|The Infamous Throat Erosion Gage
by: Bruce C. Woodford
Throat erosion is a term often used by shooters, but far too often not understood. Throughout my years in the DCM/CMP firearms maintenance van at Camp Perry, I couldn't count the many hundreds of times I was asked to check the throat in someone's rifle. From talking to the people I quickly learned many, if not most, had no idea what a throat reading really means. Most people had one concept; If the reading was 5 or more they figured they needed a new barrel. Well let me tell you, that ain't necessarily so.
The throat gage has traditionally been used primarily by the military as one method in determining serviceability of a particular firearm. The design of and readings produced from these gages are directly related to the dimensions of a new barrel manufactured to GOVERNMENT specifications. A throat gage reading of OTHER barrels does not always provide the same information due to design differences.
The following information is presented with the M14 rifle barrel in mind as that is my strong suit, but is generally applicable to other barrels.
The term throat (synonymous with lede, lead, or bullet seat) refers to an area where the lands of the rifling start at the breech end of the barrel - just in front of the "free bore" area. The lands actually start from the same surface plane as the groves, but quickly taper to full height. A measurement from the surface of one land to the opposing land will provide the bore diameter. The drawing for the NM M14 barrel calls for a bore diameter of .3002 inch with a tolerance of plus .0004 inch. In general discussion this dimension becomes .300 or simply .30 caliber.
Although the actual starting point of the rifling is determined from an internal point in the chamber the throat can be accepted as being a specific distance from the breech end of the barrel.
The throat gage is comprised of a forward section that is precision ground to a specific taper to coincide with the numbered reference lines found farther back on the shank. There are 11 lines used as reference, the forward most is not numbered but is quoted as being zero. The next 9 lines are numbered accordingly. Although the last line is not numbered - the "field reject" line - it is accepted as 10. The rear end of the gage has a simple handle coming off at an angle. The neat thing about the handle is that it keeps you from losing the gage in the really seriously worn barrels.
When inserting the gage into a new "government" barrel until the tapered part bears against the lands, the first reference line "zero" will be even with the rear end of the barrel or just slightly beyond. The throat reading is quoted as being zero, or perhaps one half. This tells you the rifling starts where it is supposed to and the bore diameter at that point is the .300 inch as it is supposed to be.
Suppose you put the gage in a worn barrel. You may find the number 4 reference line comes even with the rear of the barrel. This would tell you the bore diameter is .304 where it is supposed to be.300 inch. The "throat" is worn away by 4 thousandths of an inch. If the gage inserted all the way to the last line - field reject - this would tell you the throat is worn by ten thousandths of an inch.
So where does this infamous number 5 get its notoriety as being the point at which you must re-barrel? Well here's the story. It has been found through the years that the "average" - what ever that is - throat wears at the rate of about one thousandth per 1000 rounds fired. It has also been noted that this so called average barrel with5000 rounds through it, and reading a 5 on the gage, has lost its peak accuracy at that point. As many other factors enter the picture, this rule of thumb is often proven wrong.
I'll digress here a bit and tell you a story. Several years ago on my way to work in the van at Camp Perry I stopped by Racine, WI to shoot a Regional. My M1A was equipped with a Douglas barrel that had over 5000 rounds through it and was reading 5 on the gage after starting at zero when it was new. My two 600 yard scores were 199's, one with 3 X's and the other with 4 X's. Yes, I knew the gun was losing accuracy, but I also knew if I did my part I'd have a good score, but with less X count. The throat gage reading is just one bit of evidence. The proof is in the group sizes the barrel is capable of producing.
Now let's consider this throat issue and its real effect on accuracy. As the throat wears a little, or it was too far forward when new, yes, you have more "bullet jump". Does that really effect accuracy when in small amounts? What about in larger amounts? That's a debatable subject at best. Consider for a minute the M1 Garands the Navy converted to 7.62MM by putting an insert in the .30-06 chamber. Think about that bullet jump. Some of these rifles shot very well in spite of the rifling starting about a half inch forward from what conventional wisdom says it should. Wow, a 500 reading on the throat gage. Think about that one. Actually it doesn't work quite that way and the gage reading would only be around a 7 or 8.
Now think about this for a bit. Suppose you have a bolt rifle with a 28 or 30 inch barrel you are quite proud of. You shoot thousands of rounds through it until the throat reading is comparable to field reject. The rifling farther down the barrel is still in pretty good shape. So you whack a couple inches off the back end and re-chamber it getting the throat reading back comparable to zero. Will it become that tack driver again? Not likely. You would have been much better off whacking a couple inches off the muzzle end and re-crowning it. You would have noticed great improvement in accuracy. This is because you would have gotten rid of the worn rifling at the muzzle and the exit diameter would be back real close to .300.
