The thread titled “6x45 SAW phenolic varnish on case?” mentions an earlier thread, author unknown, in which the following post can be read:
As I understand it, the 6x50 length was developed for the aluminium case, because to avoid the risk of “burn through” (aluminium being potentially inflammable) the case interior was lined with a fireproof material. This reduced the case capacity so the case was lengthened to compensate, in order to match the ballistics of the 6x45.
There are a couple of reports of interest in case someone wants to dig deeper into the aluminum case burning issues:
AN ANALYSIS OF 5.56MM ALUMINUM CARTRIDGE CASE BURN-THROUGH PHENOMENON
Walter H. Squire, et al
Frankford Arsenal, Philadelphia, Pennsylvania, 1972
Technical Report ARAEW-TR-05003
ALUMINUM CARTRIDGE CASE CONCEPT
Armament Research, Development and Engineering Center, Armaments Engineering and Technology Center (Benet)
Picatinny, New Jersey, 2005
I found this last one in the Internet too but now I can’t seem to remember where.