Army Considers Converting 155mm Projectiles into 40mm Grenades

The US Army has patented a system to convert surplus 155mm projectiles containing bomblets into 40mm grenades.

Someone in the Pentagon thinks it would be cheaper to convert surplus bomblets into 40mm grenades than to dispose of them. I had always assumed that the cost of the secondary explosive material was only a small component of the total cost of a grenade. This seems like a scheme to reclassify 155mm disposal costs as 40mm manufacture costs.

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • Charles222

    Hnnn, interesting. The all-star 155 round of the last few years has been the Xcalibur guided round from my deployment experience; the submunition rounds, iirc, are mainly designed for breaking up mechanized columns, and the Army’s tried to replace them with things like SADARM in any case.

    • bbmg

      Given the general attempts to avoid collateral damage in modern conflict, I can see why these submunitions might not be seeing much use.

      Presumably a bunch of these adapted rounds belted for a Mk 19 or similar AGL can give immediate support to troops similar to 155mm artillery firing from afar – with the beneft of being aimed more accurately at a specific target, rather than carpeting a whole area.

      Fired at some velocity rather than just dropped I would speculate that the rate of duds would be lower too.

      • TopGun

        IIRC these round were receiving a lot of flak (pun) because of the amount on unexploded munitions they would leave on the battlefield. It makes sense for them to phase them out

  • Nick H.

    Assuming that the fuze assembly, shaped charge and explosive material were all original to the bomblet I would tend to think this would be cheaper. The current M433 HEDP cartridge costs almost $70 per round. So this round would just be the cost of the (aluminum?) collar/driving band attached to the end of the bomblet and the M118 casing.

    • Nick H.

      It looks like to nose cone is added as well and it looks like the M42 bomblet doesn’t have any kind of fragmentation matrix like 40mm anti-personnel grenades do. So this would be a dedicated anti-armor grenade rather than dual purpose.

      • Charles222

        Interesting, Nick. 🙂 I didn’t think about that. It seems like the Army is trying to boost light forces’ armor-killing abilities. It didn’t get as much fanfare as the air burst round has, but there is an anti armor round for the XM25 that’ll penetrate about 50mm of RHA; this should be able to do about the same, I’d think. That’d make three light anti armor weapons (2x 40mm GL, 1x XM25) in a squad, and nine in the platoon. All reloadable (an issue with LAW/AT4), and capable against anything lighter than a BMP.

      • Nick H.

        It looks like these bomblets do have scoring on the inside to act as anti-personnel fragments, but still not a proper frag matrix. But the listed armor penetration for an M433 is 50mm of armor plate whereas the M42 bomblet is 70mm, so still interesting difference.

      • Charles222

        70mm of armor penetration would be truly slick. Hope it can be retained in the transformation to 40mm grenade. A combination of 25mm & this would be fantastic against a road column of vehicles; you could use the Xm25s to stall the convoy by disabling the first and last vehicles, and then use the less-accurate 40mm on the vehicles stuck in the middle.

    • bbmg

      The patent mentions polymer parts, presumable acetal or similar would be suitable.

  • genschow

    Costs of disposal to be factored as well.
    Bundeswehr budget was seriously affected by having to deal with ex-DDR ammo in the 1990s.

  • Chase

    What kind of benefits could the Army gain by decreasing their 155mm disposal costs, or by increasing their 40mm manufacturing costs? What bureaucratic process might be triggered by this reclassification?

  • bbmg

    Does it actually engage the rifling though, or does it use the “drag ribbon” for stability?

    This assembly appears to be faired over in the patent, but does the fairing remain attached in flight? It would seem not:

    ” Upon exit from the gun tube, pusher 36 may separate from casing 16. Arming of fuze 12 may occur via air flow interaction with ribbon stabilizer 14″

    Doesn’t sound like it would be very accurate, presumably it would have to be used en masse from an automatic grenade launcher to ensure an effective hit, or at very close range from single shot underbarrel launchers.

    Interesting to note that spinning the projectile makes the shaped charge less effective, which is why most HEAT rounds are fin/drag stabilised.

    • Shawn

      It will probably use the ribbon, due to the come not being fluted, like the 40MN HEDP rounds for the proper formation of the jet to penetrate.

    • shawn

      Cone* sorry autocorrect.

  • alannon

    Hey, if the Army can usefully repurpose something they’re getting flak over anyway, more power to ’em. Based on comments above, even if it lacks the armor penetration or soft-target range of a standard grenade, it would still be more than capable of taking out most of the vehicles the US has encountered recently.

    Toyota Hilux are legendary for their durability, but I somehow doubt they could survive something like this.

