How long is the barrel? My initial thought would be to suggest my perennial favorite: .243 Winchester, which has the benefit of being so ubiquitous that you'll be tripping over factory ammo choices and have reloading options out the wazoo. However, if you're going to end up with a shorter barrel, maybe a more efficient use of powder is in order. I've been interested in the 6mm BR family but have had no experience with any of them. The 6mm AR or .243 LBC also look interesting, but they would need a non-standard bolt head and Grendel brass. I don't think there is a 6mm round as easy to find on store shelves and to reload for as the .243 Win, so almost any other option would almost certainly need reloading.
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