I too had the exact same issue! Annoying AF. I initially patched it by wrapping the barrel nut in some tape so it would fit snug with both the handrail and cross-bolts, but that was a short lived fix. The wobble returned and wore away at the tape after a hundred or so rounds 😕 Im sure some of the licensed/practiced gun-smiths could probably provide a better quick-fix solution for this. The best fix I found was to replace the cross-bolts with ones from a real-steal BCM, as they are a LOT thicker. I ordered mine direct from the states a while back, but you can get them from BCM (US) or in the KRM install kit from Brownells (UK).
https://bravocompanyusa.com/bcm-kmr-cross-bolts-kit-set-of-2
https://www.brownells.co.uk/BCM-KRM-HANDGUARD-INSTALLATION-KIT
On another note. Was testing out my Ace1Arms Pmags [they really quiet good IMO] only to have my AngryGun MPA nozzle spring fire out of the barrel 😕 Anyone know where I can get a spare for this rather than buying a whole new Nozzle?