Jim,
12 yards of dirt is 12x27= 324 cubic feet. The front of the berm will have a natural slope to it depending on the type of dirt. If you can get 1 1/2 to 1 slope (horizontal to vertical) you are doing good but 1 3/4 to 1 is more realistic. The slope is called the angle of repose and it is what the soil will end up at, given time and the ever present gravity. So picture this from the side:

The top of the berm will be 1 foot wide and it is 5 feet off the natural ground line. The 1 3/4 to one slope will put the front of the berm 8.75 feet from the front of the top of the berm and the back edge will be the same distance away from the back edge. The sides of the berm will form a pyramid to either side with a distance also being 8.75 left and right of the top edge. The area of a cross section of the berm is 5 x(1+1+8.75+8.75)/2= 48.75 square feet. Now if the ends could stay square and verical which they can't, you could get a length of berm equal to 324/48.75 = 6.6 feet. Unfortunately, the pyramid shapes on either side will eat up 320 cubic feet of material. So you are left with very little to build the berm. If you want it roughly six feet wide, you will need another 12 yards of material.

The tire idea would work better for you and they will stand vertically

When,I helped build one.We used old tires packed with earth.And then staggered them,much the same as you lay bricks.One tier at a time.The tires trapped the bullets quite well....4t1