I don't see the ABS body on JG as being and 'cheaper' than any other brand of AEG, including Marui. The 416 has by far the nicest looking ABS body I've seen to date.
That being said, I think that even if the rails lined up, you'd be ruining the looks of your gun and taking away it's uniqueness by getting an M4 body. Since you're in the UK, you should probably consider getting yourself an actual 416 metal body kit.
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