From my notes:

40x51mm w/ 240g projectile = 299g cartridge weight

x24 = 7.176kg / 15.82lbs 

.....

Mag weight guestimates with 24rd loadout, based on the 700g per 10rd .50 mag provided by Mr. T

@1kg per empty 6rd mag

2x 6rd mags + 12x loose grenades = 9.176kg / 20.23lbs

4x 6rd mags = 11.176kg / 24.64lbs 

@1.25kg per empty 6rd mag

2x6rd mags + 12x loose grenades = 9.676kg / 21.33lbs

4x6rd mags = 12.176kg / 26.84lbs 

.....

If anyone knows the weight of the 5x 40mm MAG D grenade dispenser, that would help with the estimates 

https://defenshield.com/product/mag-d-grenade-dispenser/

50BMG box is at best 1/2 the lenght and even 2stack is still  considerably narrower as rounds are staggered.

So 1kg would be a very light aluminum magazine

gatnerd said:

If anyone knows the weight of the 5x 40mm MAG D grenade dispenser, that would help with the estimates

I dunno if it would help.  The MAG-D looks to me like it's made of plastic.  

From the available photos, I can't tell if the SSW mags are steel, aluminum, or plastic.

mpopenker said:

As far as I know detachable magazine was briefly considered for the GM-94 early in development, but dropped for excessive "dead" weight and bulk. You can carry more loose rounds in simple bandoliers or LBV pockets for same weight, and you can top up tube magazine as you go

Obviously, you can carry more loose rounds (for the same weight) than you can carry loaded in magazines.

Despite that, most grenade launchers designed in the 21st century use detachable drum or box magazines.

A few examples:

XM25 25x39mm

LG5 40x53mm

LG6 40x46mm

The US Army wants a semi-auto Precision Grenadier System to replace the single-shot M320.

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

The mags look metal to me from the pics. The central grooves in the mag are also what I associate with metal mag designs I've seen; can't recall a polymer mag with those central grooves.

That said, a fielded version of the SSW or similar could likely see polymer mags as standard. Polymer being light, cheap, and more resistant to dents then metal. 

...

Knowing the MAG-D weight would be good as it would provide a lower baseline of weight. Being a 5rd polymer x46 mag, we can predict pretty confidently that the SSW mag would be equal or heavier in weight. 

gatnerd said:

The mags look metal to me from the pics. The central grooves in the mag are also what I associate with metal mag designs I've seen; can't recall a polymer mag with those central grooves.

They probably are metal, but in regard to polymer mags not having side grooves, I have seen a couple of exceptions to that rule.  For example:

gatnerd said:

That said, a fielded version of the SSW or similar could likely see polymer mags as standard.

I agree, polymer mags seem very likely.  Especially for any semi-auto grenade launcher fielded by the US armed forces.

gatnerd said:

Knowing the MAG-D weight would be good as it would provide a lower baseline of weight. Being a 5rd polymer x46 mag, we can predict pretty confidently that the SSW mag would be equal or heavier in weight.

I have not come across any weight data for the MAG-D, and I doubt that it matters except for academic comparisons to something like the STK SSW (which will probably never be adopted by a Western army due to the 3-rd tube magazine).

Mag on the Hydra is 100% metal , ,stiffening and contouring ribs like that are not used in plastic  mags. 

Mag-D is look very thin walled and lightweight more of a Tupper ware than magazine surrogate wouldn't be a base line for mag weight , There are good reasons why magazines weight that much, they are practically no.1 factor determining firearms reliability

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. Same as all previously  super smart electronics packed developed mag feed grenade launchers.

STK SSW aslo added a 5.7 firarm combo with shared fire controls , so was very ambitious at the time.

All these launchers are becoming interesting as the under barrel launcher is kinda becoming obsolete with lesser accuracy(absent of dedicated electronic sights) and efficiency compared to stand alone configurations hence M320 replacing much of the M203 arsenal.

I am still toying around a bit with developing a 'submachine gun' chassis for FN40GL as i think could be made to work without adding too much weight..

The Chinese 6-rds drum mag for the QLZ-87 (35x32 mm) weighs 1.3 kg (empty) and 2.8 kg loaded.

The 15-rds version weighs 2.3 kg (empty) and 6.0 kg loaded. 

Hurray! Great info, thank you, thats very helpful for our estimates. 

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

Mr. T (MrT4) said:

I am still toying around a bit with developing a 'submachine gun' chassis for FN40GL as i think could be made to work without adding too much weight

That sounds cool. Are you thinking pairing a 40mm with a SMG, or creating a more SMG form factor chassis to hold the FN40GL launcher?

I had initially thought of the MP5/M203 combo, used by Arnold in End of Days to fight Satan (wonderful silly action movie.)