This idea occurred to me while reading about Recoilless Rifles. Why not have a turret roof mounted RCLR that drops down to allow the breech to be loaded undercover -such should fit on a M113, maybe in a multiple mounting
I thought about this idea further:-
The RCLRs can be mounted on the top or the side of a turret. They use their existing mountings for elevation but traverse with the whole turret (it could be done another way but this is probably simpler). The forward part of the turret holds the gunner and his sighting gear, and probably a machine gun or cannon. In the rear of the turret is the loader and the ammo. The breeches of the RCLRs must be above the loader's compartment. Depending on the vehicle this part will be open topped or have hatches in the roof that open or descend when the weapons need to be reloaded. After the RCLR is fired the whole weapon drops about a foot or so so that the breech of the weapon is in the loader's compartment and can be replenished. The gun is then raised up again to fire. This is a pretty simple system mechanically (I could be wrong) and pop-up mounts are hardly new -most of the finer detail is better drawn than described. Obvious safety feature is a cut out so the RCLRs can't fire when their breeches are in the loader's compartment.
System will probably work well with multiple RCLR mountings. A two rifle set-up might look like a keyhole from the top. The round bit is the loading compartment and a RCLR is mounted on either side of the rectangular section. The rectangular part has the raising and lowering mechanism and would also be the position for a separate gunner and secondary weapons. You could scale this up for a vehicle with a large roof hatch like the M113, two or three RCLRs on either side of the rectangle. This would also be a pretty low turret since the guns are raised above it for firing and elevation, and with an armoured roof high elevation shots could be possible. A single RCLR mount with just a circular shield around/below the weapon should fit a jeep.
UPDATE The Japanese Type 60 anti-tank vehicle (adopted 1960) mounts a pair of recoilless 106mm that can be raised up above the hull. Traverse of these weapons is 30° left and right.
For operations such as peacekeeping or counter-insurgency the 106mm can prove to be more useful than an ATGW. Loaded with canister or HESH rounds it is far more effective as an anti-sniper system or to stop vehicles running roadblocks. Turrets such as the CETME TC-7/106(right) that mount a pair of 106mms and a HMG already exist. With a little modification an under-armour loading system could be created. If two downward opening roof hatches are placed either side of the turret the breeches could project down through these when the barrels are super-elevated. Such a system could be fitted to the M113 or HMMWVs intended for Peacekeeping.