Imo theres collectors and hoarders.

Collectors must have at least one firearm in almost every usable caliber which have played a historical part in their development over the centuries.

Hoarders, on the other hand, just pick 3-4 sentimental/combat/common calibers and then buy as many related firearms as possible. It must be noted that these hoarders can also collect firearms within their hoard. E.G. different makes/models/manufactures.

However, there are also hoarding collectors who just buy anything and everything they can get their hands on or that is REALLY REALLY COOL. This is possibly a psychological condition needing treatment... it can be mitigated by more buying.

Final answer: Just buy an 870 and glock, the rest will follow.