Is there an existing 51mm recoiless rifle?

Not that I am aware

In that case, how about 57mm?  Gun and ammo do exist.

The design and construction just need to be modernized.

https://www.youtube.com/watch?v=qsM8EtBZ-EQ

I have thought before a scaled down Carl Gustaf might be useful. 

Something like 66mm, akin to the LAW, but reloadable and more accurate. Similar to the relationship between the 84mm AT4 disposable and 84mm Carl.

66mm is .786 of 84mm, so just taking the M4 weights and multiplying by .786 as a guestimate: 

84mm

Carl Gustaf M4= 14.55lbs

Aimpoint FCS13RE = 3.53lbs

HE 441D Airburst = 6.83lbs

x3= 20.5lbs

= 38.53lbs

66mm

Carl Gustaf 'M66' =  11.44lbs

Aimpoint FCS13RE = 3.53lbs

66mm Airburst = 5.37lbs

x3 = 16.11lbs (x4 = 21.48lbs)

= 31.08lbs / x4 36.45lbs

gatnerd said:

I have thought before a scaled down Carl Gustaf might be useful. 

Nope it would be a dumb idea. 84 mm isn't very big and the weight is steadily coming down. A drop in calibre would give up all pretence of being anti armour. 

RPG 16 went the route of a higher velocity to increase range and accuracy

https://modernfirearms.net/en/grenade-launchers/russia-grenade-launchers/rpg-16-eng/

Airtronic went super lightweight materials. accuracy is poop but for launching guided projectiles it would be a starting point. 

https://modernfirearms.net/en/grenade-launchers/u-s-a-grenade-launchers/airtronic-rpg-7-eng/

Brute force and ignorance was the Breda folgore. This is a quite old system now and could be dramatically improved with lighter materials and more moder projectiles. 

https://en.wikipedia.org/wiki/Breda_Folgore#:~:text=The%20Folgore%20(Thunderbolt)%20is%20an,the%20Italian%20Army%20in%201986.

The endless search for super lightweight systems because troops are apparently too stupid to use a transport really needs to die.

Red7272 said:

Nope it would be a dumb idea. 84 mm isn't very big and the weight is steadily coming down. A drop in calibre would give up all pretence of being anti armour. 

Thats the thing, I just don't see the US ever engaging armor with Carl Gustaf's. Really, any armor not destroyed from the air, or engaged by tanks and other armored vehicles, is likely to be hit by Javelins (or hopefully in the future, something like Spike SR.) 

Where the Gustaf actually is useful is as a dismounted squad / platoon anti structure / anti personell HE lobber. 

At which point the lighter launcher + more ammo of the 66mm is likely to be advantageous.  (Assuming a 66mm actually did result in a meaningful reduction in launcher and shell weight - not a given.) 

Just use BUR or make the equivalent with an aiming module for the LAAW. Sticking someone with a 6 kg contraption to carry around is pointless. 

https://modernfirearms.net/en/grenade-launchers/russia-grenade-launchers/bur-eng/

Red7272 said:

The endless search for super lightweight systems because troops are apparently too stupid to use a transport really needs to die.

What transport do the troops have available to them that they are too stupid to use?

gatnerd said:

I just don't see the US ever engaging armor with Carl Gustaf's. Really, any armor not destroyed from the air, or engaged by tanks and other armored vehicles, is likely to be hit by Javelins...

Concur.

gatnerd said:

Where the Gustaf actually is useful is as a dismounted squad / platoon anti structure / anti personell HE lobber.  At which point the lighter launcher + more ammo of the 66mm is likely to be advantageous.  (Assuming a 66mm actually did result in a meaningful reduction in launcher and shell weight - not a given.)

Well, the M136 weighs ~15 lbs vs ~6 lbs for the M72, so it seems to me that a substantial weight reduction could be had with 66mm.

IMO, the real question is how effective would a 66mm recoilless rifle be?  For instance, how much would HE casualty radius reduce?

stancrist said:

What transport do the troops have available to them that they are too stupid to use?

You mean real armies or the US army? The list of NATO members that have not entirely mechanised their infantry is a whole one. 

And yes it was a throwaway insult at the US military hierarchy rather than the troops themselves.