Im in a 15 max capacity state and I am trying to figure out how many rounds of .300 Blackout will fit into a concentional 2.23/5.56 NATO magazine.

10 Rounds .223 = ? .300 Blackout

15 Rounds .223 = ? .300 Blackout

20 Rounds .223 = ? .300 Blackout

30 Rounds .223 = ? .300 Blackout

I have used the 30 round magazines and then the 15/30 reducers from MagBlock.com for my 2.23 rounds (also glued and riveted as to not get any crap) but I want to build a .300 AAC and I'd hate to buy all new rounds/limiters.

The mags will hold the same number of either round. 300BLK cases are (can) be made from 223/556 brass.

If the mag holds 15 223 it will hold 15 300blk

What they said and it's .223.
2.23 would be a measurement in inches, granted both of these measurements are inches but there is a difference.

Edited for clarity.

You can also shorten the 30 round mags.