I bought some ammo from a fellow here whom I assume was getting out of the 450 Bushmaster. He threw in two magazines. The magazines were stock looking Sig Sauer 5.56, 30 Round, Black Polymer magazines the two he included hold 13 rounds of 450 Bushmaster and still work with 5.56. I did a Google Search and found these and ordered 5. Just got them. These are a little different and only hold 12. These magazines work right out of the box with 5.56 AND 450 Bushmaster, no mods needed and they are only eight bucks. They are Sig Suaer magazines so they are good quality units for even a 5.56 shooter but because they work so well with the 450 Bushy I thought I would post the link here. Check them out. They work great
https://www.sigsauer.com/store/5-56-polymer-30rd.html