That sounds like a lot of custom work.

Vollmer made a 13. You might try to find something like that. My Vollmer HK13 I picked up in '97 at the Suncoast Gun Show in Lakeland, Fl. | HKPRO Forums

If you want picatinny why not a KAC RAS?

 

Due to the weight of the rifle and added thermal optics. Usually we carry the rifles around locked into the tripod, tripod slung over the shoulder. Like an old hobo stick/sack. I know KAC makes good stuff but I don't have faith in that particular rail holding up over time especially with $20K plus worth of optics on it and then serving as the interface between the rifle and tripod under automatic fire.

 

I would consider getting yourself an m-lock handguard. On it, mount an ARCA rail. The ARCA rail will give you much flexibility on what you can mount on it.

ball mount


Arca rail segment

 

Slightly OT but my first choice of HK firearms for taking out that sounder would be an HK169 and a M430HEDP round . . .

 

Have someone make you a 13e~K. That’s the same shroud, as the 23ek. Get it made as a rifle with a long barrel and put in a short barrel when your using your sear. I think the long barrel would be better on the tripod/hunting though.

Also quad rails can be welded on the top/bottom of that.

Good luck man!

 

I didn't see a stupid question, just one concerning physical feasibility. It can be done, but is usually not due to costs of material and custom work.

• By the time you've sourced a shroud the mindset is you may as well get the most bang for your buck by installing a removable barrel system trunnion (my mindset anyway especially considering the challenge of finding the short shroud).
• Installation of the shroud on an existing firearm will require some modification (cocking tube, triple tree, etc), so considering gunsmith/builder costs my mindset would be to just build up a new receiver. You can use the internals and trigger group you already have to save costs. RTG has the new 33 flats and while I haven't tried them out yet they do focus on quality gear.
• For your builder I'd recommend ghilliebear at Parabellum Combat Systems. Jeff's built several custom 13E guns that look and function great and I know he's done work on short guns as well. I can't speak to whether he's done something like this but he would absolutely be the best to talk to regarding physical feasibility and cost along with turnaround time.

I concur with The Colt Collector in the path I'd take: build out a 13EK and enjoy the ability to modify as you go.

Sounds like a fun project, please let us know which was you opt to go.