Match Grade Machine ran tests where they did velocity tests on barrels and gradually cut them 1" at a time. Results are interesting. As I'd expect on the straight walled cartridges barrel length doesn't seem to make a very big difference. Follow the link to see the results.
http://matchgrademachine.com/veloci...email&utm_term=0_359f828a91-bdad5afd5d-629583
http://matchgrademachine.com/veloci...email&utm_term=0_359f828a91-bdad5afd5d-629583