I have a LMT 40mm and a Pace 40mm/12g adapter.  I looked through my shotgun shell stash and found Winchester 3-1/2" 00 buckshot loads and thought "These would both hurt me and the launcher".

Has anyone tried the Aguila mini shells with the seven #4 buck + four #00 buck loading?  They're smaller and less velocity so you'd think they'd hurt less.

I wonder how effective any buckshot round would be fired from such a short adapter.  Weigh in on this if you're able.

40MM launchers are pretty robust, speaking of the standard mount. Not sure about rail mounts as I don't have one. I have fired all sorts of "adapters" from the 4x.410, to a specially made for me 10ga adapter. The 10ga I fired 3" magnum turkey loads. No, I wouldn't want to fire it a lot, but I did fire it over 5x with no issues. It is the most "kicking" adapter I've fired. GROG

ETA- as far as "effectiveness" goes, as effective as a sawn off shotgun at close ranges. Effectiveness drops drastically with increase in ranges.

Thank you Grog!  My LMT is a standard receiver mount.

I don't know about the mini-shells, but I've fired lots of standard 2-3/4" shells out of my 203 and they have less recoil than some of the 40mm loads.

I wouldn't expect much for accuracy or precision.  An extremely short smooth bore in those adaptors is not good for grouping.

My concern was from a colleague who owns a Serbu Super Shorty who told me that although 2-3/4" buck shells make a huge fireball and ungodly noise, the pellets would be rolling on the ground fifty feet from where he fired it with barely enough energy to knock over a Matchbox car.

Someone is exaggerating.

You might not be able to hit the broad side of a barn...from the inside, but what you did hit at 50 ft. would definitely know it - especially with the bigger pellets. Lethal, maybe not, but I'm confident it would be painful if it hit you.

YMMV

Yeah.  I always heard that 12 gauge makes 90% of its velocity in the first 10 inches of barrel.

I know that I hit a wounded deer at 140 yards with my 13" barrel slug gun and I bowled it over ass over head.  I would never have taken that shot it the deer hadn't already been hit, but it worked.

Even from a six inch barrel, I'd imaged buckshot would be pretty lethal at typical shotgun distances.

Update:

I finally got out to the nearby hills to run some shotgun shells through the 12g adapter in my M203.  I didn't find any Aguila brand mini shells but I did pick up some available Federal mini shells loaded with #8 shot.  

Wow-wee, I suppose I underestimated the clobbering power of a 12 gauge shell out of a very short barrel.  The recoil was stout and down range penetration of 00 and 000 pellets seemed fairly powerful.  They'll go right through the metal door of a stripped and abandoned Acura MDX at 15 yards and dent the inside of the metal on the other side of the car.   The pattern was somewhere like a medium pizza pan at that range.

Disappointing news is that some loads didn't fire reliably even after resetting the striker by opening the barrel a little, slamming it shut, and pulling the trigger again twice or thrice.  The Federal mini shells have a strange hemispherically-shaped primer and were quite hard.  Those shells only got dented a bit by the firing pin and performed terribly. Other shell brands using flat shaped primers seemed to fire just fine for the most part but I did get at least one FTF among those.

The shells that reliably fired were these Sellier & Bellot 2-3/4" 00 buckshot loads I had that come in a transparent hull.  Also the USGI surplus military 00 buck shells in the green plastic hull (sold all over America in five round no-frills generic cardboard cartons; I'm sure you've all seen
them) ran just great.  These rounds also cycle my Benelli M1S90 reliably whereas not all other shells will.  

Overall I am pretty impressed.  I will not make a huge habit of firing shotgun shells in my M203 because it's awkward and I can only fire just about ten before I'm wanting to call it quits.  And 3-1/2" Magnum shells are absolutely punishing.  All shells fired were really double extra loud as well.  The extra length of the M203 barrel acts like a trumpet bell and turns loudness up to 11.

For anyone else using a 12g adapter in their M203, you'll need to bring a foot long wooden dowel with you to pound out fired shells.  That 3-1/2" Magnum 00 load practically molded it's hull permanently to the adapter.