Sightren

I'm not too sure like I said I dont have much technical experience, I've seen a couple of posts saying that G&G use standard specs but others saying they aren't compatible with much so im coming up blank with that one. Thought about wrapping in PTFE tape like you said to pack out the thread as a possible stop gap solution. I got the rail from fire support uk (https://www.fire-support.co.uk/product/angry-gun-l119a2-rail--925-inch-short-version) which says it comes with the standard AEG but it may be possible that the barrel nut was wrong?

I'll start off by saying I'm relatively new to this hobby so I don't really have any technical know how in terms of AEGs. I've had this gun for over half a year now and I've been looking to shorten it down a bit as it's a bit too long at 13.5" and can be a bit of a hindrance in close quarters. So I liked the look of others L119A2 builds and started slowly customising my gun. I purchased the AngryGun rail and tried to fit it on the GC16 but the barrel nut barely tightens then slips. After measuring the inside width of the barrel nut (female thread) it was approx 31mm and on the upper receiver (male thread) it was also 31mm so the threads are just not catching enough. The stock barrel nut measured in at approx 30mm (female) so fit/tightened securely. I'm wondering if anyone else has had this problem and if so how did you work around it? Would I need to get an upper receiver?