WGR21 was fuzed on a timer. R4Ms were impact fuzes. However they flew faster and flatter and were much more lethal, especially when armed on 262s. Late model 190s (A8s for our purposes, I guess) were supposed to have used them as well as 262s. They were very late to the war, first getting a kill I think Feb of 1945.
The WGR21 is "hard to kill" only if you try using it on fighters. Don't. If you are dogfighting, leave the rockets at home. They take a MASSIVE chunk off your top speed and your climb rate. If you're going after fighters take guns only and manuver for a guns shot.
The WGR21s are used only for bombers. Big, lumbering, mostly-stationary targets. The range is about 1.2 to 1.4K, and you have to aim about 500 feet "up" because the mortar drops in an arc. Your speed and your closure rate will change these ranges, naturally, but if you match speed with the target and fly directly behind them, you want to fire just before you have closed to 1.2K, last I recall.
They were also used in the ground attack role. Something I wasn't aware of being too common until recently. They are the equivelant of a 250lb bomb, each. (going by weight, not explosive content).
