Hmm, Good Question. Seems to me that if a game warden stops you and you have a 450 B and you have 450 B ammo, is the game warden within his legal boundaries to accuse you of carrying more than 29 rounds of 5.56 ammo (that you dont have), in a 5.56 rifle, (That you don't have), I think he would be easily made a fool of in a court room. Just be sure that YOU don't have any 5.56/223 ammo in your possession and be sure to leave the 5.56 upper receiver at the house. If you are on your killing ground with a 450 Bushmaster with 450 Bushmaster ammo, you are not gonna get more than 12 rounds in a 30 round mag, (if you have a Sig mag). To be accused of this would be the same as being accused of having a Pump Shotgun with no plug, when you really have a single shot break open. Or being accused of having a 5.56/223 Remington, when you really have a 450 Bushmaster.
So, it seems that legally, you could not possibly be convicted of having over 30 rounds in a magazine.
But thats not to say they can't accuse you.
The arguement here in Texas may go something like this.
The DA -"Your Honor, this suspect, had in his possession, a rifle that shot 450 Bushmaster cartridges, but it could be converted to shoot 5.56, and therefore, the magaizine he had in his possession, while it only held 10 rounds of 450 Bushmaster ammo, Had the suspect had an AR 15 Upper that shot 5.56/223 ammo, could have then been able to load in excess of 20 rounds in his 30 round magazine that for now only holds, 10 rounds of the 450 Bushmaster, which was indeed the rifle we found in his possession at the time of the apprehension. While the suspect had no AR 15 5.56/223 Remington rifle in his possession, we would like to think He could have possible broken the law, had he the inclination.....Also...."
The Judge interrupts the psychotic DA..
Judge- "HOLD IT!!...Bailiff!"
Bailiff -"Yes your honor?"
Judge- "Escort this FORMER District Attorney to the nearest mental hospital for further evaluation".
Bailiff- "Yes your Honor".
.
.
Now up in the northeastern US or in Memphis, TN or CA, it would sound something like this.
The DA-"HE HAD A RIFLE!!"
The Judge-"Your Guilty!! Never mind the jury, they think the way we pay them to think".