What is everyone's idea of the best bullet backstop? Is it just a big mound of dirt or? I recently had a little problem that made me want to set up a proper backstop. I want it to absorb very big rifle bullets.

Sounds like you needed a bigger pile of dirt. Seriously our club as hugh berms for back stops and refills them as needed. The Marine Range at Dam Neck and the National Guard range at Butner are also hugh berms.

Define very big

.50 BMG Penetration

Medium/Penetration @ 100 yrds
____________________________________________________________
CONCRETE (solid) 9 inches
TIMBER (logs) 96 inches
STEEL (nonarmored) 1.8 inches
ALUMINUM 3.5 inches
RUBBLE (asphalt, soil, cement) 20 inches
TAMPED SNOW(19.9-24.9 lbs/cu ft) 77 inches
DRY SOIL 28 inches
WET SOIL 42 inches
DRY SAND 24 inches
WET SAND 36 inches
DRY CLAY 42 inches
WET CLAY 64 inches

I built a back yard range on my property and used the NRA Range Sourcebook for current standards of construction.

I went with a 9' high berm of soil (18' wide at the base, 50 feet long or ~450 cu yds of soil), faced with 24 inches of sand. Initially, I used just the gravelly soil from my pond excavation, and chunks of clay/rock were consistently tossed back uprange. I do CCW training and I wanted to reduce risk to clients. The sand seems to absorb everything shot into it-416 solids do not generally penetrate the ~2 feet of sand (more actually, since it at 45 degress from vertical).
Sand is cheap- $50 a 2/12 ton load here in mid MO.

Slope is important based on soil type and rainfall-you want it to stay put. 30-45 degrees is about right for most soils. Compact it with a bucket loader when placing it (or run the dozer over it every layer).

I placed some geo-fabric underneath the whole thing to allow water to flow through, but not lead or dirt. I also mixed phosphate fertilzer in with the sand to immobilze any lead ions relaesed by acid water conditions here in MO (phosphates react with lead to create a tough coating of pyro-phoshphtes-insoluble in rainwater etc.)