Actual answer - nothing illegal about owning the parts, so long as they're not installed in your semi rifle.

More complicated answer - no problem, even if installed, so long as the resulting combination doesn't operate automatically.

ATF has specifically addressed this issue and stated that simply owning the parts is not illegal.

Also, storing them away for SHTF is silly, unless you also have the machine tools and the know-how to modify a semi-auto receiver in order to install the factory auto sear. A complete set of full auto parts in a semi receiver results in three selector positions - safe, fire and single-shot (due to hammer follow).