I'm trying to determine which has more ass kick factor for an M203.  An M576 round or a shotgun shell adapter.  

How many 00 buckshot pellets does the mil-spec M576 contain?
How many 00 are in the standard 3.5" 12 ga. round?

Sketchy internet info indicates (27) 00 pellets for the M576 and (18) 00 pellets for the 3.5" 10 ga.  If this is true, it appears the M576 wins.

Hell no to  3.5" 10ga.

I'd rather load up some low pressure buckshot rounds than fire those beasts.

Your internet info is wrong.

The M576 does not use OO Buck. It uses #4 buckshot.

There are 20 pellets in the small inner cup that contain the shot.

12 ga adapters work just fine, so do the M576 rounds. Since they contain different shot, your other question is moot.

The 12 ga adapters use a very short "barrel", no longer than 4-5". You don't get a lot of power from that short a barrel, and your shot pattern is pretty wide after a few yards. That's why the 576 was developed. To keep the shot pattern reasonably close at 20-30 yards. The 12ga adapter will not do that.

GROG

Thanks for the input Grog.  I was hoping you'de chime in.

Federal Classic 12 ga 3-inch #4 buck contains 41 pellets.  This is significantly more than the 20 pellets used by the M576.  
However, even though the 12 ga round has double the pellets, it spreads too quickly in the large 40 mm barrel and is therefore less effective than the M576 at 20-30 yards.  Is this correct?  
Therefore, if ones plans to use buckshot wih his M16, would one be better with an undermounted SBS than an M203??    

Yes. The shot from the M576 carrier is not released until it exits the 40mm barrel and the carrier slows enough from air resistance to rfelease them. The shot pattern will hold tight until that happens. When fired from an adapter with a length of 5", a 12ga OO buck will start spreading when it is released from the adapter, after 5" of travel.

If you want to add the ability to launch shotgun payloads to the M16 platform, a "Masterkey" mount serves this purpose better than an M203. (You can carry FAR more 12ga rounds than you can carry M576 rounds, plus the cost is much cheaper!) To get the effectiveness of the shotgun, you need a longer barrel than an adapter. There are actually two ways you can do this. First is the Masterkey mount for an 870 type shotgun. Second is this:
Nick Ferris Adapter
This adapter transforms your 40mm barrel to a 20ga shotgun barrel (rifled for slugs). You can then shoot 20ga standard shotshells, including buckshot, slugs, and the exploding practice pictured on the site.

As far as weaponry goes, a shotgun is meant to fire shotgun shells, the M203 is meant to fire 40mm grenades. When you take ammo from one weapon system, and use an adapter to fire it in another, there is a trade off. Especially with shotgun shells in a 40mm with adapter. Imagine the difference between your standard length 12ga 870, and a sawed off 5" barrel single shot 12ga. For short range "oh crap" shots, the adapter will work fine. So will the 12ga. For the medium range shots... the adapter will not work so well. The shotgun will.

GROG

wow, only 20 pellets of #4 buck in a 40mm round, surprised.  I thought it was this massive shotgun round.

I forgot the nomenclature for the 40mm buckshot round but I was understanding it to have 9 pellets of 00, same as a standard 12ga round.

CD

 

I like.
Where can I order some?

so the beehive adapters are substantially more lethal than the 40mm shotgun round?

the xm1001 is a mk19 rd, the M203 just dosent have the velocity.

i dont know how well that would work because people woul aim too high. The other problem is how the flechettes seperate from the canister.