Not illegal if the distance from the road is good enough, target backstop/down-range is cleared the requisite amount, in accordance with any county stuff, etc. It's your land. Welcome to Free America.
I used to do this no problem. Works great. And it was for 3 reasons:
- provided a better angle to get further distance
- provide for a larger downward angle, safer down range
and
- able to lay prone or sitting position without being on the ground (covered from the hot summer sun while shooting from the open tailgate on a pickup with camper shell, and not having to lay on the ground with all the ants and bugs, or 2 ft tall grass/jaggers in a field).
I was able to make a nice spot to lay in the truck bed with a moving blanket, good rest for the rifle, reduces glare on the scope since under cover over-head, etc. Don't have to clear out a grassy area and build an elevated platform, that's what the truck is for. Work smarter, not harder.
If you have an open pickup bed, you can stand or set up a chair or whatever. Whatever's clever.
For all the crazy anti-gun folks reading this post, calm down and go away. Take those images of the DC sniper out of your head, get real, and move on. Go target shooting in August in Georgia in a field somewhere and see how much work it is to set up a range, and sit on the ground with the ants and mosquitos and chiggers and gnats and flies and bees and spiders. Sweat your brains out just to put a few rounds down-range even just to sight in your deer rifle and if there's any brains left, you'll figure out a better way to do it.