The Japanese ‘knee mortar’ to my understanding was not designed to be shot from the knee. Ian McCollum at Forgotten Weapons have done videos on it, and I’m pretty sure I recall him mentioning that if you actually tried to use it on your knee, that it would not go well. Don’t know where the ‘knee mortar’ nickname came from, but to my understanding it’s an inaccurate nickname.
You are correct. It was never intended to be fired braced from the knee and it could break bones/dislocate your knee if doing so.
Rifle grenades work off blanks or shoot through designs with real cartridges. Not enough energy to prohibit firing from the shoulder (As far as I recall).
The M203/320 40mm grenade launcher works in the same fashion, it can be fired like a mortar for indirect fire or laid in from the shoulder.
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Well put. I hold no Military or Police training, but this seems accurate.
apparently supposed to be braced on the ground too -
(from 90th IDPG: The M8 Grenade Launcher)
They certainly COULD be used from the shoulder - here’s an interesting video and shows both ground and shoulder bracing - explaining why you would use one or other…
Special Forces used to use the M203 for expedient indirect fire via ground bracing.
The M203 uses a hi-low pressure system that gives much softer recoil than the large cartridges used to launch WW2 era grenades - and fires a much smaller projectile so it is not really comparable.
Anything can fire indirect - it is just a matter of ballistics.
I see, that’s interesting, but I’m pretty sure the grenade launcher sights on the M1 Carbine are meant to be used while standing. I also believe that the much smaller propellant charge of a .30 Carbine blank would deliver much softer recoil than that of a .30-06 or 8mm. So, if it occurs that the Kar98k and perhaps also the M91/30 grenade launchers require crouching, I think the M1 Carbine should still be serviceable from the shoulder even while standing. Then, to make it a bit more fair, the full-sized rifles given rifle grenades should be able to deliver their payload farther and more accurately. That just seems realistic. That, or the rifle grenades are launched basically the same but the payload for the full-sized rifles is greater, meaning a bigger boom, but the actual rifle grenades and charges used should be researched and that should be the basis of how they perform. Seems only realistic though that the recoil for the M1 Carbine would be lesser, and that the full-sized rifles either can push their rifle grenades farther or if not then they surely have bigger/heavier grenades that would produce a bigger bang.
The launching cartridge was not a “blank” - it was a more powerful cartridge than a regular round since it had to fire a large grenade.
The M1 carbine used the M6 Grenade Cartridge - I have no idea what it was loaded with - it certainly looks smaller than the Garand one. They all fired the sdame grenades so whether it was less powerful than the Garand version can probably be determined by knowing what ranges the 2 weapons were capable of firing grenades to.
Of some interest may be that the M7 booster - a small pellet (cut down .45 cartridge I read somewhere) could be placed inside the M1 Garand muzzle to give another 100 yards range - but was strongly recommended not to be used in the M1 Carbine.
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That’s pretty interesting, yup, and as you mention it would be helpful if more info was given. As for the .45 cal M7 Booster, I have NO idea how that works, but what I can tell you is that the rim of the .30-06 and the rim of .45 ACP are basically identical. This is also the case for 9x19, 7.62x25, and 5.56/.223. I don’t know how commonly known that is, but what is probably more commonly known is that .308/7.62 Nato (7.62x51) is simply a .30-06 (7.62x63) cut down by 12mm (about half an inch). .30-06 is a bigger cartridge than 8mm Mauser, but the 8mm Mauser was loaded more hot in WWII than M2 Ball .30-06. It might be due to limitations in the M1 Garand design (which was initially going to hold 10 rounds of a sub-.30 cal cartridge that would be softer shooting but ONE of the higher-ups saw this and said “Nope, cut that shit out, it’s .30-06 or it’s nothing” so the M1 was scaled-up and even then could only fit 8 rounds, plus a bend was put in the op-rod which may have weakened the system slightly), I’m not entirely certain, but it seems to me that basically with the potency of M2 Ball, .30-06 was bigger than it needed to be, so they swapped it for the civilian .308 cartridge, gave it the designation 7.62 Nato, and got rid of all the empty space inside for the more compact cartridge with M80 Ball 7.62 Nato being pretty much identical in function to M2 Ball .30-06 but being smaller. Smaller cartridge means smaller magazines, smaller receiver, less material used, a little bit of weight/bulk saved, etc. Good idea, really. If you can have the same oomph in a smaller package for a cartridge, do it!
Anyhow, yeah, .45 ACP and .30-06 (along with .308/7.62 Nato) has basically the exact same rim so it makes sense to me. Though I have no sweet clue how you go about putting a .45 cal ‘booster’ in the same chamber as a .30-06 blank, let alone a .30 Carbine! What on Earth?! .45 is 50% bigger than .30!
The muzzle is .30 calibre, again, .45 is 50% bigger! I dunno, I’m clueless as to how on Earth that ‘booster’ works with .30 cals… my mind just can’t wrap around that.
The M7 gets placed in the muzzle of the rifle grenade launcher - so the bore of the rifle itself doesn’t matter.
It has about 20 grains (I think) of black powder - about 1.3 grams - sealed by a waxed paper disc.
The base of the M7 faces outwards - on the side of the grenade. The open end faces down the launcher/rifle bore.
When the flash of the grenade cartridge reaches it it burns the paper disc and sets off the powder, providing an extra boost.
BLACK powder? Surely you mean smokeless powder…
I see, okay, that helps me to envision how it can be mounted and used. Trying to figure out how it works, I kept getting interrupted by the fact that two things cannot occupy the same space at the same time.
Sort of sounds like, depending on if the M7 leaves with the grenade or not, that it sort of turns the grenade into a borderline rocket-propelled grenade. Though I guess the propellant needs to be IN the grenade itself in order to be an RPG, so since the M7 is separate, fair enough, not an RPG, even if it’s a rather weak one. Neat! Learning new stuff about smallarms and ammunition is invigorating! If they make rifle grenades its own stand-alone add-on to firearms that can be upgraded just like firearms, one of the steps in upgrading a rifle grenade could be the inclusion of one of those boosters for better range and possibly improved accuracy in launching. It’d be nice to give realistic reasons why handling is improved, like the compensator you can attach to an M1 or M2 Carbine via a wing nut, just like the grenade launching adapter itself. “With the compensator attached, recoil management is improved.” That’d be nice, though I don’t know what’d be used as an excuse for reducing the M1 Garand’s recoil. Maybe the addition of adjustable gas somehow helping? Pretty weak, but it’s something, but I don’t know if the adjustable gas plug came about during WWII or not until after.
The Russians were quite forward thinking with their SVT-40 in that regard; built from the ground up with adjustable gas to help if using different types of 7.62x54r ammo, or if it gets dirty and needs more pressure to cycle reliably, etc. Love the SVT, but the M1 as well, lots of pros/cons to both semi-auto rifles 
Black powder was still used for a lot of applications such as small igniting charged in all sorts of munitions in WW2.
but on looking around the sites I found just say “powder” - so that’s all I know eg Mark II rifle grenade cartridge
as for making the grenade rocket propelled - not really IMO - A rocket carries the propellant inside it’s body - this is a seperate item to the grenade. More like the cartridge that fired the Panzerfaust.
I’m pretty sure the booster would not do anything good to accuracy!! lol
However there are developments of the M7 grenade launcher that could be improved - the original ones blocked the gas mechanism on the Garand - so it became a single shot rifle, but in 1945 an improved M7A1 was produced that blocked it only when a grenade was fired, so the launcher could be permanently attached and the rifle operated normally.
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