I have had no desire to but a sniper coil because I have done well with the 5x8 in trashy areas. Most targets I find are 4 inches, but I have found some as deep as 8".
Takes practice, but with the DD you can rotate around multiple targets and pick out the good ones often.
The average depth I have seen/heard of for the sniper is 4". NEL makes one that may get a little more.