Chop blocks and game-improvement irons share a similar component. If you want to do either well, you’ve got to get that center of gravity (CG) low.

We’ve all seen the idea in action with cavity-back iron designs, but it’s what we don’t see in the latest irons that’s taking the idea a step further. By completely hollowing out the area behind the face, designers are able to save weight and lower the CG by widening the sole or adding heavier inserts or even enclosing the back of the face, much like in a hybrid design. It has been done by Adams Golf with its Idea set (look for the next generation soon) and by MacGregor (the M565 and M455).

Two new examples are the upgraded Nike Slingshot OSS and the forged Mizuno MX-900. Both offer hollow designs that attempt to push the CG lower and farther back from the face. Doing that in an iron design can help the face close and the ball fly higher. The revamped Slingshot incorporates some impressive metallurgy, including a new face insert made of a thinner “cryosteel” (which undergoes a cooling process that makes the steel harder and stronger), a laser-welded face, a wider slingback and a 16-percent wider sole than on the original. Nike designer Tom Stites says the moment of inertia (resistance to twisting) has improved 10 percent over the original Slingshot ($700, nikegolf.com).

Mizuno’s MX-900 expands on some of the things learned from its successful Fli-Hi iron-like hybrids. It utilizes a hollow construction and a high-strength steel forged face and neck in the 3- through 7-irons, and a one-piece mild carbon steel in the short irons, but it’s the long and middle irons that are a club design cornucopia, including a 12-gram tungsten plug in the wide sole to lower the center of gravity, internal heel-and-toe weighting and a multithickness face ($800, mizunousa.com).