Does it make any difference what ammo I use to break in a .223 barrel? Will I see any difference in long term performance if I break in the barrel using FMJ ammo instead of other types?

I don't even look at groups from the first 100 rounds that I put down the barrel.

I'm more concerned about, letting the barrel cool down between shots and cleaning the barrel after each 3-5 rounds.

With haveing shot benchrest for years and owned barrels by kreiger, Shilen, Lilja, and then some lesser known custom barrels for my rifles, some costing more than the rifle itself and we have come to find out it really does not matter on the barrel break in what you do.

I attended a seminar with G.David Tubb, holds and broke more long distance records than anyone known, said that if the barrel was not manufactured correctly at the shop, nothing will make it shoot better. I asked him bout barrel break in and thats what he told me.

I myself fire the first 25 shots with cleaning between each shot, soaked patch in solvent, plastic brush, then a clean patch. Then I do it for the next 25, every third shot. Next 50 every 5th shot, then take it home for a THOROUGH cleaning using barrel foaming agents and bronze brush. All cleaned from the chamber using a bore and chamber guide. I could go into using a bore scope to show imperfections and lapping compound agents on cast bullets but that is far and beyond what we need in a hunting rifle. I even do this with our sniper weapons at work.

As Sheridan said, don't even beging to look at groups for the first 100 shots. Just take along a friend, let the barrel cool between shots, and just make a hole in a berm with them. Don't even take a target so to speak. Sounds like alot, but it really not for what your asking your weapon to do.

I have to disagree with the others.My CZ 527 .223 shot well right out of the box even with low cost ammo.This was back during the Obama scare and primers were very hard to come by.Otherwise I would have just bought brass/primers right off.With any gun regardless of age you should take care not to overheat the barrel.As far as what you should try that depends on the twist rate.Faster twist-heavier bullets will shoot better.

Just don't break it in with moly bullets. Any others will work just fine.