I have not tried this mag, but reviews seem fairly positive. If, repeat, IF this is one of Thermold's "variable capacity" mags, capacity of mag ought to be 26 rds with base of mag "collapsed", and 30 rds with base of mag "extended." Pix show base of mag extended. This is speculation, as text is not informative, and it might simply be a 26-rd mag. FWIW, I had a similar, "variable" mag for .223, and it worked all right. Might still have it.

https://www.budsgunshop.com/product_info.php/products_id/57876/thermold+26+round+black+mag+for+mini+thirty

Here is same item direct from Thermold: https://thermoldmagazines.com/r-m-30-26/ Note price difference.

Here is pic of 30/45 rd mag for AR platform, for reference: https://thermoldmagazines.com/ar-15-m-16-30-45/

Disclaimer: no financial interest.