Hi everyone, I've been playing airsoft for about a year now but this is my first time actually trying to modify my gun hardcore. I have a full metal AGM M16. The reviews on it were great so I purchased it from airsoft gi. Yesterday I recieved a package of internal parts that I planned on putting into my gearbox. This package consisted of 1) Guarder Complete Bore up Kit (Cylinder, Air Nossil, Cylinder Head, Piston Head) 2) Matrix Polycarbonate Piston with Metal teeth 3) Matrix Shims (.01mm &.02mm) 4) Systema Bucking I purchased the bore up kit because earlier in the year I had installed a Madbull 650mm tightbore barrel. I was looking for some extra bb distance and was told that any barrel over 600mm should probably be using bore up. So I opened up my gearbox and installed all the parts listed above. However the systema bucking would not fit in my hop-up unit. I tried everything but just resorted back to the stock bucking, I'll try the systema again soon. Anyway, my real problem is that once I put everything back together I realized I was double feeding. I will fire 1 shot on semi 5x out of 10x. However for the rest of the shots I will fire anywhere from 2-5 bbs at once. I was wondering if you guys could offer any incite as to why this is? I have not used any new mags and have not had any problems with my mag in the past. Thanks everyone! Hope you can help! Also, when I first opened up the gearbox, it exploded. I put everything back together but I noticed this small spring out of place in the corner of my gearbox while shimming (see image). It is the smallest spring I have ever seen and have seen no one mention anything about it in any videos I have watched. If it has any significance please let me know! Thank you again!