You probably will find best accuracy with 240 and lighter bullets, from what I have seen. Microgroove and slow twist prefer short bullets. Its a waste of time to try the solid copper in it, such as Barnes. The bullets are too long, even 200 gn. and shoot salad plate sized groups.
240 XTP would be a good bet to start.
Best thing is to bore foam the bore for about 24 hours, if it is used. You never know the history of what was shot through it or how it was cleaned. The microgrooves don't shoot well if fouled bad.
