You have to change the bolt head... which requires disassembling the bolt. A big allen bolt (or the older big slotted screw) is all that really holds it together. Take that out, remove the cocking pin, dump out the firing pin assembly and sleeve, use said firing pin to push out the cross-pin that holds the bolt head on, and remove bolt head and bolt baffle. Substitute bolt head and baffle of choice, and reverse the process (hope you paid attention to how it came apart!)
A shorter cartridge like a .223 or a .222 has a different bolt baffle also... basically it has a 'tail' on one side that hits the bolt stop so you don't have to cycle the bolt all the way back as far.
The magazine box and follower are going to be different as well. The magazine box will have a filler welded or riveted in place to accomodate the shorter rounds, and the follower will be shorter as well. Getting the magazine box off is... not an exercise in precision gunsmithing. I'd recommend removing the trigger assembly (good education any way) and a large rubber mallet or deadblow... which is why I recommend removing the trigger (in case you miss). I about fell over the first time someone recommended it to me... it just doesn't seem right, but it works. The issue is the magazine box is spring steel, and its 'clipped' into slots machined in the receiver. You need to compress it somehow and get it loose at the same time. Some people have made up jigs to push against the recoil lug and compress the mag box in a controlled fashion just enough to pop it loose. Since nobody that I know of makes something to fit this specific application commercially... the rest of us give it a smart whack w/ the mallet and it pops right out. Installation is the reverse... except now you have to push it in, and whack it. Fingers can be in jeopardy, particularly if your name is 'Lightning'
Getting the factory barrel nut loose is probably the hardest part... after getting the magazine box loose and re-installed. Once you get it loose, you can spin it forward an inch or so, unscrew the action from the barrel, remove the factory stamped (and probably warped/bent) recoil lug, put on a new SSS competition recoil lug (Savage lugs are keyed, so its pretty obvious the right way it goes), screw the action back on the barrel, and then insert a GO gauge in the chamber and snug up the action to the barrel by hand. Snug up the barrel nut, open the bolt, swap the GO gauge for the NO-GO gauge, check that the bolt handle WILL NOT DROP all the way closed on the NO-GO gauge, and then give the barrel nut wrench a light tap w/ the mallet/deadblow. Put it back together and go shooting.
If you don't have a set of Go/No-go gauges for your desired caliber, some people use a fired case for the 'go' and the same case w/ a thickness or two of tape on the case head - you only want about 0.003-0.004" total thickness. It works, but if you were to get over-zealous tightening things down, you've got enough mechanical advantage w/ the action threads to possibly compress the case headspace dimension a tad... thats part of the reason headspace gauges are *steel*, and solid.
You can get a lot more help on the subject over @ savageshooters.com
