The obvious solutions to the recoil problem would be either:

1. To produce a lower-velocity version of the 30 x 29B ammunition (modifying the case belt to ensure that the HV loadings would not chamber)

2. To combine a very short barrel with a very long muzzle brake (not so comfortable for anyone near the shooter!)

autogun said:

2. To combine a very short barrel with a very long muzzle brake (not so comfortable for anyone near the shooter!)

Due to the low muzzle pressure of launched grenades, muzzle brake are not very effective. That was the problem with the Barrett "payload rifle", the recoil was brisk and can't be mitigated with a muzzle brake.

stancrist said:

You seem to be missing the point: Despite the supposed advantages of fixed, tube magazines being touted by some in this thread, pretty much every multi-shot grenade launcher developed during the last two decades uses detachable magazines.

Which could be also written this way:

"You seem to be missing the point: Despite the supposed advantages of box magazines being touted by some in this thread, the only multi-shot grenade launcher adopted by a western army during the last two decades uses a revolver action".

schnuersi said:

The problem seems to have its roots in the shape of the 40 mm catridges. Its simply no where near optimal for any feed system. IMHO this is why the results of trying to get a multi shot 40 mm GL are all underwelming. A clean slate approach seems the most promising course of action in this case.

I actually think Rheinmetalls 40x51 is a pretty smart approach. It uses the same 40x53mm grenades used in the grenade machine gun, but fired at a tolerable 100m/s for shoulder launched use.

This is nice because

-The ~240g grenade of the x53 has proven to have pretty decent punch / about as much bang as we could expect from a shoulder fired grenade launcher; at the same time we probably dont want to go much lighter in shell weight

-Having x51MV and x53HV use the same shell allows for a much better economy of scale for the production of high performance fragmentation / airburst shells. 

-100m/s velocity allows for better range, but still maintains plunging trajectory for better attach against trenches / better effect with base fused forward fragmenting shells. 

-Certain types of x51 launchers would remain backwards compatible with existing warstocks of 40x46mm

....

Frankly 40x51 is what a smarter OICW program would have produced. 

EmericD said:

Which could be also written this way:

"You seem to be missing the point: Despite the supposed advantages of box magazines being touted by some in this thread, the only multi-shot grenade launcher adopted by a western army during the last two decades uses a revolver action".

Uh, no.  It cannot also be written that way.  That says something completely different than what I wrote.

20 and 25mm are just whole different form factor than 40mm

You can literally cut down a 50bmg case , flare the neck straight and stick in a 20mm cannon projectile, stick whole thing in a 50BMG action, fit 5 rds in 5rd 50BMG magazine mount a thin walled 20mm barrel. the main effort is then spent to shave weight from .50cal bolt and receiver making them out of aluminum with a steel locking insert for the new low-pressure round. 25mm is not that much larger a magazine similar to those used in 50 bmg would have 4 instead of 5 rounds in it.

Tubular mags for small caliber like that make no sense as capacity is more down to OAL than diameter, OAL is nearly the same for the 20mm vs 40mm grenade , while box magazine  is much more limited by round diameter, Same goes for Milkor style revolver its hardly of much use to these small caliber grenade launchers.  

Form factor changes things in similar ways in rifles and shotguns, first are impractical with tubular mags second are  rarely made with box mags that make them unwieldy or offer even less capacity than tubular magazine

Mr. T (MrT4) said:

Same goes for Milkor style revolver its hardly of much use to these small caliber grenade launchers.

A 30mm Milkor could be interesting. Either a reduced drum size, reducing the bulk and weight of the weapon.

Or keep the same basic drum size and boost it to a 7-8rd drum. 

The 30x113mm could be a candidate for a shared projectile. 

gatnerd said:

Frankly 40x51 is what a smarter OICW program would have produced.

I agree that a ~250 g grenade and ~100 m/s seem about right.
But I am not convinced that it has to be 40 mm in diameter. There are 23 mm shells ~200 g. So it should be doable to get a usefull 250 g grenade of 25 mm diameter or less. This would offer conciderable advantages for feeding. Not only for magazine or drum options for the squad support weapons MV ammo but also for the HV AGL ammo. The latter would also benefit from the improved aerodynamics.

EmericD said:

Due to the low muzzle pressure of launched grenades, muzzle brake are not very effective. That was the problem with the Barrett "payload rifle", the recoil was brisk and can't be mitigated with a muzzle brake.

Not quite what I had in mind. My aim would be to minimise the recoil by reducing the MV, because for a grenade of any given weight, it is the MV which mainly determines the recoil. The barrel would therefore be shortened to whatever length results in an MV of, say, 100 m/s.

This would result in lots of gas escaping at the muzzle at much higher pressure than usual, so some form of muzzle brake/suppressor would be useful in minimising the muzzle blast.

stancrist said:

Uh, no.  It cannot also be written that way.  That says something completely different than what I wrote.

Yes, that's the irony.

And it's true, too.