Lost wax casting of AR uppers and lowers requires some pressure to insure the proper resolution. A knowledge or sprues, risers, and runners is also vital. I recall that Ruger pours their AR alloy from a 10 foot drop for the needed pressure. Yes, Ruger casts or has cast AR uppers and lowers for other manufacturers. One of the big challenges of the process is dealing with warpage. Some parts can be straightened others are tossed back into the melt.
Casting wax is harder and has a higher melt temp than say candle wax. A friend makes his own wax shapes and ships them to California to be cast in nimonic alloy (small turbine parts).
Other than as a fun casting project I would suggest using the forged AR blanks and machining them at home, much less grief.
Have fun !!!