When I'm using BB's to load my cores, I'm pretty much always using the same method of attaching the the BB cord to the solid handle:
I peffer using stainless steel as handle foundations (V2A steel) with a diameter of 6mm.
Since I'm using 550 paracord as a core string, the front portion of the steel handle needs to be tapered down in order to be small enough to accept a strand of 550 paracord.
I prefill the core strand with the needed amount of BB's before sliding it over the tapered part of the steel rod and I grind two grooves into that taperd steel portion.
Before I slide the core strand over the steel rod, I apply epoxy glue to the tapered portion (up to the second groove. The reson why I don't cover the entire tapered portion with epoxy glue is this: the epoxy glue will start to "rise" towards the thong when sliding a core string over it and as soon as the epoxy glue would reach the flexible part of the whip, the entire core would be ruined.
After I've slid the core over the tapered part of the steel handle (and while the epoxy glue hasn't dryed yet), I'll bind those grooves with artifical sinew.
The following picture shows these first attachment stepts WITHOUT EPOXY GLUE since I've taken them on an older construction, but you'll get the idea of this concept.
The attachement is far from beeing done - the next step for me would be to apply epoxy glue from the "outside" of tapered steel/paracord portion and to wrap it once with sinew(closed loop binding). The sinew wraping will avoid "glue bumps" and assure a round shape.
After the epoxy glue has dryed, I'll make sure that all BB's are as close together as they can be, double check if the BB's reach out as far into the thong as I want them to and then I'll secure the end of the BB's with a constrictor knot (just like I did it on my experimental whip).
Now the core is ready for the next step - the binding.
To be continued...
All the best
Ben