The 4.5 sniper coil is a concentric. It works good for the trashy areas to provide better target separation, having less targets under the coil, or actually separating a target from junk, depending on how close/distance a non-desirable item is from a desired target.
A larger coil in a trashy area covers more ground and can also have more targets (desirable & non-desirable) under the coil at a giving moment - think of what the machine thinks with 2, 3 or more targets under the coil. More, the confusion of mixed signals, hence masking, and the machine limitations.
When I hit a good signal and have a difficult time getting a repeatable signal, I'll narrow my sweep width, sometimes so narrow some call it the wiggle, and I'll zero in by walking around the target to get a sweep angle that may eliminate a nearby non-desirable target hopefully giving me a good repeatable signal. Sort of like a wagon circle walk. I'm known to dig iffy signals as there are just so many variables and digging the iffy has paid off. Yes I dig up more junk, but if I get nice finds in the area, I try not to pass up a good possibility. Of course the ultimate is to dig everything in all metal, but we all have limitations one time or another. And days when I'm less into it, then I'll up the discrimination a notch or so. But as I get older, I get slower and get more patience. Folks I hunted with, it's common to hear that I'm the slowest they have ever known detecting, and my a good bit. I really like to hunt all metal and listen with quality headphones. Been my way for years, now I'm trying some silent search machines which I'm getting more used to, but after having a threshold tone and manual ground and then change all that, it's like learning a new trick.
I see I got away and caught myself - back - the 4.5 is a concentric, and a DD coil due to it's ground matrix just is better designed for high mineralization. I myself haven't compared the 250 stock coil and the 4.5 sniper coil in high mineralization and therefore cannot give a best answer - except the 4.5 is not a DD coil.
I see the 4.5 is a top performer for the trash areas, and it gets good depth for it's size. I'm just small coil slow worker when I can be.
In my earlier years when I first got an extra coil for a machine, I would use the stock, clean up what I can, and if the area does me good or has very good potential, I'll hit it again walking grids at different angles, and if it continues to hold my interest, slap on the extra coil (usually smaller) and re-work the area. Good areas are worth the effort. Then a different machine can produce different results.
Maybe a link on coils - and I'm sure there is a ton of coil info on-line:
http://www.treasurefinders.net/types_of_coils.html
And ones info on the 250 - mentioned coils:
http://www.thetreasureleague.com/Ace250_Tips.htm
If your getting the results others are on this forum and elsewhere, then your machine-soil conditions should be fine for it's preset.
Some of the common mistakes I see others who hunt with me is trying to run sensitivity to high for conditions, and not cleaning debris between the coil and coil cover, and sweep speed/also not keeping the coil flat to the ground, poor sweep overlap. When checking out an iffy, besides different angles , sweep width, speed, try raising the coil and re-sweep at different heights and speed, - get the narrower coil matrix working on what may be a good target. Then let me work slowly behind their path - one of my favorites going with newbies
