That was using my Nail Board and I replaced the Indian Head Cent with an 1836 Capped Bust Half-Dime, That was in March of last year. Oregon Gregg and I used a Racer, Racer 2, CoRe, Relic and Impact and all the smaller coils they offered ... then. We had a few surprises as we evaluated all the models with all the different search coils and two or three accessory coils Gregg had on-hand.
I had used the 4X7½ DD Impact coil for some urban Coin Hunting and it did fine in typical modern trashy conditions. When it came to the 'tough challenge' we did, the Impact and 4X7½ DD came in last getting 4, maybe almost 5ish hits out of a possible 8. The Racer & Racer 2, using the small 'OOR' DD coil, gave us a 2nd surprise as that combination only gave us about 5 or maybe 6 out of 8 hits on the Half-Dime. Those same models using the round 5" DD that came with the Racer 2 produced 7 out-of-8 as did the Relic w/5" DD.
The overall winner when, using a factory manufactured search coil, was the CoRe w/OOR' DD as it was the only one to give 8-out-of-8 hits. I kept the 4X7½ DD only for use in Coin Hunting sessions. Gregg reported the results on the ahrps org forums and that got me an e-mail and phone call from Dilek. In less than a month I had the prototype sample 5" DD for the Impact mounted and put it to the test against all the others, again, in the same scenario and it gave performance about on par with the Relic, Racer and Racer 2 using their 5" DD coils. The 5" DD for the Impact was soon in production.
Today I mainly use the 7" Concentric as a general-purpose coil on the Impact and the 5" DD when I get into a dense iron contaminated environment. I used all my Impact coils on the Multi-Kruzer and found it also displayed the better performance with the 5" DD in really dense nail and other ferrous based trash.
Just my experiences with those coils, and for the Multi-Kruzer the only two accessory coils I want are the 7" Concentric and 5" DD to handle almost all my needs with that model.
Monte
[size=small]PS: Gregg & I also tried a couple of different brands/models and none matched the performance of the CoRe w/'OOR' DD. We also tried some aftermarket search coils and that left me a little surprised, too, because the NEL Snake and Sharp DD coils also produced 8-out-of-8 hits when mounted on his Racer 2. All detectors used comparable settings as well.[/size]