Calliope asked some good questions, so here's some answers...
The conundrum was this: Getting better projectiles to kill tanks into smaller/lighter guns - preferably that the troops could carry themselves and not require motor transport. Especially light troops, like airborne forces.
What to do, what to do.
Conventional guns are tubes, sealed at one end. Open the sealed end, stick in your cartridge, close the breech, fire. Newton's observation that for every action there is an equal and opposite reaction then takes hold. You send a lump of steel in one direction, the barrel wants to go in the other direction, less the impact of various inefficiencies such as friction and heat transfer from propellant to tube, etc. There are just limits to what you can do here. You can improve the performance of a given gun system by using shaped charges instead of solid shot. Of course, that presents a different problem, since shaped charges don't like to be spun. You can re-tube it to a 'squeeze bore' gun where you use special ammunition that swages away going down the bore, which will achieve a greater velocity, at the cost of greater ammunition cost and complexity and tube wear. You can be more efficient than that and put a sleeve, or sabot, around the smaller projectile and fire it from the same bore (the process used on most tank guns today). All of this is fine - except it doesn't make your gun any smaller, and aside from adding a muzzle brake to it, it really doesn't help your recoil any. If just improves performance of the existing system against more resistant targets.
You can use a rocket. Once the shaped charge was developed, that became practical. This time, instead of sealing the tube and pushing out a projectile, you seal the tube and let the gases vent out the open end. Same thing - only this time the projectile sits on the end of the tube and the tube flies with the 'jo. This is the concept used by the bazooka, and it worked, though you suffered some limitations in ammo types, because the state of the art at the time pretty much limited you to shaped charges, which limited the tagets you could attack. And the ammo was expensive.
So, what else can you do? Well, the first recoilless gun (Argghhh! I can't find my copy of Hogg/Batchelor's Artillery!) was developed during the 1700's. Not terribly practical, it would have achieved it's recoil-cancellation by firing the projectile one direction, and an equal weight of shot the other. Difficult to employ tactically, yes? But what if you could use the gases? Meter them out the rear of the piece, so that the thrust from that canceled the thrust from the projectile? And thus the recoilless rifle was born.
The rest is in the extended post.
The 57mm was the first US gun. The Burny gun was the first Brit piece, which took a different approach, of using multiple venturi tubes.
The photo above is the breech of my gun. The black 'sausage shaped' holes are the vents, more technically known as venturis. Making use of the 'venturi effect' these vents are carefully shaped and sized to precisely meter out the gases from burning propellant to produce just the thrust needed to counter recoil. Since the bun requires no other recoil-absorbing mechanism, you can save a lot of weight. And once the principle was proven, it was easy to scale 'em up to bigger calibers.
So, Calliope wanted to know why pre-engraved rifling? That was to ensure uniform, predictable resistance from round to round. Variations in the bronze used for rotating bands, inconsistent seating by loaders under combat pressure could result in teeny-tiny delays in how fast the projectile starts it's trip down the bore. Since the gases aren't suffering those delays, accuracy and range could be effected. You could even have forward recoil. So, the rounds were pre-engraved, and the casings had lugs on them to prevent improper seating of the projectile. The fuzes were shaped so that is was easy for the loader to get the round in the chamber without fiddling, and a tiny twist one way or the other was sufficient to seat the round properly.
The breechblock, which, as John noted, is of the interrupted thread type, only requires a quarter turn to open and close. Because the chamber is vented, the gun can be lightweight overall and the breechblock can be much lighter.
The breechblock supports the end of the cartridge, keeping the base aligned with the firing pin.
Lessee, what have I missed...
How do we get those gases out the vents? Simple. Since the cartridge case does not, unlike in small arms and larger guns, perform any sealing of the breech, we punch it full of holes, so that the gases of combustion boil out of the casing into the breech, moving the projo on it's way, and exhasuting themselves out of the rear. Having lost a trouser leg to the backblast of a 90mm reckless rifle, I can attest you should keep soft and chewy bits out from behind the gun while firing.
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