverick but was hoping to pest with it shooting 10 gr JSB KO MK3 slugs, but that is not going to happen so pellets it is.
I am thinking FX need to reinvent their barrels,,,,,, and it seem they might do that.