Most people will say to do your own work but I have yet to see a home conversion that met good standards. I used to work on them but I dont do that anymore.
Im not sure of anyone offhand except maybe Engage Armament in Kensington.
Here's one based in Damascus (just south of the HighSchool on 27) I've got one piece (a semi-auto pistol) with him now so I don't yet have the results, but given my conversation with him, the strong recommendation of the folks at the GunShack in Mt. Airy and the condition of his workshop - I'm expecting good things.... (and i thought his price was reasonable as was his target turn around time)
John's Gunsmithing Shop Licensee Name: Tyvela, Einar John
24636 Lunsford Court
Damascus, MD 20872
Phone: 301-253-0839
Served in the USA for 27+ years, salt of the earth. Worth a call to see if his background includes the type of work you are looking to have done.
I just left a message for Rich Freed out of Damascus. I went to Cresap today and my Marlin 1894 Cowboy totally locked up...with a live round in the mag!
If i dont hear back from him i'll give engage a call
Also, if you're not opposed to shipping out your gun, there are some folks on saiga-12.com who can convert your gun for a reasonable price. (I can personally vouch for Jack Travers.)
Check out www.saiga-12.com, scroll down left side for Lone Star arms. Check out his website completely including testimonials. He conveted mine to a model call "The Signcutter" It has never had a fail to fire or a fail to eject, even with cheap target loads. You can get the job done cheaper but never better. Good luck with the S-12, you're gonna love it. Start saving money for shot shells.