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A good guided SAM would, surely, be more effective than anti-aircraft guns?
I'm not sure. The best naval surface FC radar was, as I recall, about +/- 25m or so in range accuracy and less in bearing accuracy. Dunno if that's close enough without having to use a ridiculously large fragmentation warhead.
Then there's the cost of missiles and launchers vs. guns and shells.
Missiles like Terrier or the American Standard Missile need to detonate within about 50m to have a 30% chance of a kill, but are quite small as fart as arhead size is concerned . They basically use specialised ballistics to achieve this. the missile is rotating on its axis when detonated. When it detonates, it sends out a concentrated "spiral" of elongated shrapnel that for a time travels through the air, rather like the propellor arms of an aircraft. In that way, the probability of a hit is much enhanced. i doubt the Germans would have been aware or able to produce that effect in 1945, which means their kill radii for SAMs would have been a lot less, or, as you say a missile of very large size .
The point about this is that it is not radar accuracy that is necessarily the dominant constraint affecting AA effctiveness, its the effective kill radiius of the ordinance as well, and postwar, a lot of additional changes were made with missile techs to improve that lethality.
I think German heavy flak was on the right path with radar guidance and shorter shell flight times. Add a proximity fuze and the 12.8cm Flak 40 is probably all you need for knocking down WWII era heavy bombers.Compared with the 88mm FlaK 18 36, the 128 used a powder charge four times as great which resulted in a shell flight time only one-third as long. This made aim against fast-moving targets much easier.
My question is was the equipment too large for use in a missile