The truth about Multi-cal lowers?

[Ronin13]

So I have a POF P415 (based on the H&K 416) and it has a multi-cal lower. Does that imply that I can just slap on an upper of any AR caliber that's compatible? I'm thinking that I want to keep the 16" 5.56 upper I have now for "general purpose" use, and then get a 21" upper in .308, if possible, and outfit it with a 10x scope and an AAC flash hider so I can put a suppressor on it (overall length could come out to about 25 or so roughly). Is this even possible or is only a 6.8 compatible?

[Hoser]

You cant put a 308 upper on a 223 lower.

[Circuits]

So I have a POF P415 (based on the H&K 416) and it has a multi-cal lower. Does that imply that I can just slap on an upper of any AR caliber that's compatible? I'm thinking that I want to keep the 16" 5.56 upper I have now for "general purpose" use, and then get a 21" upper in .308, if possible, and outfit it with a 10x scope and an AAC flash hider so I can put a suppressor on it (overall length could come out to about 25 or so roughly). Is this even possible or is only a 6.8 compatible?

You can put any upper that will fit. Cartridges too long for the magwell like .308 or .50BMG won't be magazine fed, though, or would need some kind of top or side magazine.

The caliber marked on the side of a lower is usually meaningless, and there because ATF requires licensed manufacturers to put something there.

[King]

Yep, you would need an AR-10 lower in order to have a functioning mag well in .308. Or just buy a complete setup like the LT OBR or LWRC Repr.

[HoneyBadger]

Yep, you would need an AR-10 lower in order to have a functioning mag well in .308. Or just buy a complete setup like the LT OBR or LWRC Repr.

Multi means that you can use any caliber with an OAL that fits in the magwell. Examples: 5.56, 6.5, 6.8, 300blackout, even 5.7.
I switch my 6.8 upper and my 5.56 upper depending on which lower I want at that moment. One of my lowers is very lightweight, the other is more for bench shooting.