One of the particularly popular transferrable machine guns out there is the H&K auto sear. Since H&K grip and trigger assemblies are interchangeable between 9mm, 5.56mm, and 7.62x51mm guns (ie, MP5, HK33, and G3) a single registered full-auto grip assembly can allow someone to effectively have three machine guns for the price of one (albeit only one at a time). However, there are some potentially very expensive and dangerous pitfalls for someone acquiring one of these without a full understanding of the law and the technical details.
In short, an H&K upper receiver that has the ability to fit a factory full-auto grip assembly is considered a machine gun by ATF, just like an AR lower with the third pin hole drilled. If you have a registered standard factory full-auto grip, any gun it can attach to is also by definition a machine gun. Instead, the proper way to do this is to register a grip assembly that is modified to fit onto factory semiauto receivers. This sounds pedantic and strange – and it is – but it is a complex and tricky area of law and ATF rule making. If you are planning to put tens of thousands of dollars into a transferrable H&K, you really ought to have a clear understanding of these issues before you make a purchase!
Not a legal issue but moving a trigger pack between different caliber HK’s would usually require a change of some ejector parts to function properly.
“a grip assembly that is modified to fit onto factory semiauto receivers”. This must be for HK factory semi-auto receivers. Other semi-auto receivers are very different. They are made without the front mounting hole of grip assembly. Instead they have a shelf that intrudes into fire control group where the full auto parts go. You can not mount a full auto fire control group on to them. I make my own semi-auto clones from demiled G3 and CETME mod C guns.