Which caliber of short action M70 classic would work best for converting to 257 Roberts: 22-250 or 308 based catridge? Any feeding modifications necessary? Would a standard length 30-06 based cartridge convert easily to 7X57? Any feeding modifications necessary for this one?

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Which caliber of short action M70 classic would work best for converting to 257 Roberts: 22-250 or 308 based catridge? Any feeding modifications necessary?

If you were going to 250-3000, I'd think hard about finding a 22-250, but if you're going to Bob, I think any 308-based case will work with little feeding modifications. HOWEVER, CRF (I presume that's what you are talking about) can be finicky about round changes. So, get some Bob shells and give it a try before barreling - at least you can get an idea of what you're looking at around the mag-to-extractor exchange. The modifications will vary from gun to gun...

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Would a standard length 30-06 based cartridge convert easily to 7X57? Any feeding modifications necessary for this one?

This would likely require some work (again, assuming a 70 CRF action). I'd go with a 280 and be done with it, shouldn't require any feeding work...

The 06 sized action should work fine