The ATF does actively work field cases, and you have the FBI and DEA who would likely be making such charges in conjunction with their normal cases. For example, suppose the DEA takes down a drug trafficker who is in possession of an illegal suppressor or other NFA item, the DEA would make such charges. I would think Customs would also be an agency that would routinely run across such items. HSI could conceivably come across such items in human trafficking cases, and I suppose U.S. Fish & Wildlife probably even comes across them from time to time. - Trafficking = "Larger cases"
Such a law would prohibit your local sheriff's office from being able to take gang related felon in possession of a firearm cases to federal court. -They can't do that now. (unless dual sworn and attached to a federal task force.) But such a law would not prevent them from giving the case to the feds for prosecution. It is only a symbolic law and really does nothing. We encounter cases of sawed off shotguns and short barreled rifles routinely, but the feds defer to a state prosecution because it is not a big enough case for them. For example, a felon with a gram of meth and a sawed off shotgun -they typically will not even bother with it.
As for immigration, the Trump administration began a new program called Immigration Warrant Service Officers. These are local personnel who are issued federal credentials to execute federal immigration warrants. They can't initiate immigration cases, but the program satisfies the requirement to have a federal official execute the warrant. -Like I said . . 287g program.