We gbb m4 keeps shooting more than one bb and sometimes none at all?

[Magpulmad056]

We gbb m4 keeps shooting more than one bb and sometimes none at all? Any ideas what could be wrong? It is brand new, was using the below 10 degrees neurpol has or whatever, also if anyone could tell me how to load the magazine quickly as the speed loader from another gun is rubbish haha, thanks!

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Found the problem it's the little spring at the back of the gun where the buffer tube joins, Does anyone have one or know where to buy one? Thanks!