Many shooters do not realize the wear that takes place at the muzzle and how important that end is to accuracy. Here is a brief view of what happens. The gases pushing the bullet through the barrel possess a greater velocity potential than the bullet does. At the very instant the bullet breaks seal with the muzzle the gases blow past the bullet. This "blow by" effect at the time of exit tends to wash away metal at the muzzle. As the muzzle erodes, the gases start leaking around the bullet before it is really supposed to be free from the barrel. Any irregularity in the way the gases leak past the bullet will have effect on accuracy. The condition of the rifling at the muzzle and the crown have far more meaning than a few thousandths of wear at the throat. From experience I have found a throat to muzzle wear ratio of about 2 or 2« to 1. With a throat reading of 5, you'll usually see about 2 or 3 thousandths of wear at the muzzle. This amount of wear at the muzzle will account for losses in accuracy.
When I am asked to check the throat in a rifle I first determine what kind of barrel I'm dealing with. Assuming it is a government barrel, when I insert the throat gage I am thinking about the muzzle. Suppose the gage reads a 3, or slightly more, but the shooter tells a convincing story about how the rifle used to be a tack driver but has gradually lost accuracy. I then check the muzzle for erosion. Once in a while I find the muzzle has worn at a greater rate than normal which would account for the complaint. This greater wear than normal could be caused by an inconsistency in heat treat, or even from cleaning rod abuse. Whatever, if the muzzle is open too much, it isn't going to shoot well.
By the way, here's how I check for muzzle wear. Originally I turned a piece of drill rod into the configuration of the front of an M14 flash suppressor and brazed a flash suppressor nut to one end of it. Using the gage lab at Rock Island Arsenal I selected a barrel with a proper drawing dimension muzzle as my bench mark. Then I went through a process of modifying the length of my suppressor like tool until I could thread it onto a barrel, insert the throat gage and get a reading of zero at the end of it. This worked fine for checking muzzles that didn't have a flash suppressor already in place. After this I went through a process of painting one side of the taper of a throat gage with layout ink, inserting it into the muzzle, and marking how far in the taper went. Then I would measure the diameter on the taper adjacent to my mark. As time went on and I gained experience I got to where I could insert the gage into the muzzle of an M1 and estimate the amount of wear very close. When checking an M14/M1A for muzzle erosion, I use a throat gage for the M1. I carefully insert the gage into the flash suppressor until I feel contact with the front of the barrel. Making mental note of the reading at the end of the suppressor, I then center the gage and slip it into the bore. By noting the amount of additional gage travel and subtracting one reference line I can determine very close the amount of muzzle wear.
Now some comments on throat readings from non-government barrels. For the most part, little meaning can be extracted from a throat gage reading alone. To start with, think about all the various folks out there making barrels. We know that a throat gage reading is in part determined by the bore diameter. What are the starting bore diameters for all these different barrel makers? Your guess is as good as mine. I can tell you that one particular variety put out by a fellow in Wisconsin is 2 to 3 thousandths less than .300 and his barrels are noted for their quality. If the throat is located correctly on these barrels, a new barrel will throat at well less than zero. These barrels can have many rounds through them before they ever read zero, giving the false impression of a new barrel.
Then consider a barrel blank of some other manufacture that has a bore diameter of around .301. Suppose the reamer used to form the chamber cuts the throat a little forward of where it is supposed to be. This brand new barrel, and a tack driver at that, may throat at a2 or 3. It could be reading at that nasty 5 on the throat gage when it has only a couple thousand rounds through it and there is a lot of "goodie" remaining - very little wear at the muzzle.
As I mentioned earlier, the throat gage reading is only one piece of information. Most of the time when people are concerned about a throat reading, it is relative to accuracy and accuracy itself is a relative term. If you have a service grade rifle that will hold 2 to3 MOA, I'd say you have a pretty accurate rifle. If that accuracy was coming from a full blown match rifle, I'd say it leaves a lot of room for improvement. No doubt the barrel is the foundation from which you build an accurate rifle, but many, many other things go into it.
To wrap up this discourse, I recommend you not get too consumed by the throat reading. Expect a realistic amount of accuracy, something equivalent to the overall combination of the rifle and ammunition being used. And consider this; often a great deal of accuracy can be restored by utilizing proper cleaning procedures and making sure you have removed possibly years worth of fouling. But then that's a whole nother story for another time.
Keep favoring center.
Bruce C. Woodford