Despite what ‘Grand Dad’ or that weird uncle with the wild hair and stash of cheap crap guns might have told you. Making a gun fully automatic takes a little more than just ‘filling the thing down juuuust right‘. It’s taking a high pressure process that happens several times a second, usually 10-15, and making sure that all the correct things take place for it to happen safely.
This means timing. This means correct part weights and pressures. This means having parts in place so that nothing happens out of sequence. This means building in such a way that if something does happen out of sequence, the explosive failure (yes, it will be explosive) happens in such a manner as to minimize and avoid injury to the operator.
This is why automatic and burst fire triggers have extra parts. It’s why you can’t easily convert a semi-automatic into a fully automatic firearm, because you need these extra different parts. I know Nancy P. and the Gun Control Gang say otherwise, but since they’re functioning morons in this regard you should ignore them.
Every (successful) variation of fully automatic have design considerations implemented to make certain everything goes in order and on time (unlike USPS and FedEx quite often).