Yeah, yeah, it's an old post. No, there's no way that "destroying the secondary sear" could make a semi-HK become full-auto. The secondary sear is what MAKES them full auto. If it did have a secondary sear, then it was already full-auto. Semi-auto guns do not have that secondary sear. 2 things happen when the selector is moved to full-auto - a block to the trigger sear is removed, allowing it to be pulled back further so it won't interfere with the hammer. Then, every time the bolt moves back to the locked position, it pushes a trip lever that releases the secondary sear that releases the hammer. Without the secondary sear, the hammer would be in continuous contact with the bolt head, and might never even develop enough velocity to drive the firing pin into the primer. I've never removed my registered sear and tried to shoot it to see what happenes, so I'm not sure if it will fire or not. I suspect that it will not, because if it would, then sear-less trigger packs would be considered "machine gun conversion devices" by the BATFE, and they are in fact not. It is the sear itself that is the registered machine gun conversion device.
Furthermore, the bolt carrier needs a special ramp to release the trip lever. Full-auto versions have the ramp, semi-auto carriers do not. So if you put a full-auto pack onto a gun with a semi-auto carrier, it would not fire in full-auto, and destroying a sear would not change that. Just more evidence that the rifle was already full auto.
So, while it may be possible that a full-auto trigger pack could still fire full auto without the secondary sear, you can't destroy a required part for full auto function, and then say that the destruction of that part caused it to be full-auto.
I have an MP5 with both an SEF trigger pack, and a 3 round burst pack, and I've taken them apart multiple times, so I'm very familiar with their operation.