    • charles222

      I actually wish Toyota sold the Hilux here. They seem like nice trucks :p

    • mosinman

      if it will take out a toyota hilux, then what does that mean for the LADA technical? ;D

  • gunslinger

    wait, i’m a bit confused. turning a 155mm projectile into a 40mm projectile? what happens to the other 115mm?

    or does the 155 have a ‘shotgun’ type load of smaller projectiles that are being used in the 40mm form factor? i.e. there are say, 6 30mm bombs inside a single 155 shell meaning there will then be 6 40mm “new” projectiles?


  • Chucky

    Hold on, the M42 submunition doesn’t HAVE an inertial safety or timer. Nor does that patent for the 40mm conversion. Sounds like a bad idea to me.

    • Chrontius

      Not all 40mm grenades are fired from rifles. Isn’t there a version of the CROWS station set up for the Mk.19?

      • Chucky

        Sure, but people are still gonna handle those rounds even if they aren’t necessarily holding the weapon that fires it. Without an inertial safety, the rounds can blow without being fired ie if dropped.

    • bbmg

      I believe there is a delay mechanism, the drag ribbon spins to activate the fuse but it is not dangerous before that – after all, if it can be fired out of a cannon without detonating, I doubt you could set it off by dropping it.

      • Chucky

        If they’ll be putting a ribbon it better be strong enough to take a lot of abuse because it’s sitting on top of a sizeable powder charge. Plus, if it will use the ribbon system as a safety, it can only be used on single shot launchers because I’m pretty sure that ribbon sticking out will jam any action, revolver, pump or automatic.

      • bbmg

        Have you looked at the diagram at all? The ribbon is folded away and covered by the rear cap, and only exposed when the round leaves the barrel. In theory you could play tennis with the loaded round in perfect safety.

      • Chucky

        You’re right. I mistook the ribbon as a solid cap protecting the detonator. But how will they assure the actual end cap will separate?

      • bbmg

        That seems to be a minor problem, one that has been solved before with other ballistic projects.

        They could for example cut grooves in the side as on remington accelerator sabots:

        The centrifugal force from the rifling would force the “flaps” out, greatly increasing drag making the lightweight cup decelerate much faster than the rest of the projectile.

  • Tony

    If they can make the deal work, it will save money and the environment.

    • Bill C.

      Rrriigghhtt, the environment.

  • Lance

    I wouldn’t want to trust a bomb-let made for artillery for a small GL. But who knows. it is a good way to save tight DoD dollars though.

    • charles222

      Remanufacturing process, dude. It’s not like they’re just shoving M42s into 203s/320s and calling it good. :p

  • Mike Knox

    Wee, Retrofitting heavy ordinance components into small arms, What could possibly go wrong?

  • Big Daddy

    This is a bad idea and being used to show that it is not good to cut defense spending. They will start to come up with all these dumb ideas to save money trying to literally scare people and politicians into changing their mind. They won’t…..

    The cost of a 40mm is high compared to a 50 cal round for instance, I bet this way would eventually cost more. Considering the start up cost of making assembly lines and testing who are they kidding.

    The first accident trying to make them this way or using these rounds will stop the assembly line shutting it down and wasting even more money. Not to mention somebodies life.

    So I wonder exactly who would want the job of taking apart bombs? Nobody right, so you would have to start making robotics specifically designed for that job right. How much would that cost.

    This is just plain BS.

    • bbmg

      People put these things together, handle them and store them without fearing for life and limb because of the built in safety mechanisms, I don’t see why robots would be needed for the job of disassembling them.

      As to assembly line cost, we’re talking about adding a polymer nose and tail to the existing round, I can’t see that being very expensive per round with modern injection moulding techniques. Besides, it will probably be outsourced to a Chinese firm who will do it for peanuts anyway.

      • charles222

        correct-a-mundo, besides that we can’t outsource something like this to China. :p

  • snmp

    155mm with submunition (Cluster bombs) are against some War Laws, But if You have the same bullet in Submunition and 40mm : The proof of use of forbbien weapon could be difficult

    • charles222

      We’re not signatory to the ban on cluster munitions, and remanufactured M203 rounds aren’t going to be like the M42s they’re converted from.

      • snmp

        You need to have in mind that’s for others countrie that’s war crime , and you could lost supports of allies, could be face of trial aftermatch ……

    • charles222

      As long as the US is never signatory to the ICC, your point is worse than irrelevant.

  • Pepin the Short

    Now that’s thinking like a Russian!

  • mick

    They will have to change the fuze firing mechanism. If they try using the grenade/bomblet as it is currently loaded, they will run into a problem when the tape ribbon causing the removal of the firing pin from the sliding detonator, allowing the sliding detonator to align the firing mechanism, this action will cause a shift the center of balance throwing the accuracy off. It seems like a good Idea but it needs more thought by ones who understand cause and effect on how munitions work and the end result desired vs. received.