Kinda like Keltec CP-33 grew a long forend and a butstock  , likely something designed from the ground up but using of shelf pistol parts like maybe Sig 320 fire control and mags .

FN Grenade launcher in stand alone chassis is kinda just begging for it with its design.

stancrist said:

I just don't see them considering a 3-round tube magazine as being anywhere near adequate.

It depends on the planned primary role of your weapon. If you need a rapid area denial fire mission, them semi-automatic action and a large capacity magazine is optimal

if you need something else, i.e precision application of HE or other type of grenade (such as less-lethal) against a point or near-point target (door busting, for example, or room clearing), other options might be more preferable

gatnerd said:

If you had to ballpark the 6rd SSW mag, would you say 1kg, or more in the 1.25kg range? 

Not easy to answer, but I would bet that no one is going to use a 6-rds box of 40 mm grenades.

EmericD said:

I would bet that no one is going to use a 6-rds box of 40 mm grenades.

Concur.  The 6-rd mag looks too unwieldy.  5-rds or less looks more usable, such as on this French 40mm launcher.

Alsetex Cougar 40mm Grenade Launcher - Modern Firearms

Mr. T (MrT4) said:

STK SSW not being adopted is probably more down to at the time maturity of the firecontrol and the associated bulk not the tube mag , Bofors was at the same time offering a 5rd tube mag feed weapon.

No army adopted the Bofors grenade rifle, either.  And in the two-plus decades since, there been virtually no interest in tube-mag grenade launchers.  The development has been on launchers with detachable drum or box mags.

No one adopted 40mm box feed launcher ither !! Only thing seeing adoption so far in scale are chinese drum feed launchers, everything else uses significantly smaller caliber 25-20mm grenades and aside from Korean K11 nothing is even remotely close to being fielded in any numbers , 25 and 20mm grenade considerably changes magazine form factor so its hardly comparable to 40mm  concepts

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.  It doesn't matter that a drum or box magazine can't be "topped off" and may weigh as much as two or three rounds of ammo, it's still seen as a superior feed system versus a shotgun-style tube.

Pointless to argue , there were at least 5 designs developed with tubular mag one time or another so it seemed promising enough to try , GM-94 , ST Kinetics SSW, Bofors SSW ,Metal Storm 3GL ,Arcus 40, and there are hardly many more box feed grenade launchers in 40mm caliber so its a bit hard to claim box mag rules them all , and as none are in use it's a moot point anyway?

At present is seems Milkor ,GM-94 and the Chinese Grenade launchers are only ones in wide service .

We can't put the 25-20mm grenade launcher concepts developed in the past into the same category, they are closer to a low pressure 50BMG rifle than 40mm GL , and even these more or less failed to enter service.

40mm GL 5rd box mag is at least 1+kg , equal to 4-5x 40mm grenades ( each weighing 190-230g) , so its not that trivial considering that soldiers' combat ammo load is typically in the range of 3-3,5kg

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. 

Mr. T (MrT4) said:

...there were at least 5 designs developed with tubular mag one time or another so it seemed promising enough to try , GM-94 , ST Kinetics SSW, Bofors SSW ,Metal Storm 3GL ,Arcus 40, and there are hardly many more box feed grenade launchers in 40mm caliber so its a bit hard to claim box mag rules them all...

1.  I clearly stated that I was talking about weapons developed during the last two decades.  The GM94 and Bofors SSW are 20th century designs, and so do not count.  And the 3GL does not even have a magazine; the rounds are all loaded in the barrel.

2.  I never limited my remarks to grenade launchers only in 40mm caliber.  You are trying to make it sound like I said something different than I actually did.

3.  Again, I never claimed that "box mag rules them all."  What I repeatedly said is that most development has been on grenade launchers with detachable drum or box mags.

Mr. T (MrT4) said:

At present is seems Milkor ,GM-94 and the Chinese Grenade launchers are only ones in wide service .

Again, I was addressing what has been designed and developed, not what has been adopted and fielded.

Mr. T (MrT4) said:

We can't put the 25-20mm grenade launcher concepts developed in the past into the same category [as the] 40mm GL...

I strongly disagree.  There is no fundamental difference between 25mm and 40mm grenade launchers.  Both are designed to fire bursting munitions.

25mm HEAB - https://youtu.be/w-w_oNouMWo?t=57

40mm HEAB - https://youtu.be/BPaC1LTrHy0?t=545