Just as important, if not more important than Bit Protector Mold Variations, are the slight variations of Running Core 'Prongs'. These are the two protrusions that are the locking mechanism for your RC to connect tightly to your BP and keep the HMS Beyblade bound together. 

Note: There is no one-size-fits-all approach. Some combinations of BP and RC simply don't work that well together, while others do. It should be determined on a case-by-case basis.

This is a topic you can find absolutely zero information on elsewhere. It may be anecdotally mentioned in passing somewhere, or simply known to veterans without any form of actual documentation. Regardless, it is not easy to come by or look into.

I've found two common designs for HMS Running Core Prongs, and one much less common Brand-specific design:

First Mold (Mold 1)

• Present in the earliest releases, Wolborg MS' Bearing Core is the latest. Unlike BPs, a given Beyblade's RC design seems to persist regardless of when it was produced. Example being RBA3 Dark Effigy MS, which despite being released after Mold 2 RC prongs began production, retained its Mold 1 RC prongs just like MA-04.

• Very thin relative to the other two variations.

• Features a softer top-edge, rounded off.

• Weakest of the three, and most prone to breaking over time.

• Through natural play and wear, tends to slowly shift upwards instead of staying straight out at a 90 degree angle.

• Works fairly well with Small Type BPs but the pair can sometimes run a bit tight. Has trouble with Large Type BPs, usually.

Second Mold (Mold 2)

• Present in the second half of releases, earliest being Phantom Fox MS.

• Slightly thicker and bulkier than Mold 1, does not feature as prominent of a rounded edge, instead more defined edge work.

• A hair wider than Mold 1.

• Generally stronger than the first mold, though it too tends to shift upward through normal use and play. More durable nonetheless.

• Works well with Small Type BPs, but again, they can sometimes run small. Works decently well with Large Type BPs too.

Hasbro's Third Mold (Mold 3)

• Absolute behemoth of a prong design.

• Originally the RC pictured was a Grip Flat Core RC, until I transplanted an Ultimate Version rubber onto it (not legal for competitive play, RTR has a long way to go).

• Present only in the earlier Hasbro release waves, when they were still roughly following Takara's release order.

• Likely a response to the fairly weak Mold 1 design present in Takara's starting releases. Improved design to ensure these Beyblade RCs didn't snap.

• This mold is not present on later releases, also likely a response to the lack of popularity of HMS. Later RC releases were essentially Takara RCs with localized stickers, all sporting Mold 2 prongs, completely unaltered. Probably in an effort to keep costs low on a poorly received product line.

• Easily the largest in height, yet it does not extend out as long as either Mold 1 or 2. This is likely to decrease the chance of prong fracture.

• Prong edge is more like Mold 2, less rounded and more defined. Width is equivalent to Mold 2.

• Likely the most durable and uncommon of the three, the only one I have a single copy of at all. 

• Works well with both BP sizes, Small and Large. In some cases Small may still be too Small, but in general this Mold is the most likely to be compatible with all sizes. Large tend to be a bit better, personally.

• Because this is the most rare mold, having only been released in select HMS Hasbro releases (an already rare breed), it isn't worthwhile or straight-forward to go hunting for them. Especially since it would only encompass, at most, Bearing Core and Grip Flat Core in terms of competitively viable RCs. 

• While transplanting a more valuable rubber piece into a Mold 3 Grip Flat Core RC is wise, like I've done in the picture shown, it isn't considered legal. If you don't care, and just want to make sure your favourite Attack RC is probably not ever going to break through normal play, go for it. 

Likely the most well-known RC variation is the one between Grip Flat Core and Grip Flat Core (Ultimate Verson), one coming with the original A-126 Dragoon MS, the other with MA-01 Dragoon MS (Ultimate Version). While the two are very comparable there are a few differences that may make you pick one over the other.

A-126 Dragoon MS' Grip Flat Core

• More easily controlled 

• Less likely to skip the Tornado Ridge in BB-10

• Not as explosive in power, meaning certain ARs don't reach their full potential (Jiraiya Blade, Metal Upper)

• Very beginner friendly

MA-01 Dragoon MSUV's Grip Flat Core (Ultimate Version)

• Less controllable overall

• More prone to skip the Tornado Ridge in BB-10

• Highly explosive power, makes every AR shine as best as it can 

• More ARs become viable on this RC (Jiraiya Blade, Metal Upper)

• Requires some practice to get the hang of

In general, veterans to the HMS series will opt for GFCUV over the original, because the added power and speed cannot be understated. Since there is a bit of a learning curve to it, and GFCUV is slightly less user friendly in the BB-10, many beginner or novice players might have a better time using the original GFC. If you prefer the precision and can live with a little less of a lively Attack combo, the original Grip Flat Core may be more your speed. If you like to play a bit dangerously, or are confident in your launching skill, Grip Flat Core (Ultimate Version) is a must.

Here is the variation between the three "Flat Core" RCs. Technically Metal Weight Flat Core, found in Thunder Dragon, can be viewed similarly to BC2, it is a deliberate departure from the initial design and not necessarily an undocumented mold "change". It is still included to be thorough.

