Chrome lining a surplus barrel?

[Kimio]

Hello

I am in the process of putting together a G1 FAL from a parts kit that I purchased a while back. The thing is, the barrels that typically come with these kits from my understanding are not chrome lined.

Now, if memory serves, chrome serves to help lengthen the life of the barrel among other things. Seeing as there are no new production barrels (At least of the G1 variety), I'd like to try and make sure I get as much out of mine as possible (Though the likelihood of me actually shooting one out is slim).

If it is indeed possible to do this, what would it involve and who would I want to send my barrel to if I wished to have the treatment of having the bore chrome lined?

I'd appreciate any input or any corrections in the event that I'm mistaken on something.

Thank you

Kimio

[Jim Watson]

I do not know of anybody doing aftermarket chrome lining of barrels.
The old Marker Machine Co. did a lot of them but we are now told it is impossible because it will change the bore dimensions.

The New Wonder Treatment is nitriding barrel bores. Ask around.

[Unclenick]

Yep. You can't chrome line because it will reduce the bore diameter about four thousandths, making a .224" groove diameter into a .220" bore. When AR type barrels are designated for chrome lining, they make their groove diameters .228" to allow for that.

Part of the reason for that much thickness is that you can't put chromium directly on steel, as the plating solution is strongly acidic and etches carbon steel away faster than the layer can deposit. As a result, you have to first plate nickel onto the steel, to resist the acid, and then the chromium actually is plated onto the nickel. The nickel, IIRC, is about half a thousandth thick to seal off direct contact with the steel, then the hard chromium as about one and a half thousandths thick. That is about the right thickness to prevent heat stress cracking from extending down to the steel substrate, and is probably as thick as they think they can go without barrel heating causing the plated layers to separate due to differential expansion. You end up with about two thousandths of plated metal, total, but since it is all around the inside of the bore, that subtracts four thousandths from the diameter.

Nitriding is more similar to case hardening in that it penetrates into the steel, forming a case rather than thickening the surface. It can be accomplished at a temperature of 900°F-1200°F. That's the temperatures range in which barrel steel is normally stress relieved, so it doesn't tend to distort the steel the way heating and quenching it can. Sizing dies for reloading are typically nitrided rather than conventionally heat treated for this reason. This page describes the gas nitriding process, and the hardness range examples it has for 4130 and 4140 steel are right where 4150 barrel steel will fall, too: a Rockwell C scale 28-32 for the steel itself and Rockwell C scale 52-58 for the nitride case hardness at the surface.

The page points out that the very surface, which is the most brittle part, is sometimes lapped off. If you are planing to firelap a barrel you also plan to have nitrided, I would do the nitriding first for that reason.

[CowTowner]

Um, Uncle Nick? Which page would that be, Sir?

[Jim Watson]

Gee, the hard chrome platers' sites I checked claim a buildup of only .0002" to .0005" per side and only use a nickel strike on aluminum, not steel.

Further, I had an old article describing a chrome lined 5.56 barrel which was so rough as to foul badly. The owner had the plating stripped and lapped the bare steel bore. So he then had a .225" barrel instead of .224". No four thou buildup there.

I think Unclenick is describing decorative or "bumper" chrome, not the industrial hard chrome used by knowledgeable gun shops. And perhaps used as a horrible example by nitriders.


Nitride treating a used barrel from a surplus kit brings on some problems, so I would talk before I bought.

[Dfariswheel]

The industrial hard chrome as used in gun barrels and on the outside of guns is bonded directly to the steel. In a gun barrel any "base coating" of nickel would cause the plating to not stand up to use.

The only plater I know of who offers plating inside chambers and bores is Armoloy of Fort Worth.
Price listed for a rifle bore is $45.00

You can contact them and ask, but I suspect what they can do is not like the standard plating done inside chrome lined rifle barrels.

http://www.armoloyftworth.com/Armolo...plications.htm

[Unclenick]

I put the missing link in. Not sure how I missed doing that, but I did.

