The 527's had three different trigger guard/magazine well and magazine lengths. Smallest was the 17/22 Hornet, 221 Fireball and 222 Remington length. Longest was the 6.5 Grendel/7.62X39 magazine length and trigger guard/magazine well. The middle length accepted the 17 Remington, 204 Ruger, 223 Remington and I think the 300 Blackout rounds. I don't actually know which of the three different lengths the 300 Blackout uses though. From experience you can use the 223 magazines for the 17 Remington, but the 204 Ruger has to long of a shoulder distance to fit in the 223 magazines. The 204 hits the 223 magazines side stiffeners. I don't have any experience with the 300 Blackout in a CZ 527 but while looking for some other magazines I noticed a picture of the CZ 300 Blackout magazine and it was pretty much a smooth sided (no 1/2 moon stiffeners) magazine. I'm mostly just curious if it is possible that the 300 Blackouts magazines would be usable with the 17-204-223-300 rounds. This would save some expense if you had different caliber 527's in the middle magazine length and were looking to pick up a few spare magazines. The four different cartridges all use the 222/223 parent case base size so it would only then matter if the 300 Blackout magazines feed lips would work with the other three rounds. With the discontinuation of the 527 rifles and parts to follow eventually this might prove useful in the future. I'm sort of thinking out loud here.
