The LAR-15 magazines are made for 5.56x45, the M305 is made for 7.62x51. Cartridge sizes are incompatible.
However, the Kel-Tec SU16 in NR and can use the LAR mags. As does the Norinco Type97NSR and NRR. Also the IWI Tavor TAR-21. The Mossberg MVP an as well, but that's a bolt action rifle.
For AR type magazines, my personal preference is the Alberta Tactical Rifle Supply (ATRS) AT15 pistol magazines. They are black polymer, and slightly taller than the LAR mags are so they fit better in pouches and just seem a little easier to grab on to. Unfortunately all their gen1 mags are sold out, and the gen2 have been facing delays.
However, since you are considering the M305, Northern Republic Magazine is expecting a shipment of Australia International Arms (AIA) M10 .308 Winchester magazines in various sizes. These were supposed to be full capacity, but the RCMP have recently ordered that they be limited to 10. NRM is expecting batches of 10/10, 10/20, and 10/30. These magazines are not designed for the M305, but will function if you so desire. The laws are concerned with what they are designed and intended for, not what they are actually put in.
Cool, thanks.
Isn't it possible to use mismatched ammunition and magazine? Somehow I've gotten it into my head this is possible with some pistol magazines, and therefore maybe rifle magazines?
Sometimes. A huge factor is cartridge length. You may be able to get the magazine to function with shorter cartridges, but it is difficult to make cartridges with longer overall lengths work. Cartridge diameter is a different matter, but easier to work around such as how 9mm will generally fit into .40S&W magazines. In some cases the feed lips will need to be tweaked for proper feeding. It's a very case-by-case basis. Beretta 92 mags work fine for 9 in .40, but MecGar Walther PPQ mags have feed lips about 1/64th of an inch too wide to hold 9mm reliably in .40 mags.
As an example, 300 AAC Blackout can be made from .223 Remington brass, trimmed down and necked up to fit a .308 bullet. Because the cartridge has the same dimension body, but just a slightly shorter overall length they will fit and function just fine in .223 mags.
50 Beowulf is the most extreme example I know of, approx 13 cartridges of .223 will fit in a mag intended for 5 of 50 Beowulf.
In some cases external adapters can be made to change what gun the magazine will fit into, someone on here made an adapter to use XCR-L pistol mags in his SKS. Because the magazine itself was not altered, it is legal to use it in that manner.
Then there are also some internal adapters that can be used for increased reliability with smaller cartridges. I think GSDesigns on here sells some .22lr adapters for SIG P226 mags, so you can use the same magazine body as 9mm, just swap out the follower. I really don't know where that falls if you were to do something like made a 50 cal to .308 adapter to fit 15 or 20 cartridges into a magazine. Theoretically it should be fine because you're not increasing the capacity of the cartridge it was designed for.