Well, I'm showing my age. It's been 40 years since I did any chromium plating and that was in a lab environment, but I remember being frustrated by the hexavalent chromium and sulfuric acid solution etching steel faster than I could plate. At that point in time the only difference between the decorative and industrial hard chrome plating was not the plating solution but higher plating temperatures and current density for the latter to get thicker layers. Today they seem to use less toxic trivalent solutions for decorative plating. When I looked at an old lab notebook I found I could not meet the normal current density of hard chrome plating with the equipment I had, and that's the part I had forgotten. So you are correct that no nickel intermediate coating is involved.

A quick perusal of military plating specs shows the decorative type is class 1 and the hard chrome type without underplating is class 2. From QQ-C-320B (1974, last version before SAE-AMS-QQ-C-320 replaced it):

3.4.2.1 Thickness. The minimum, maximum or range of thickness for class 2 plating shall be as specified in the contract, purchase order or on the applicable drawing (see 6.2). If a thickness is not specified, the minimum thickness for the finished part shall be 0.002 inch or 2 mils (51 μm).…

So that may be one reason for the 2 mil number. I don't have a barrel drawing to verify it. I did run into one thread on THR (sorry, I didn't copy the link, but you can search it out if you are interested) in which it was mentioned that the barrel plating was 0.002" thick and that Armalite had to lap bores before applying it. (That makes sense as the hex chrome plating solution has poor throwing power and an uneven surface will get an unevenly thick plate. You have to form the lead anodes to mirror the surface you want to plate evenly, I recall.) But there are a couple of other reasons to think 0.002" is probably right.

It turns out, in plating big gun barrels, the process is more elaborate. The bore is first electropolished to remove microscopic unevenness, then is reverse current etched to activate the surface before the plating is actually applied. For the big guns the chromium plating is 0.002"-0.006" thick. That's in a table on page 30 of this document. The document is about finding alternatives to electroplated chromium and is an interesting read because it covers the problems with plated bores and has a flow chart of the plating process early on and one for barrel making that is in the slides that start about half way through the document.

The last reason to think 0.002" chrome plating is minimum, even in small arms barrels, is mentioned in the text of that last document. Part of the protection mechanism provided by the chromium is not mechanical wear resistance. It is providing a thermal barrier to exposure of the steel substrate to the high side of the temperature gradient that develops during firing and which penetrates about two thousandths of an inch into the bore. (I got that from a study a couple of years ago; if I can find the link, I will come back and put it in). When the steel is exposed directly to that transient gradient, it detempers the surface martensite, weakening it, and that is part of the mechanism of forming the alligator skin heat stress cracking you see in bore throats.

Another aspect of heat stress cracking is the differential expansion of the steel at the surface side of the gradient that finally breaks those alligator skin squares off. Chromium has only half the linear temperature coefficient of expansion of steel, so it doesn't endure as much stress from the heat exposure as steel does. In other words, the chromium is acting as a more temperature tolerant thermal barrier as well as being a wear barrier. Since cannon and small arms are exposed to similar powder combustion temperatures, it makes sense to suppose the minimum coating thickness of chromium will be of similar magnitude to that of the big guns. While a thin plating of chromium can get you the wear resistance, it will be insufficient to achieve the thermal protection.

[Dfariswheel]

After I started having some problems with Armoloy, I had customer guns plated by other companies.
In talking to one, I mentioned Armoloy plating chambers and bores. The manager told me that he thought all that Armoloy was getting was "a little color in the bore", meaning it wasn't doing much good.

I did have a few guns plated in the chambers and bores and it seemed to work well, but they were all pistols, no rifles.

[Unclenick]

I don't think I've never seen a pistol barrel throat with the alligator skin surface cracking you see develop in high power rifles. Probably partly the lower pressures and temperatures and partly the much lower total heat content in the smaller powder charge being unable to sustain a sharp temperature gradient. So a pistol probably only needs the mechanical wear resistance and not the thermal protection anyway. If that. I've not noticed the military moving to chrome line pistol barrels.

[Jim Watson]

Actually the current USGI Beretta is specified to have a chrome lined barrel.
So did the last big lots of 1911 replacement barrels in the 1980s.
A Smith & Wesson made 1911 barrel with chrome bore is kind of a strange thing, but you could get them for $6 from Gil Hebard in the 1970s.

[Dfariswheel]

Chrome lining a pistol barrel is more about corrosion resistance and ease of cleaning.