So, it sounds like HK went further in putting in full auto carrier block than they did in the SP5K, doing basically what Zenith/MKE did, instead of a little nub like the SP5k. This is going to make it pretty difficult to use with a sear pack, unless someone can magic up a full auto carrier that can clear the block, but somehow have enough material to trigger a trip.
I've seen on Facebook that at least of a few big name MP5 smiths have already stated they won't be modding any SP5s to remove that block. My guess is everyone is gun shy over the ATF blocking up the Zenith modifications.
Here is what the ATF said, according to Zenith:
https://zenithfirearms.com/blog/removal-of-zenith-full-auto-bolt-carrier-blocks/
On May 17, 2019, Mr. Curtis sent BATFE’s formal reply, several excerpts from which are reprinted here (bold and underline are BATFE’s emphasis):
Previously, importers requested ATF allow a forward pin-hole in semiautomatic firearms, in order to give their firearms a more authentic appearance. These semiautomatic firearms continued to incorporate a “blocking shelf” in order to prevent the installation of an unmodified machinegun trigger pack. However, this “blocking shelf” was smaller than the original full shelf; therefore, a secondary blocking feature was required to prevent a semiautomatic firearm from being easily converted into a machinegun.
ATF has permitted semiautomatic receivers to include a forward pin-hole only if the receivers contain both:
A smaller blocking shelf, still preventing actual attachment of an unmodified machinegun trigger assembly; and
An internal blocking feature which prevents the installation of an unmodified machinegun bolt assembly.
Any HK-type receiver that contains a “blocking shelf” that is insufficient to prevent the installation a machinegun trigger assembly, constitutes a machinegun receiver, and will be classified as a “machinegun.”
Zenith semiautomatic firearms also incorporate a blocking bar in the rear of the receiver, which prevents the installation of an unmodified machinegun bolt carrier. This is consistent with other semiautomatic HK-type receivers, where the presence of a forward pin-hole and the smaller “blocking shelf” require a second blocking feature.
In your correspondence, you request permission to remove the secondary blocking feature in order to install unmodified machinegun bolt carriers. FTISB cannot approve this request because the Zenith Firearms without the necessary blocking features is a “machinegun” receiver.
That all being said, looking at what the ATF actually responded to Zenith about doing the mods, I'm not sure they really apply to the SP5.
I haven't seen an SP5 in the wild and the pictures I have seen aren't that great (though someone must have some detailed disassembly pictures out, or will soon). But looking at this picture from HK USA's website, it's very clear that the SP5 must have a full shelf. It doesn't have a second pin hole. If that's the case, the ATF message to Zenith shouldn't apply. They were hung up on the Zenith guns have double push pins and stating that the Zenith micro-shelf wasn't sufficient. But since the HK appears to have a full up shelf, like the older HKs and pretty much all the US built clones do, then I would think that means it would be okay to remove the carrier block.
That being said, I'm not sure most of 07/02s are going to want to put their licenses on the line in case the ATF decides to just make up more legislation out of thin air and declare removing the block is creating an unregistered machinegun.