A few comments on recent posts:

The M61 is an extremely reliable and long-lasting gun. However, the
little 20mm shells only weigh 102g compared with 180g for the 25mm
GAU-12/U, 260g for the 27mm BK 27, 270g for the 30mm GIAT 30M791 and
390g for the Russian 30mm guns. This means that the M61 has to score
many more hits than 30mm guns to have the same effect on the target (I
have read an estimate of around 20 hits needed to bring down a modern
fighter).

This lack of hitting power was recognised by the USAF as early as the
1960s and led to the development of the 25mm GAU-7/A, which was
intended to replace the M61 as the standard fighter gun, and was
planned for installation in the F-15. This fired 200g projectiles for
a considerable increase in effectiveness. However, the advanced,
combustible-cased ammo ran into technical difficulties which could not
be solved in time so the gun was scrapped and the M61 soldiered on.
Its survival for so long can only be put down to the fact that fighter
guns are much less important nowadays with the development of better
missiles, so it hasn't been worth the cost of developing a new one.

More recently, as has been pointed out, the 27mm Mauser was selected
by both of the JSF contenders as providing the optimum balance of
characteristics for an aircraft gun, despite being a foreign design
produced by a country which is not even a member of the JSF consortium
- that tells you how good it must be compared with the home-grown
product.

Incidentally, there is still some mystery about the current situation
- I have it on good authority that GD (given the job of integrating
the BK 27 to the F-35) have proposed using the GAU-12/U instead
(allegedly for cost reasons), but every publication I have seen on the
F-35 still mentions the BK 27. Can anyone point to a definitive
reference?

Tony Williams
Military gun and ammunition website: http://www.quarry.nildram.co.uk
Discussion forum at: http://forums.delphiforums.com/autogun/messages/