I am trying to convince a friend that chrome lined barrels are the way to go in a Ar15 system. He argues that the Sig 550 line of rifles has no chrome lining either but just a hammer forged barrel & thinks that just polishing the chamber will have the same effect on reliability.

So, please offer your opinion in the final thread on chrome lining- or not!

Sig 550's were designed and built for an Army that hasn't fought a war in almost half a millenium (and works in a very limited area of operations).  Consider that.

While chrome lined chamber/bores are required by the Armies with the most combat experience with the weapons (i.e. the US Military).  Also not the Russians (with lots of recent military experience) also demand chrome lining on their military weapons.

Polishing will help - but its still not as good as chrome lining.

Actually, chrome lining was not instituted in and of itself to aid extraction.  I's just that, in the humid jungles of Viet Nam, the chambers were rusting badly enough to make them rough enough to resist extraction.

If the barrel has been properly cut, chrome linig will not make the empties come out any more reliably. If the thing isn't well cut, why would he want it anyway?

BTW, the habit of some people seem to have, of mirror polishing the chambers, is not a very good idea.  The miniscule tools marks you'll find even in well cut chambers are an aid in helping the case grip the sides of the chamber, taking some of the burden off the locking lugs of the bolt during firing.  That's why even the best makers of firearms will leave some cutting lines in there, even if it takes a magnifying glass to see them.[:D]

If you think that I suggest looking up the coefiecient of friction of the chrome lining vs the bare steel.

Also consider you be extracting as deposits build up.

Chrome lining is definately an aid to extraction.

Chome lining mainly helps for military users.  If the soldier is out in the field getting rained on for a week or two on patrol and only has time and security to externally wipe down his weapon to prevent external rust, the chrome lining will save the bore from rusting.  

Bores on military weapons don't see as much attention as actions (the gun functioning 100% is the concern not bench rest accuracy) soldiers spend far more time cleaning the chamber than the barrel.  If he wants a barrel that he can use in that sense he should get the chrome lining.  If he is a semi-auto range shooter or a varmint hunter who likes match grade barrels he should get something match grade with no chrome lining.

I think you totally missed my point.  Having had my first bore chrome plated in 1955 (M1 Carbine, by Marker Machine in Illinois) I'm completely aware of its' lower coeffiecient of friction. Please note, I used the word "reliably" in my first post; not "easily".  Any weapon that would need the aid of a chromed chamber to function well is not something I'd want to own [:D]

The simple fact is that AR's function just fine with unchromed barrels, as they do with.  They don't wear out extractors, or anything else, any faster; IF the chamber has been properly cut.  They do give a bit less throat erosion over an extended period of use, and they are less subject to water damage in humid environments.