Everyone reading this has likely relied on the stated weight on a bat as part of the purchasing process. Whether we are in the store, shopping online, or talking directly to a bat maker, we almost undoubtedly start talking in terms of length and weight, drop, etc. Why do we look at the weight? Because it's important, that's why.
In his recent book, The Science of Baseball, A. Terry Bahill lists bat weight as one of the critical factors in optimizing bat performance. He definitely isn't wrong, it is a critical factor. (We have our own thoughts on a more important metric, but that will be the focus of another blog post.) Here's the problem, though... When you buy a bat, the weight isn't often as advertised.
We do our best to hit the stated weight, and we are normally VERY close; within a 1/4-ounce of our mark. However, in some recent reading, I came across a Bat Digest article from last fall. When you are done reading this, you can dive into that article for some more specifics, but let me assure you it was pretty alarming. To summarize their work for you, the average non-wood bat difference between actual weight and stated weight was 1.17 ounces for BBCOR bats and even worse for USSSA bats.
I know that until reading the article, I had just taken for granted that the material consistency and manufacturing process for non-wood bats would tighten the variance to where it would be indiscernible by the hitter. Apparently, I was way wrong. When you are talking a 1.17 ounce difference from one bat to the next, of the same model, you are talking about a significant difference.
The actual weight difference, though, wasn't even the most alarming thing in the article. When asked about where the difference in between actual weight and stated weight was found, some companies claimed it was the grip and others the end cap. When it comes down to it, that is a HUGE difference, and if you don't know where it is for the bat you are ordering, it will impact your swing.
So, what's in an ounce? If that 1.17 ounces, on a 33-inch bat is found in the end cap, then that bat will swing heavier. How much heavier? There are a lot of factors to consider, but it could swing as much as 10% heavier. How do I know? Let me explain...
The closest metric we have found to swing weight is Moment of Inertia, which can be read about at length by diving into the work of people like Dr. Daniel Russell, Dr. Alan Nathan, and others who have explored the science of hitting, and the bats used to do it. We have seen non-wood bats with listed MOI as low as the mid-8,000s oz*in^2. The difference of 1.17 ounces, found in the end cap of a 33" bat is roughly 850 oz*in^2; that is about a 10% increase in how heavy that bat swings.
Again, what's in an ounce? In a world of ever-increasing velocity, quite possibly the difference in a double to the gap, and the walk of shame back to the dugout.
The moral of the story is to know what you need in a bat, and know who you can trust to deliver on it.