I pulled these quotes:
The MG 131 was a light weapon, but this was achieved by combining a modest muzzle velocity and a light projectile. Despite its limitations, it was used in German fighters until the end of the war, because it was not possible to install the MG 151 in the engine cowling of small fighters such as the Bf 109 or Fw 190; only the Do 335 and Ta 152C had the MG 151 as cowl gun. The MG 151 was a much heavier, much more powerful weapon, and it replaced the 20mm MG-FF as centreline armament on the Bf 109F. During the war a copy of the MG 151 was designed in the USA, modified to fire a very powerful .60 (15.2 x 114, 76.5g) round. But this T17 gun never reached service, and only about 350 were made.
Only at the very end of the war did some fighters carry the MK103 gun. Installations in the wings tended to be inaccurate, because the enormous recoil twisted the wing; centreline installations as engine cannon were designed for the Ta 152C, Do 335 and Bf 109K, but evidence that this was turned into hardware exists only for the Do 335 and the prototypes of the Ta 152C. The fighter designs that were on German drawing boards in 1945 sometimes made provision for the MK 103, but the favorite weapon was the MK 108.
from this website:
http://www.geocities.com/CapeCanaveral/Hangar/8217/fgun/fgun-in.html Also, these numbers(gun, shell type/weight, ROF, Muzzle Velocity and gun weight):
MK 103 30x184B(330g) 420rpm 860m/s 146kg
MK 108 30x90RB(312g) 600rpm 505m/s 64kg
ShVAK 20x99R(96g) 800rpm 860m/s 42kg
Hispano Mk.II 20x110(130g) 600rpm 880m/s 50kg
MG 151 15x96(57g) 700rpm 960m/s 42kg
MG 151/20 20x82(115g) 740rpm 710m/s 42kg
MG 151/20 20x82(92g) 740rpm 800m/s 42kg
Type 99-2 20x101RB(129g) 490rpm 625m/s 34kg
Browning.50 M2 12.7x99(48.5g) 750rpm 870m/s 30kg
MG 131 13x64B(34.6g) 900rpm 730m/s 17kg
Breda-SAFAT 12.7x81SR(36.7g) 700rpm 760m/s 29kg
[This message has been edited by juzz (edited 01-17-2000).]