The 17 cm gun was developed to meet the need for a long-range mobile gun. Although the gun and carriage weigh 19.25 tons, the piece can be put in and taken out of action very quickly and with little effort.
The tube, usually transported separately from the rest of the weapon, is of the built-up type consisting of a main tube, locking ring, breech jacket, and breech ring. A manually operated breechblock is of the horizontal sliding wedge type with six rollers mounted almost flush with the bottom face to decrease the surface contact area. Extractors and firing mechanism are of conventional heavy German artillery design.
One unusual feature of the Mrs. Laf. 18 is that two recoil mechanisms absorb the shock of firing. A variable hydropneumatic recoil mechanism with the buffer cylinder within the cradle, and the recuperator cylinder mounted above the tube permits recoil of the gun; and another recoil mechanism mounted in the bottom carriage permits recoil of the upper carriage.
The gun is mounted on the 21 cm Mrs. Laf. carriage which has four hard rubber tired wheels. As there is no spade, the trails rest on an oblong float which houses an arc that permits 16° of traverse by means of a traversing handwheel. The firing platform when fully lowered fits into two sockets in the bottom carriage. Three rollers on vertical legs rest on the platform stabilizing the weight. The rear roller is mounted on a jack screw threaded leg. When more traverse is desired than that permitted by the arc, this roller is screwed down, lifting the float clear of the ground. One man, with the aid of a trail spike, can then swing the entire carriage about the firing platform 360°.
The prime mover for this weapon is reported to be the Sd Kfz. 8 semitracked vehicle.
SPECIFICATIONS
| Caliber | 172.5 mm (6.79 ins.) | |
| Weight of weapon (firing) | 19.25 tons | |
| Length of tube | 46 cal. | |
| Breech mechanism | Horizontal sliding block | |
| Firing mechanism | Continuous pull | |
| Recoil mechanism | Two independent hydropneumatic systems—variable | |
| Length of recoil of the piece at 50° (normal) | 33.86 ins. | |
| Length of recoil of top carriage at 10° (normal) | 49.21 ins. | |
| Rifling | Increasing R.H. twist—4° 16′ 40″ to 5° 58′ 42″—48 lands and grooves | |
| Chamber capacity | ||
| Shell KGr 39 | 2,538.6 cu. ins. | |
| Shell KGr 38 (Hb) | 2,513.2 cu. ins. | |
| Muzzle velocity | 138 lb. shell—3035 f/s | |
| Range (maximum) | 138 lb. shell—32,370 yds. | |
| M.V. | 150 lb. shell—2821 f/s | |
| Range (maximum) | 150 lb. shell—30,621 yds. | |
| Elevation | 50° | |
| Depression | -1° | |
| Traverse (carriage) | 16° (360° by using the pivot platform) | |
| Sights | Independent type using combination of range drum and panoramic sight mount—left trunnion. | |
| Ammunition | Two semifixed high-explosive rounds | |
| K. Gr. 39 | 150 lb. | |
| K. Gr. 38 (hb) | 138 lb. |
German: p. 103