On the PU forum, there have been tests using 9/16" ID shaft collars for barrel tuning. The results have been impressive. Appears most of the work has been done on older Mini's, making this even more mentionable. Worth a look, and maybe a try for about $4.00
http://www.perfectunion.com/vb/ruger-mini-14-mini-30/46560-mini-14-barrel-tuner.html
