Points worth noting are that the Koreans have built the launcher around the rifle, rather than just hanging a rifle under the weapon. They also seem to have used a rifle with a barrel of sufficient length to produce decent ballistics.
Fixing The OICW.
In concept theOICW is basically that of a grenade launcher with a laser range-finder. The range-finder lases the target and sets the round to explode at that range. The grenades are of 20mm calibre so that they will fly flatter, faster and farther than the standard 40mm. The original idea was such weapons would replace rifles. More likely is that they will serve as fire-team support weapons.
Quite a simple idea, and fine if you can get the programmable fuse to work.
Since the idea of the OICW was made public, General Dynamics have produced a 40mm GMG using Programmable Fused Air-burst rounds and the mechanism has also been demonstrated with 30mm cannon rounds.
A 12 gauge shotgun is already nearly 20mm calibre. It should be possible to build such a weapon that fires the 20mm OICW round, and it maybe possible to interchange components so the weapon can fire 40mm grenades or 37mm riot munitions too. Like the Striker shotgun this weapon should be capable of ejecting fired cases, and the port used for this could be used to load empty chambers. Such a launcher would be easier to manufacture than a semi-automatic weapon, and more compact.
Singapore is offering an OICW type weapon using a 40mm round. Negative feature is the use of 4.6mm or 5.7mm PDW rounds, which will be even less effective than the 5.56mm.
I'll term my variation of the OICW the Ph1-OICW to distinguish it from the official model which I'll term the US_OICW.
With a laser range-finder to eliminate the guesswork such a weapon would be pretty accurate with just a simple optic scope or iron sights. Fancy scopes and vision devices can be added on later when they are perfected.
An interesting sighting system has been introduced for the grenade launcher for the FN F-2000 rifle. A rangefinder lases the target and an LED lights up when the weapon is raised to the correct angle for firing. A similar system could see broader applications, including for long range machine gun fire.
The weapon would have a dial on the side of the Fuse Setter Interface to select different fusing modes. There would also be a two stage trigger, the first pressure activating the ranging laser, the second firing the round.
Both the OICW and OCSW may occasionally need to make curved trajectory shots, and should therefore be provided with a gas port to bleed some of the propellant out from behind the projectile. For this mode of fire the computer in the sighting system should display the correct angle of elevation for high trajectory fire for the range lased.
An Alternate Solution to the Problem of Close Range Firing. Information about the official 20mm OICW ammo is not too common. An educated guess would be that a HEDP round would be the likely choice. In fact the current round available is a simple HE round with Air burst fusing (HEAB).
The official OICW prototype has a 10" barrelled underbarrel rifle for close range defence. This adds about 3.5lb to the weapon's weight and probably doesn't do much to improve the balance either. Another thing that seems to be overlooked is the an M855 round fired from a ten inch barrel doesn't reach enough velocity for the bullet to fragment, so stopping power will be considerably less than that of either an M16 or M4.
The need for a co-axial weapon can be eliminated if the 20mm round used is based on the Krupp Universal shell. This resembles a shrapnel shell with the charge at the bottom, and with the shot also held in a matrix of explosive -we'll call the latter charge "A" and the base charge "B".
For troops in the open the B charge is used to produce a conical pattern of fragments.
If the troops are behind cover then the shell is exploded overhead using the A charge, producing a spherical pattern. This setting is also used for firing into a window during street fighting.
Against soft skin or lightly armoured vehicles there are several options:- firing a conical pattern just before impact, or firing both charges on impact.
For close range fighting the weapon may be able to project a conical pattern like a large shotgun, or fire the B charge on impact with the target. If the target is very close then the weapon may fire a round with the fuse unarmed. Given the size of the projectile this should still have considerable effect.
Alternate solution to close range firing is to issue canister rounds as was done with single shot grenade launchers in Vietnam. A 20mm OICW canister round would be slightly larger calibre than a 12g round, and being longer could probably carry more shot or flechettes.
The CAI S418 40x53mm Air Bursting Munition designed for use with Singapore's ST Kinetics Air Bursting Munition System is constructed with a base-mounted programmable fuse and the majority of its preformed fragments in the ogive of the round so they are projected into the forward arc. This may be a simpler alternative to the Krupp Universal type shell design.
One thing that also must be addressed is the default setting of the round.:- i.e. how it should still work if the electronics malfunction or are fried by an EMP. This will probably be impact detonation, so the weapon can function like an old grenade launcher, and should work purely mechanically. The default setting for the Universal shell would be impact detonation in a spherical pattern.
Ph2-OICW. If you must have a 5.56mm co-axial gun, make it an integral part of the weapon rather than an add on so that it can be part of a full offensive system rather than just a close range defence. This will make 5.56 barrel is long enough for the round to reach a decent velocity. Linking it to the 20mm barrel will stiffen it and allow it use its bulk as a heat sink during sustained fire. This weapon could be based on a configuration like theCrossfire shotgun/rifle.
The resulting weapon would therefore be a combination of both grenade launcher and light machine gun.
UPDATE A .45 Machine pistol of the form proposed here could be designed to be attached to the OICW. This would make a far more effective close defense weapon than any short barreled 5.56mm.
Whatever the form of OICW that is finally adopted it is highly likely that a canister round will eventually be produced. The Russians already produce a 23mm shotgun, the KS23, so a 20mm round (8 gauge?) is not that wild an idea. In addition to the OICW there should also be a compatible underbarrel weapon with at least a three round capacity for door-breaching.
The most recent releases on the XM29 now give the maximum range as 500m.
This does beg the question "What advantages does the 20mm round have over the existing rounds?"
As has been mentioned above, a Programmably Primed round has already been produced for the 40x53mm HV Grenades. Could not the same round be fitted into the 40x46mm LV rounds for use from modified M203s and Revolver grenade launchers? I suggest an alternative system to the 20mm OICW on this page
Most recent news is that the XM29 is to be dropped and will be replaced by a stand alone air-burst grenade launcher of 25mm calibre, the M-25. The intention appears to be to use the same 25mm shells as the OCSW. A round based on the Krupp universal shell would be a very useful. One can hope that there will also be a canister round, and 21 balls of OO arranged in three layers of 7 is not unreasonable. Such rounds are discussed on the blunderbuss page Intention appears to have a weapon with a box magazine. The Russians have the Izmash JSC MP-1331K, a variety of shotgun that has both a box magazine and a tube mag and the user can switch between the two. It occurs to me that an XM-25 loaded with a box magazine of grenades could also have a reserve tube magazine loaded with several canister rounds. If the grenadier finds himself suddenly in close combat he can select these rounds should he not have time to load a box of canister. Possibly this magazine would automatically be selected should the laser rangefinder detect that the target is too close for grenades. In a CQB operation he might have a box of canister loaded but have a few grenades "in the tube" just in case.