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.

autogun said:

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.

I don't think that increasing the MV of a grenade from 78 m/s to 100 m/s will make the muzzle brake working... you will still have ~0.5 g of powder to drive your ~240 g grenade (40x46 mm; 40x51 mm and 40x53 mm are all using much less than 1 g of propellant), with a HI/LOW chamber pressure system, so don't expect those 0.5 g of powder to produce any significant recoil-reducing effect.

EmericD said:

so don't expect those 0.5 g of powder to produce any significant recoil-reducing effect

I'm not. The primary recoil-reducing effect would come from reducing the MV by shortening the barrel (this is in the context of something like a RAG-30 firing 30 x 29B ammo).

That then leaves the gas exiting the muzzle to be disposed of. On further reflection, I think that a suppressor might be more appropriate than a brake - or some kind of combination of the two, as the US seems to be playing with, for their new small arms.

EmericD said:

Yes, that's the irony.

And it's true, too.

LOL.  If you want irony, try this revision of what you wrote:

Despite the fact that the only multi-shot grenade launcher adopted by a western army during the last two decades uses a revolver action -- or perhaps because of it -- pretty much every multi-shot grenade launcher developed during the last two decades uses detachable magazines.

schnuersi said:

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.

Concur.

schnuersi said:

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.

250g is almost twice the weight of the 25x59mm grenade. 

Seems like it would make for a very long 25mm grenade?

schnuersi said:

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

I'm curious how much aerodynamics / shell shape plays for the range of these subsonic munitions? Even the 'High Velocity' 40x53mm has velocity comparable to a .38 Special snub nose revolver or mid grade air rifle....Especially as the grenade probably needs a rounded, blunt nose to maximize the amount of fragmentation pellets that can be packed into the nose for forward firing airburst.

...

I say 40x51mm is what OICW should have been mostly in terms of what I see as the logical, incremental development chain that should have been used for developing counter defilade / airburst grenade launchers:

1. Develop FCU for MK19 AGL (large Gen 1 FCU size less a hinderance for AGL)

2. Develop Airburst 40x53mm ammunition (larger shell offers more room for Gen 1 fuse)

3. Work on refining both: improved frag shells, smaller fuse, reduced FCU size / improved performance 

4. Field rifle mountable FCU as technology when tech improves enough to allow miniaturization. Begin use with existing 40x46mm M203 and ammo.

5. Develop something like 40x51mm airburst, using airburst projectile developed in steps 2-3, but at a tolerable recoil level for shoulder use

6. Field either improved 40x51mm underbarel launcher and/or dedicated 40x51mm multi shot launcher paired with FCU from step 4

Instead they jumped straight from M203 to semi auto 20mm airbust high velocity rifle paired with 5.56 integral micro assault rifle...with predictable poor results.