I just purchased a Ballistic Advantage barrel with a.223 Wylde chamber with a 1/8 twist. When breaking in a new barrel what round should be used? I plan on shooting everything from light .223 varmint to heavy 5.56 OTMs.

http://ballisticadvantage.com/blog/ballistic-advantage-barrel-break-in-procedure/

Didn't see anything about bullet weights. I'd go with some basic NATO spec stuff to start and save the more expensive rounds for later.

This.

First, you want to surround the rifle for at least 24hrs w/ ripe Spanish onions. Then your going to want to set ups some rocks in a pentagon shape, dont forget the salt piles... Ah screw it, just do what Rob recommended

Burn sage and tobacco and let the smoke run through the barrel, then wash with sparkling water and lube with virgin olive oil. Then shoot 500 rounds of wolf bi metal and never clean after that.

Shoot one round then clean for 3k rounds. Another method is to just go shoot it. Both seem to work identical.

I've recall an article which stated breaking in was created as a method to sell more benchrest match barrels.  In the days when stainless match barrels had a lifespan of 1200 rounds for best precision, a 100-200 round break-in would take away from the barrel's life span, prompting for sooner replacement.

While barrel break-in is real, it is a voodoo science that depends on what phase the moon is in.

FPNI (as so often happens).

I'm a fan of swabbing the barrel after each round for the first 5 rounds, then again after 5-10 rounds for 30-50 rounds. This is usually the zero process and first range trip. Then I consider it broken in. I usually don't do this for non-SS barrels, if it's chrome moly just shoot it. I clean all new barrels before firing incase of rust preventatives and dust/grit burrs form machining and rifling.

I have only broke a barrel in once and that was shot one then clean for 5 rounds followed by a couple shoot 5 then clean. I haven't done a barrel break in on anything I own currently and don't see any difference.

BA is right about break-in being about removing reamer marks from the throat, and not to somehow improve the bore or rifling.

If it's a stainless match barrel follow the manufacturer's break-in procedure.  If done right, you will have polished the throat, have removed the copper buildup that tries to attach to those reamer tool marks, and will get much longer time between bore cleaning.

Here is the method I used on my Lilja Recce barrel.

https://riflebarrels.com/support/centerfire-maintenance/

This is a sub 1/2 MOA match barrel made to benchrest standards.  I doubt the break-in improved accuracy, but the barrel has very little copper fouling now and cleaning is easy and infrequent.  It allows the barrel's inherent accuracy potential to be reached and maintained longer before cleaning is need again (when groups start to open up).

If its not a match grade barrel shooting match ammo, don't bother.  Just shoot it.  Same with a nitride barrel.  Nitride is going to be too hard to successfully remove the reamer marks.

I believe it was McMillan themselves that admitted that.

All my precision barrels are nitrided, so break-in seems even more pointless. Just shoot it and clean it when you see accuracy dropping off.

You are correct. From all I've read, the only benefit, as mentioned above, is that it may buy you more time between cleanings.  The cost, of course, is barrel life and upfront effort.  If you can work break-in into your shooting that actually provides useful practice, I guess it is a wash.