I had a big bore subsonic built on a P14.
It's a .458x1.5 Barnes with a 16 inch bull barrel.
Brass is .375 H&H, cut down and inside reamed to thin the case wall.
I put together a die set for it from my odd dies lying around. Sizing die is a normal .375 H&H sizer. The seating die is a Redding expander die body with a 45ACP seating stem in it. The decapper is an old RCBS .38 special decapper and the decapping rod is the perfect size to do case mouth expansion.
I've got 500 and 600 grain lead waiting to test it. I'm expecting to fill what's left of the case after bullet seatng with CFE Pistol. I have an old Lynx M4x20 with NATO reticle ready for it .
Mine was the first in SA, the caliber was not on the SAPS system.
I got a call from my DFO on Friday, looks like the license for it has arrived.