Not commenting with any specificity on this case, just general insight: with regards to certain firearms/device charges, charges are often held from being laid until a forensic firearms laboratory has the opportunity to examine the purported firearms and devices, test them, and certify their characteristics and capabilities. For example: any one of us can jam thirty rounds into a C7 magazine, but for prosecutorial purposes a forensic firearms examiner would verify that a magazine could hold a certain number of rounds of a given caliber and feed them into a firearm. Or that a given firearm does in fact chamber and fire a cartridge of a certain caliber, and that a bullet thereby fires.
Like any lab process there’s a lot of demand for this, and so there’s some delay. I won’t speculate as to whether any additional charges will be added here, but if later we were to see charges added related to certain types of firearms or devices, I wouldn’t find that delay weird. An initial arrest and laying of charges for bail purposes doesn’t preclude adding or dropping charges later.