The best article I have found was authored by Bruce Woodford who I have known for over 30 years.
http://www.fulton-armory.com/\faqs\M14-FAQs\TEGauge.htm
While the article doesn't say so Bruce taught the National Match Armorer's course for the M14 rifle at Rock Island Arsenal for a number of years and he is now retired on his farm in Iowa. As it turns out I just talked to him just last night on this very subject.
The article also does not say that Bruce shot for the Army Reserve Rifle Team and he was one hard gun to contend against in those days. My background is the Army Small Cal Wpns Lab (Picatinny Arsenal) and Small Arms and Ammunition Test Director at Aberdeen Proving Ground.
The gov't never had a muzzle erosion gage and was basically concerned primarily with throat erosion because this was the logical place to measure as it shows the most "wear" thusly one finds throat erosion "TE" gages at a fairly common rate. I have one for M1 and M14 I picked up at gun shows.
As Bruce points out in his article the Army MTU removed barrels that showed 5 or more from their match rifles. I was told the same thing by MTU armorers I have known over the years being a active high power shooter since 1973.
There is more to it than what Bruce outlines and I can supply a piece of that puzzle.
Accuracy (we called it dispersion) acceptance for the M1 Garand was 5" at 100 yards fired from machine rest. The point that a barrel is worn out for military standards is/was 7.5" at 100 yards or a loss of 200 FPS velocity from a control lot of ammo.
At Aberdeen I shared a office cubicle with Otto Haenel who was/is a legend in himself from the Proving Ground having been in the test business since 1948 at Aberdeen. He died in 80s time frame after working for Ruger as their test advisor (for lack of a better term) for a number of years. He was recommended to Steve Vogel (Bill Ruger's son in law who was in charge of military sales) by me when he asked me if I knew someone from Aberdeen who might come to work for Ruger to test their designs under the same "conditions" they would see at Aberdeen and I recommended Otto to Steve. Ok back to the "rest of the story".
Otto told me that during the late 40s they wanted to field a TE gage for the M1 Garand but there was no round life = wear test data so Otto was assigned the test and he told me how it was conducted.
Otto ordered in ten new unfired M1s from Springfield Armory and they were measured to determine the location of the throat on all ten rifles and they were numbered on buttstock 1-10.
First Otto obtained a truck load of M2 ball ammo all the same lot number and shot initial dispersion groups and chronographing with each rifle which normally consisted of three NRA master class shooters shooting 3 ten shot groups with each rifle and the data on the targets was recorded and the targets filed away. Then each rifle fired another 910 rounds and had the barrels measured again, recorded the readings and then did the same 90 and 910 round series to get to 2000 and it continued to 10,000 rounds. Bottom line he burned up a little more than 100,000 rounds (with final dispersion tests) and the readings were averaged and they came up with the ring spacings on the erosion gage which as Bruce Woodford says grows at the rate of one ring per thousand rounds.
For a match rifle we know about 5000 is "it" for a normal match rifle and it starts to "open up" and is thusly retired but as you will read, many barrels are replaced as worn out when actually a little TLC just might bring them "in".
What Bruce did was to start measuring the muzzle wear and he determined a gage of the same taper used in TE gages showed a movement of about half that that showed at the chamber end thusly 5 rings on chamber end tends to equate to about 2 1/2 rings on the muzzle end and Bruce had hundreds of barrels to conduct his personal study on.
From the military standpoint it was a pass/fail criteria at depot inspection or field service rejection as cutting off the muzzle to where the gage gives a "0" reading is not possible on military barrels and considering the gov't had/has a "unlimited" budget the barrels are replaced like we change underwear.
Couple normal shooting with kids cleaning rifles from the muzzle and letting the cleaning rods rub the muzzles you can literally take a rifle with a new barrel and achieve a 2,3,4 or worse reading with a muzzle gage on a barrel with only a couple hundred rounds as troops were not taught the importance of not letting the cleaning rod touch the muzzle AT ALL ! ! ! !
This is good news to bolt gun shooters because as Bruce points out a barrel can be shortened and recrowned to achieve an original "reading" and not really affect anything save for perhaps having to drill another hole for the front sight block set back.
There is good news though for guys with 30 cal and other rifles and other calibers.
http://www.superiorbarrels.com/
has muzzle protectors for 30 cal and larger barrels that clean from the muzzle (M1, M1A, M1 Carbine, lever guns, pump guns etc) and they have erosion gages for other calibers. I have figured out a way to use these same erosion gages for measuring muzzle wear which is basically how we measured erosion at Aberdeen.
Say you have a 7MM barrel and you have a throat erosion gage for 7MM and you want to see if you have muzzle wear. You simply place rifle in vertical position WHEN BARREL IS NEW and very gently insert the 7MM gage in the muzzle till it stops. Then take a caliper and measure the distance from the front of the muzzle to the first ring on the gage and record it in your data book.
Every time you measure throat erosion you measure muzzle erosion and track your wear. You should get 1/2 the muzzle movement less than you see on the chamber end.
For those that already have a TE gage for a M1 or M14 you can save the extra bucks for the Muzzle Gage from CMP by using it in the same way with a caliper.
As Bruce points out in his article the "barrel makers" do not have to produce anything to any specification but their own as there is no inspection so the internal bore dimensions of barrel makers can vary considerably from maker to maker and barrel to barrel.
For instance I received two barrel blanks from the same manufacturer a few years ago that were "identical" in outside configuration but that is where it stopped. I chambered both barrels with the same chambering reamer headspaced to snug on a "GO" gage and ran my 30 cal erosion gage in at "0" rounds to make the initial recording and the difference was 1 1/2 rings between barrels. What does this mean? Basically the bore as machined was "looser" and chances are one will not last as long as it is bigger from the "get go" and you are at the mercy of the luck of the draw.
Bottom line is Bruce is absolutely right in all aspects, muzzle wear can and will take out a barrel quicker than TE because the troops were/are not taught to care for their muzzles and more barrels have been ruined in cleaning than were ever "shot out".
You have to really care for your barrel to obtain maximum service life from it and protecting your muzzle from damage is a vital part of it. Thusly a bumper sticker I once saw comes to mind once again and it read:
IF YOU THINK EDUCATION IS EXPENSIVE, TRY IGNORANCE.
Ain't that the truth? haha.
