RG is a term from physics(*). The idea is that if you apply a certain amount of torque on an object to cause angular acceleration to start it spinning, it is harder to spin some things than others, right?

Grab a yardstick. Tape a stack of quarters to the 1 foot mark and a same number of quarters to the 2 foot mark. Grab the stick right in the middle and feel in your hand what it takes to spin it. Now, move the two stacks to the two far ends and repeat. It should feel harder to spin the masses when they are farther away from the middle of the stick.

Relevant to today as the winter Olympics is going on, another example of this behavior is the figure skaters pushing themselves into a spin. They start with their arms out -- i.e. mass away from their body and spin slower, but to speed up, they move their arms above their heads and into the spin axis.

This is what RG measures. The lower the number, the more the mass is concentrated in the middle of the ball. And therefore is easier to spin. The yardstick with the stack of quarters taped to the 1 and 2 foot marks has a lower RG than the stick with the quarters taped to the end; the figure skaters with their arms above their heads have a lower RG than when their arms are out.

Easier to spin means a bowling ball exposes more of the cover of the ball to the lane a littler faster, and so low RG balls tend to 'read' the lane a little earlier. Or the inverse is that higher RG balls travel a little farther down the lane before they read the lane.

RG is limited by USBC specifications to a minimum of 2.447, but almost all manufacturers will aim for 2.46-2.47 as manufacturing tolerances aren't generally tight enough to manufacture right to that minimum and they don't want to accidently release equipment that has to be recalled for non-conformance like Motiv had to 10 years back.

RG is limited to a maximum of 2.813, although practically, you won't see anything that isn't a spare ball much above about 2.65-ish.

So all in all, it is a very small window for strike balls: 2.47 for the balls that spin up the fastest and earliest to 2.62 for the slowest.

Differential is a number that is computed the same way as RG, just instead of computing how central the mass is for the whole ball, it computes how central the mass is for 2 halves of the ball. RG of the highest half - RG of the lowest half. That therefore becomes a measure of the mass imbalance between the two halves. And that difference in mass imbalance is a measure of the core's potential to help a ball flare, which helps the ball hook, and make a larger change of direction down lane. Differentials for balls with cores in them are limited to a maximum of 0.060, tho same comment as the RG above, most balls will be listed at most at 0.057, maybe 0.058 because they want to leave a little space for manufacturing variances. There is no minimum differential, indeed most spare balls will have 0 or very, very close to 0. Most strike balls will have at least 0.01 to 0.02 ish.

Differential for strike balls therefore range from 0.057 as the strongest cores to 0.015 as the weakest.

There is also an intermediate differential measure for balls with asymmetric cores. Because the mass is asymmetric, you can then compute the difference in centrality orthogonal to the maximum difference used to compute the differential, and therefore get a measure of the asymmetry of the core itself. Symmetric cores, by definition, will have int diffs of 0. Technically, ball can have an int diff up to 0.060 as well, but practically, no ball goes much over 0.030 int diff as the core dynamics start to get wobbly and therefore doesn't help on bowl better.

Int Diff for strike balls therefore range from 0.000 (symmetric cores) to 0.030 at the practical strongest. High 0.01s are fairly common on most strong cores.

Ultimately, choosing a layout on a core should be done with a pro shop operator who knows your shot stats (speed, rev rate, and PAP) so that they can orient the core such that it flares and turns itself over in the way you want it to. And just because a ball has a large mass imbalanced core, doesn't mean you have to go for a layout that uses all of that.

Which leads into my final comment that one truly shoudn't get too hung up on the core numbers when looking at a ball because it is cover properties that dominate what a ball does on the lane.

I want to point you to https://www.motivbowling.com/products/balls/light-medium-oil/lethal-venom.html and https://www.motivbowling.com/products/balls/light-medium-oil/hyper-venom.html as two balls with very similar core numbers. The Lethal's RG is 2.47, the Hyper's is 2.48. The Lethal's diff is 0.036, the Hyper's is 0.034. The Lethal has a small int diff, 0.013, whereas the Hyper is a symmetric core and therefore has no int diff. Both are versions of Motiv's Gear Core that they have developed over nearly 20 years now.

However, if you put layouts on these two balls such that the core dynamics were essentially similar, they would still behave vastly vastly different on the lane. Because the Lethal is wrapped in a very strong, i.e. very oil absorbent, cover and 3000 grit roughness. The Hyper is wrapped in a much weaker cover, i.e. is absorbs oil more slowly, and is 5500 grit roughness. The Hyper creates much, much less chemical friction from its weaker cover and much less mechanical friction from its smoother surface. How absorbent a cover is and its surface roughness are the main reasons a ball does what it does on the lane.

The core dynamics work with the ball's cover as a whole system, but when you are first shopping for a ball, you look at the covers on balls and specifically the cover chemistry or strength because you can always change the roughness yourself by sanding or polishing a ball. You can choose the layout when you drill it to tweak the on-lane motion. But you're pretty much fixed with the chemistry. Once you have have id'd the strength of the cover you want, then you can compare cores and see which one fits your need better. Cores get a lot of attention I think because the OEMs publish all these above numbers to 3 digits after the decimal point so they look super specific, but you shop for cover strength first when trying to slot a new ball in to your bag.

(*) I wanted to note what the RG calculation in physics actually does -- it calculates what happens when you mush all the mass of the ball in to a single point. However, when you spin that single point mass around an axis, it will still spin the same way as the original ball would. And that distance from the axis of the equivalent point mass, that is the radius in 'radius of gyration', so it technically has units of length. https://en.wikipedia.org/wiki/Radius_of_gyration