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I have 2 SA-28. I had the same issue on the first one. The only choke that fit was the one in the barrel. The other mossberg chokes didn't even fit. I had a gunsmith check it. He said the same thing as IC told you. I ordered an IC and it did not fit. I shipped the barrel to IC and they did the same for me. The second newer SA-28 has no issues. I heard that Mossberg corrected the machining discrepencies and all is well now. The guns shoot great. They do shoot consistently low with aftermarket chokes as well.
Here's a pattern picture of the Indian Creek .525 with Apex 9's. 40 lasered yards.Sent from my moto g power (2021) using Tapatalk