Where are these found in the market?

This

 

Here: https://www.gunpartscorp.com/ad/1368250.htm
As for Stoner63a's question, I have not heard of anyone making a subcal for the 84mm. It could probably be done easily, if you had the money to blow on one.





 
 

The problem I see is, the CarlG has the firing pin striking from the side, the M67 90s we had, the primer was centered in the casehead, struck from the back inside the venturi, vented out the perforated casing holes sideways then out the back.



I don't know if there is enough spring energy to drive a floating piano wire firing pin to a 12ga etc chambered adapter centered in the bore.



OR a vertical pin that strikes an angled block to direct the FP energy 90 deg?



Simple blank adapter would have the 12ga extra loud black powder shell inserted sideway into the case, with a T at the chamber end that directs some smoke/blast out both ends, like a Roachcoach burrito. This leaves about 5/16" chamber wall thickness and about 1 1/6" in front of a 2 3/4" shotshell for the T.



Turn a replica casing in 7075 T6 aluminum 3 3/4" OD round bar, right, then the index slots on the case rim, drill an pilot hole centered on the original flashhole spacing, cut a 2 3/4" chamber,then bore a 3/4" hole front to rear for the vents, maybe bore it oversized then press in a stainless tubing section with 3/4" port in the side for the shotshell chamber, the stainless will reduce the blast erosion out the casing side, even be replacable if needed. Bore some 3/4" holes into the adapter front face symetrically to reduce weight.



The CarlG venturi assy holds in the adapter, covers the adapter's appearance, where the orig casing vent gets blown out the casehead.

Turns out there is a sub-cal adapter made by Saab for the 84mm (although this is far out of the hands of us lowly civilians).
It's called the "7.62 mm 553B Sub-Calibre Adapter" and uses a separate primer to actuate a firing pin that sets off a special 7.62x51 tracer cartridge that is loaded to match the HEAT shell's trajectory.
The subcal unit:




Here you can see where the side mounted primer goes, along with the safety on the rear:




And the special cartridge and ignition primer:









 

On the 553B subcal adapter there are three parts that can be set up in 3 combinations.  Weight of 553 is 8 lbs and lenght of 24 inchs.

Combo 1:  7.62mm Tracer (lower velocity at around 340 mps) and primer
Combo 2:  Primer and Backblast charge
Combo 3:  Primer, 7.62mm ctg and backblast charge

The 553B can be used in both the M2 and M3 Carl G.  As stated 553B is used for training using the sites (fire control device/optic or irons) set for the 551 or 751 HEAT rounds.  I usually see the 7.62mm round with a white tracer tip instead of the red shown.


CD

A member built a sub cal unit for the disposible one shot version of the 84mm:  the AT4:

http://www.ar15.com/archive/topic.html?b=6&f=2&t=416612

As for sub caliber units for the M2 and M3 84mm, there are at least 5 types:

-6.5x55mm Swedish (original - configured as above with optional backblast simulator)

-7.62 NATO pictured above  (here is is being fired without the backblast simulator charge:   https://www.youtube.com/watch?v=9MhX1pCuVUA

-9x19mm  (here is a YouTube video of it being fired:  https://www.youtube.com/watch?v=Q4v1_byHetI

and also:   https://www.youtube.com/watch?v=linZrCRS3t0

-20mm true recoiless (pictured in the Scribd manual).  Here, at the 25 second mark, you can see the newest 20mm true recoilless unit and fixed tracer round:  https://www.youtube.com/watch?v=BOrZqZa14P8

The 20mm sub cal previously used a non-rifled barrel and a 20mm practice projo that looked like an elongated airgun pellet, followed by a caseless blue recoiless charge and a priming charge/primer on the side of the unit.

Finally, there is aslo a  63mm rocket assisted sub cal.  Don't know much about this one.

OP:  I would avoid building anything over .50 caliber - even if it uses a shotgun shell (remember what happemed to the "Street Sweeper" and USAS-12 shotguns?) and I'd make it at least 16" long just to avoid hassles - like the kind of hassles that "behive rounds" could create re: additional stamps.

As for the 90mm sub cal in 7.62 NATO:  here it is:   http://thedonovan.com/archives/2012/04/the_90mm_recoil.html

Its not explained on the page, but that unit's design  is very clever:

-it uses a FULL POWER tracer round and is designed to blow four holes right through the case shoulder (which destroys the case for reloading , BTW). That accomplishes 2 goals at once:  the tracer bullet's trajectory is lowered to MATCH the 90mm projo's trajectory (about 900 FPS) and it produces a small amount of back-blast noise (though far less than an actualy 90mm round I'd guess).  As you noted, the unit features a FP at the rear instead of the side as in the 84mm.