A-123 Gaia Dragoon MS' Flat Core (Original)

• In-between diameter of the three

• Lightest weight of the three

• Decent speed and controllability 

RBA1 Thunder Dragon's Metal Weight Flat Core 

• Largest diameter of the three (Takara Variation)

• Heaviest weight of the three 

• Most controllable

MA-18 Magical Ape MS' Flat Core (New Revision)

• Smallest diameter of all three

• Middle weight, good balance 

• Technically just a strictly better Flat Core

In this section we will cover brand-specific changes to RCs. 

A-124 Driger MS' Semi-Flat Core

• Hasbro Semi-Flat RC has a rounded tip, instead of the more edged Takara mold

• This results in a less aggressive RC, it isn't prone to the same kind of change seen mid-match in Takara's Semi-Flat.

• Sonokong's Semi-Flat RC (not pictured) is a slightly modified version of Takara's tip design, with a slightly more conical/taller shape to it. Essentially a marginally more prominent Semi-Flat.

A-125 Draciel MS' Sharp Core

• Hasbro's Sharp RC is noticeably less sharp, in fact much more of a "ball" shape, meaning it has a completely different type of movement to Takara's, and is also marginally easier to knock out.

• Sonokong's Sharp RC is also different from Takara's, actually doubling down on Sharp, making it even more pointed. This creates even more docile movement than the original, but also even more of a sitting duck.

A-126 Dragoon MS' (Not pictured) Grip Flat Core 

• This deviation is not a visual one, but instead a difference in material. Hasbro's Grip Flat Core RC is marginally harder than Takara's rubber-wise, and it is also more prone to become less aggressive faster, which is not ideal. 

A-131 Dranzer MS' Manual Change Core 

• Very noticeable deviation. Hasbro's Manual Changer Core was given a huge boost in diameter. The tip is significantly larger than Takara's version, resulting in even more uncontrollable aggression when in Hole-Flat Mode.

• When in "Sharp Mode", the MCC looks more like MFB's Defense Spike tip than it does a legitimate Sharp RC. It has reduced stamina, and more importantly it is also less controllable than the Takara version, making it much easier to knock out, even less usable than the original.

Note: If you can take pictures of your own Sonokong RCs and their potential deviations, feel free to message me directly on Discord or WBO.

RBA1 Thunder Dragon's Metal Weight Flat Core

• Hasbro's Metal Weight Flat Core is identical to the original Flat Core, but Takara's original design for this RC has a wider tip.

• The two are identical in weight, but the wider tip of Takara's version gives it a bit more speed and nuance. In general it is slightly harder to flower pattern due to the increased tip diameter. 

• Hasbro's alteration to this design aimed at staying consistent with Flat Core means it behaves almost identically to the original Flat Core, but with a bit less speed and more room for control, yet not quite the same amount as the Takara version.

• There is no definitive version, if you prefer a bit more conservative and easier to control tips, the Hasbro version is a better pick. If you want something a bit harder to use, but more rewarding, pick the Takara version. Neither are particularly viable considering better options exist for Attackers.

While they technically aren't the same part or fill the same roles, Bearing Core (MA-08 Wolborg MS)and Bearing Core 2 (MA-22 Jiraiya Blade) share the same bearing housing. The difference between the two is actually more than just BC2 having a rubber tip, but it is actually noticeably taller thanks to a significantly longer tip. Where  Bearing Core's tip ends is roughly where Bearing Core 2's convex shape begins. 

It is possible that had Bearing Core 2 perfectly emulated Bearing Core's original tip, it would have been much better for Attack, the reason being that being lower would help aggressive ARs angle better. Conversely, it would probably be slightly worse for Defense, as this extra height means just a bit less head to head contact between an Attacker's AR and the Defender's AR, allowing for the CWD to play a bigger role for the Defense type.

Somewhat similar to Bearing Core 1 and 2, these parts are slightly different in name, but their designs are very much related. The only serious difference is that Shooter Change Core Alpha (MA-23 Bloody Devil MS) has a Semi-Flat Mode while Gamma (MA-24 Shining God MS) has a Sharp Mode. Both Alpha and Gamma have a Hole Flat Mode, which is likely the more viable option, unfortunately. Semi-Flat and Sharp are far too docile and the low-hanging CWD makes the RC prone to scraping, making them rather poor Stamina or Defense options. While the two RCs are not particularly competitive, they are interesting and sometimes fun to use.

Unlike many of the changes here that come down to manufacturer changes or reimagining a certain base design, the Bunshin Core RC was deliberately released with three variations of its tips.

The original Phantom Fox MS Bunshin Core comes with a Semi-Flat tip. 

This final one is easily the most niche in the fact that this variation seemingly only presents itself specifically in the Three Kingdoms releases of the Metal Sharp Core RC. Instead of having a slightly spiked or rounded metal tip, these tips are completely flat.
Essentially, it behaves like a taller Metal Flat Core, with all the downsides of being  MFC at MSC height.