Just brought home a new Savage in .17HMR, I have a box of the 17g & a box of the 20 g, does it make a difference wich one I break it in with?

Nope, won't matter. Enjoy your new firearm!

[/align]Only if you break in and site in at the same time. Or at least get it close until it is broke in.

There will be ponit of impact differences. I wouldn't worry about breaking the rifle in, ive had my .17 for 3 or 4 years and ive shot a couple hundred rounds through it and it shoots the same now as the first group it fired.