The Royal Armouries released
a video a few days ago on the BSA 28P. Rather unsurprisingly, Paul already has it in his
Best Assault Rifles That Never Were, but the video mentions that the earliest prototypes did not include the cyclical rate inhibitor of the rifle that was written up. For serial number 1 (and an unknown number after, but probably no more than 6 because SN 7 has the inhibitor):
Change ROF from 5 to 10
Change burst recoil from 9 to 19
Yes, it's going to spit bullets everywhere but where it is aimed. Based on trials results, that seems to be historically accurate.
The write-up also mentions that it used rifle grenades. Per the video, the one it was designed around was the British anti-tank grenade No. 85, which was equivalent to the American M9/M9A1. It used a scaled-down bazooka-like shaped-charge warhead capable of penetrating 2" (~51mm) of armor.