The Jagdtiger used what I believe was the best overall anti-tank gun design of the war, the 12.8cm
Panzerjagerkanone 44 (L/55), also known as the Pak 80. Some confusion over the name of the weapon stems from the fact that the name was changed in mid-1944 from Pak 44 to Pak 80, causing confusion in many reference works. You will find the gun called either name in official documents from late 1944 and on.
As is common at this time this weapon underwent several name changes. At various times it was known as K 44, Pak 44, Kanone 81, Pak 80 and Pjk 80.The equipment numbered in the 8x range were temporary equipment issued usually in small numbers and not regarded as standard issue service weapons. Hence Pak 80 and Pjk 80 were temporary names as were K81/x. Had this equipment been accepted into full service it would have had a Pak 4x designation attached. The only difference between the Pak 44 and the K 44 was mode of operation-the weapon itself was identical.
The 12,8cm PaK 44 and 12,8cm PaK 80 is the same gun with two different designations. At various points in the development of the Jagdtiger, it was also referred to as the 12,8cm PaK 43, 12,8cm Panzerjäger-Kanone (PjK) 80, 12,8cm StuK, 12,8cm Kanone (K) and so forth. Also refer to Pak 43 or PjK 44.
In principle, the towed version of the same weapon was known as either the Kanone 44 (when used as field artillery) or PaK 44/Pjk 80 when used as an anti-tank gun.
The 12,8cm PaK 81 is something different again. The 12,8cm K 81/1 was the Jagdtiger 12,8cm gun mounted on the carriage of the French 155mm GPF gun while the 12,8cm K 81/2 was the same weapon, mounted on the carriage of the Soviet 152mm M1937 howitzer.
We are basically dealing with two similar but slightly different guns: The weapon intended for the towed PjK 80/PaK 44/K 44 and the gun intended for mounting in the Jagdtiger which also ended up in the two K 81 towed guns. The main external difference is that sleeve (if that is the correct term?) of the Jagdtiger gun has a "ribbing" on it to make it fit in the Jagdtiger gun mantlet. But you cannot judge which is which based on the name alone.
All of these designation referred to the same, basic 12,8cm L/55 gun. Not to be confused, of course, with the 12,8cm Kanone L/61 which was used in the PzSfl V built in 1941 on the basis of the VK 3001(H) tank prototype.
Length of piece (L/55):7023mm.
Length of bore 6623mm.
Length of rifling 5538mm.
Weight of gun and carriage 10.160kg.
Elevation -7° 51' to +45° 27'.
Muzzel velocity (AP):950m/sec,(HE):750m/sec.
Shot weight 28.3kg.
Shell weight 28kg.
Armour penetration 219mm at 500m (0°), 202mm at 1000m, 197mm at 1500m.
Maximum range (HE) 24.410m.
Barrel live 1000/2000 rounds.
Manufacturer Fried. Krupp AG, Bertha Werke, Breslau.
Designed during 1944 as a dual-purpose field and heavy anti-tank gun.
Rheinmetall also built a prototype.
Krupp Pak 80 had an elevation limited at -7° to +15°.
These information from the book "Small Arms, Artillery and Special Weapons of the Third Reich "Terry Gander and Peter Chamberlain near Macdonald and Jane's London.