FWIW "Proper 260 ammunition" will fit in the magazine box. It is manufactured to the SAAMI specification and the Remington magazine box allows plenty of length for that.
Optimised 260 ammunition (like VLD handloads) won't. Same can be said about ANY of the 308 based cartridges. If you want to throw a VLD, you need more magazine length to get the velocity you want (or use a different case).
I've had no problems loading a 130 VLD to about 2.82 and fitting in a Standard Rem length SA mag box. YMMV.
