My first guess was this is looking at 40x51mm medium-velocity grenades.  But on a closer read, I think they want something much more demanding :

"After trigger pull, time of flight to target no greater than 3 seconds to 500 meters."

40x51mm usually has an MV around 120m/s, which isn't fast enough for this requirement.  You'd need 40x53mm at 240m/s to approach it.  But the only shoulder-fired weapon firing that is the Chinese LG-5, which is twice the weight target for PGS and nowhere near as compact as requested.  So, meeting this requirement is likely going to require another new type of ammo. 

This is exciting news. 

I've been hoping for something like this for awhile. 

Off the shelf:

Milkor 40mm w/ Multi Ray 800 FCU = 16.14lbs

24x 40x51 MV = 15.84lbs

Total = 31.98lbs

In terms of achieving the target weight of 14.5lbs, thats doable.

Rippel XRLG40 = 11Lbs

w/ Multi Ray 800= 12.54lbs 

Reducing capacity to 5rds could further reduce weight, although would complicate firing 3rd bursts. More likely, a Finite Element Analysis re-design of the launcher would find extra weight savings. 

In terms of the velocity requirement, that seems improbable. 40x51 is already on the upper end of tolerable recoil.

Per Emeric:

"A 250 g grenade launched at 100 m/s will need 8° of elevation to reach 258 m in 2.78 s, with a falling angle of 8.5°."

gatnerd said:

0mm w/ Multi Ray 800 FCU = 16.14lbs 24x 40x51 MV = 15.84lbs Total = 31.98lbs Per Emeric from a previous discussion: "A 250 g grenade launched at 100 m/s will need 8° of elevation to reach 258 m in 2.78 s, with a falling angle of 8.5°."

This solution doesn't satisfy at the time-of-flight requirement in the RFI.  To get to 500 meters in less than 3 seconds, you'll need an MV of at least 200 m/s, and possibly more.  That's not happening with a 40mm grenade in a package that fits the desired size and weight limits.  This demands a new ammunition nature to work, I believe, probably more like 30mm.

I expect they'll likely soften that time of flight requirement.

The 25mm XM25 was 210ms velocity, and even with the lighter grenade and nice semi auto design, recoil looked brisk in videos I saw.

https://www.youtube.com/watch?v=W-7L0Frj6vQ

Meanwhile, current 40x46 already has pretty marginal lethality, so it's hard to see them going to a much lighter grenade.

gatnerd said:

Meanwhile, current 40x46 already has pretty marginal lethality, so it's hard to see them going to a much lighter grenade.

The casualty radius of the 25 & 30 mm grenades studied during the "PAPOP" program were around 0.5 m against protected targets... absolutely not enough.

is this, finally going to be a use for Metalstorm technology? (I say this only semi-seriously . . .)

I'm trying to imagine alternative approaches that could be applied.  Maybe a 40mm GL-compatible course-corrected rocket for the counter-defilade round to get the necessary velocity without punishing recoil.  Other bits of the requirement are within the capability of a 40mm MGL with improved ammunition.

taschoene said:

I'm trying to imagine alternative approaches that could be applied.  Maybe a 40mm GL-compatible course-corrected rocket for the counter-defilade round to get the necessary velocity without punishing recoil.  Other bits of the requirement are within the capability of a 40mm MGL with improved ammunition.

Or maybe just a heavier 40 mm grenade with a bounding function? At longer ranges a quadcopter with a grenade might work as well without limiting it to being launched by a GL.

"Meanwhile, current 40x46 already has pretty marginal lethality, so it's hard to see them going to a much lighter grenade."

I've wondered why the U.S. uses the HEDP but not a dedicated HE projectile in both their 40 X 46 and 40 X 53 grenade launchers.

A well designed HE would have a better lethally radius. With airburst fuse or bounding ammunition it would be even better.

17thfabn said:

I've wondered why the U.S. uses the HEDP but not a dedicated HE projectile in both their 40 X 46 and 40 X 53 grenade launchers.

HEDP was opted for in order to have one ammo type that could be used against both personnel and light armored vehicles.

HE is ineffective against light armor, so grenadiers would have to carry reduced amounts of two ammo types:  HE and HEAT.