Double feeding and firing auto on semi

[L33]

This is driving me insane, if anybody can shed some light on these issues I'd really appreciate it

In short, I've a brand new G&P gearbox and hop unit in my M4 that is double feeding and shooting fully automatic on semi....

Double feeding
The rifle fires absolutely perfectly, but having the barrel pointed down even slightly causes a second BB to roll out. The following shot then dry-fires (as there is no BB in the chamber) and the issue repeats, so:
Pull trigger - BB (full speed) - BB (rolls out of barrel) - Pull trigger - Dry fire - Pull trigger - BB (full speed) - BB (rolls out of barrel)

The hop, bucking and nub have been refitted 3 or 4 times now, everything is clean and nothing is damaged.
The bucking holds a BB (dropping one in the hop it doesn't roll through unless tapped)
The barrel/bucking is held securely in the hop, it doesn't move in and out
The hop is firm against the gearbox.
The gearbox compression is excellent.
The tappet plate/nozzle are moving normally and aren't damaged or worn
Hop is dialled all the way to 'off'
Have tried G&P, CA, Ares and TM magazines

Shooting full auto on semi
Safety works, but both semi and full auto selected is full automatic fire. It isn't runaway (i.e. it does stop if I release the trigger).

The cutoff lever is able to move and is not worn or damaged
The selector plate moves well and isn't snagging on bearings or anything.
The trigger contact block moves as expected
The receiver selector switch is working normally and isn't worn
The gearbox fits perfect in the receiver

I've tried just about everything I can think of for both issues!

Specs in case something stands out as a culprit
G&P 8mm gearbox, rear wired
G&P M120 motor in magpul POE grip
G&P 1 piece metal hop unit
G&P hop bucking and nub
Madbull tightbore barrel
G&P receivers
Madbull 11.5" outer barrel
7.4v 2600mah lipo

As I said, any help is much appreciated!

[L33]

Re: Double feeding and firing auto on semi

For the sake of documenting the solutions in case someone else has the same issues:

I solved the full-auto on semi by replacing the trigger switch & cutoff lever with ASG ultimate ones. The G&P switch is pretty low quality plastic and their trigger spring is stiffer than most others I have which appears to twist the trigger in its locating holes slightly. I think this was causing the switch assembly to skew, and the trolley getting stuck between the switch contact prongs. Most likely could have been solved with a weaker trigger spring and/or a stronger trolley spring, but I'm not sure for how long. Replacing the switch with an ASG ultimate one has fixed the problem though, they seem much more sturdy so the issue should not return.

The double feeding issue was solved by replacing the hop rubber & bucking. I tried 2 brand new G&P rubbers & buckings, both had the same problem. Why on earth G&P rubbers don't work with G&P hop units and a G&P gearbox is beyond me, but it didn't in my case.
Replaced with a much harder bucking and a madbull rubber and the problem is solved.
Side note: the madbull rubber was too long so needed trimming, and took a lot of time & patience to perfectly line up the rubber to barrel as it wasn't very well centred.

Gun now firing absolutely perfectly, 343-345 fps every time and accuracy is great.