Well it is going to be more difficult than simply shaving the top off the bolt to clear the last top nubbin that is for sure...
It looks as if HK put special effort into making certain that the new SP5 will not be able to be a sear host. A new video on youtube goes into a disassembly of the SP5 and at about the 2:00 mark the video goes into comparing a semi action bolt carrier to a full auto bolt carrier. The semi auto bolt carrier has metal milled from the bottom so that it won't trip the trip lever on a full auto pistol. Other than that the bolt carriers are identical.
I'm not saying it can't be done somehow, but HK definitely chose to make it next to impossible to host a sear in the SP5.
Here's a link to the video https://www.youtube.com/watch?v=0jxSpgNfCV0, and a picture below showing the differences between the semi auto and full auto bolt carrier. The full auto bolt carrier is on the right.
Sent from my iPhone using Tapatalk Pro
The same could be said for cutting the shelf off a 94, are there any clones being built without the shelf? No. Machine gun. So I guess if you are suggesting you could cut the lower nubbin off and add a semi shelf to the magwel then yes I guess you could probably do that, but why would you do that? Modifying the receiver in a manner they could interpret as easily convertible sounds dangerous to me even if it only shoots 1 shot per trigger pull, the bolt is what needs to be modified in my humble opinion.
The primary difference from my vantage point is that US made clones don't have push pin holes in the receiver and nor is one of those push pin holes exposed on the right side of the shelf. Looking at the HKPro members detailed video I am not sure if the solid pin through the shelf and receiver on the SP5 is even welded in, it looks like HK put the solid front push pin in and then just maybe peened/staked part of the shelf to keep it from falling out. Red arrow pointing to what appears to be a small stake mark vs. a weld.
In terms of using an SP5 as a sear host, I was looking at one of my MP5s this evening and how the sear trip lever interacts with the carrier. The SP5 rear carrier block based on the pictures in this thread appears to be about half of the width of the trip area on a full auto carrier. Unfortunately, that carrier block is also right in line with where the full auto trip lever is in the trigger pack.
So if you were to mill away part of the trip lever engagement ramp on a MP5 full auto carrier enough to clear the carrier block in an SP5, it looks like you would be left with about ~50% of the sear trip ledge on the MP5 carrier in order to clear the block. Unfortunately that is also the critical area on the MP5 carrier where the sear trip lever in the pack hits the trip ramp on the carrier.
However, if you were to thicken/widen the sear trip lever in the pack on the left side.... say by cutting the triangular portion off the end of a spare sear trip lever, and welding or soldering that cut piece of trip lever to the left side of the existing sear trip lever in the pack (essentially doubling the sear trip lever area thickness on the left side), it looks like there may be enough of the trip ramp left on a MP5 full auto carrier to engage a modded/thickened sear trip lever, even after removing part of the sear trip ledge on the carrier to allow it to clear the carrier block in the receiver .
I don't have a SP5 in front of me to measure the thickness of the carrier block and exactly how much of a full auto carrier trip ramp needs to be milled away to clear it.
It looks like there could be a cottage industry of SP5 "sear ready" thickened width sear trip levers and corresponding SP5 "sear ready" full auto carriers with part of their full auto trip ledge milled away in order to clear the block in the receiver, but leaving enough material to engage a thickened sear trip lever so that it will all work.
That way the receiver could stay "as is" without modification but you would need a modified MP5 carrier to clear the SP5 block as well as a and modded sear trip arm that could engage the reduced in size trip ramp on the modded carrier.
We would need somebody with a SP5 and full auto carrier to measure how much of a full auto carrier trip ramp needs to be milled away to bypass the block (and ultimately how much of the ramp is left) to confirm if this potential sear host workaround is a viable workaround.
Removing the block in an SP5 seems akin to drilling a 3rd pin hole in an AR lower.
With that being said, you all are politely Internet fighting with each other over unknowns. The SP5 will be a great sear host.