Re: McMillan A3-5 Build possibly.
You can't go wrong with a Surgeon XL or Stiller TAC338 action. If you are planning on shooting 250 gr pills, go with a standard 10 twist. If you are planning on running 300 gr SMK's or Bergers, go with a 9.3 twist. Go with a Bartlein barrel (5R or 6 groove).
I and a few friends have excellent luck with 300 gr Bergers with Vit N570 powder.
For a break, being biased a little, go with Surefire. There are many effective breaks on the market, but Surefire have been really generous at supporting and sponsoring many shooting matches around the country. It would be great to support those that support the shooting community. Plus they work great.
If you are at the infant stage of gathering up all your components, dies, reamers, barrels, action, etc., you might want to do a little research on the 338 Norma or 300 Norma. Look at the "Beyond 1000 yards" link on this forum and see what others are saying about these two cartridges.
Call Marc
http://www.spartanrifles.com/ to build it for you. He may have an XL action in stock or may be able to get you one at a good price.