I would buy the U.S. ACE 300 for ordinary dry park and yard detecting. I would pay more for the U.S. ACE 400 if I wanted to listen to rejected iron or would otherwise buy its larger DD coil. Those are identifiable differences you can price. Neither detector promises an extra number of inches on every coin.

Decision Garrett ACE 300 Garrett ACE 400
Stock coil 7 × 10-inch concentric 8.5 × 11-inch Double-D
Operating frequency 8 kHz 10 kHz
Interpreting iron Numeric target ID and normal tones Adds switchable Iron Audio
Locating a signal before recovery Electronic pinpoint Electronic pinpoint

What Iron Audio actually buys

With Iron Audio engaged, the 400 lets you hear ferrous responses that discrimination would otherwise silence. Garrett describes using the low tone as another clue around items such as iron bottle caps. It requires interpretation. It does not remove the cap from the ground or guarantee that the next higher tone is a coin.

On a picnic lawn where crown caps consume most of your time, that extra clue may justify stopping over a doubtful signal. In a quiet yard where you intend to investigate nearly everything, an extra way to classify iron changes fewer decisions. The 300’s pinpoint function and numeric display still do their jobs there.

Compare checkout totals, not accessory counts

Start with the package you would actually take out. Give an included bag value only if you would buy one separately. Give the 400’s coil value only if its footprint suits your sites. If the offer pairs the 300 with a handheld probe and the 400 without one, either add the same probe to the second total or remove its value from the first. Then ask whether the remaining premium is worth Iron Audio.

Compare the Garrett ACE 300 carrying-bag bundle on Amazon with the Garrett ACE 400 detector on Amazon . Check the selected package before comparing totals.

Both models have waterproof search coils and control housings that should stay out of water. If submerging the machine is a firm requirement, neither package meets it. Compare the U.S. versions identified above; international models marked 300i and 400i have their own documentation.

Manufacturer references

This is a desk review of the cited documents. We did not field-test this equipment.

Related reading: Garrett ACE 300 Review: Target ID, Performance, and Value .