From the NRA website –

“The NRA is aware that Defence Infrastructure Organisation (DIO) has announced a new restriction on the use of the 6.5 Creedmoor cartridge on fixed ranges on the Salisbury Plain Training Area (SPTA). This restriction applies to NRA Clubs licenced to use SPTA ranges.

The NRA is working with our colleagues in DIO to better understand the restriction and any broader implications for civilian users of MoD ranges elsewhere in the United Kingdom.  As we get more information, this will be communicated to members and Clubs.“

As at 2/10/24 no other explanation has been forthcoming.