Re: You mean I can build an open source rocket to hit targets several miles away? Cool!
You can trade paylod for range, and you can still launch it with a different trajectory.
The german 8.8 gun was designed as an AA gun (and later found its way as an antitank one), which does require a "straight" trajectory (and thereby an high muzzle speed) to be effective. Long range guns don't, otherwise they would be limited by obstacles and earth curvature. Antiship missile take advantage of the sea being almost flat and surface waves not being radar friendly. Their terrestrial counterparts need to fly higher, often slower, and with a fairly more complex guide system to deal with the more complex environment. Most unguided rockets simply use a ballistic trajectory because it's the simplest to get past most obstacles at high speed.
Low level bombers were a big bet in '70s and '80s strategies - but in the first Gulf War they demonstrated to be very vulnerable to plain old AA artillery barrage fire. After many Tornados were downed, the allied forces switched to medium altitude bombing. Guided bombs and stand-off ones gave better result when air superiory is established and medium/high altitude SAMs are already disabled/destroyed, or taken care by elecronic warfare and SEAD teams. Today, more effort is put in low visibility and electronic warfare to blind/deceive enemy sensors than just flying low and fast.
It is still true it may be far easier to infiltrate a single low flying plane, but an unamanned one would require far more complex technology, or remote control, not easy to achieve BVR without a powerful enough radio transmitter - or using some for of repeaters.
Anyway, I don't find very sensible to detail some "open source rocket science" in these times.... ok, it's secret matter, but helping mad people with ready to made components looks a bit silly to me.