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Throat Erosion Gauges

Throat Erosion Gauges
Throat Erosion Gauge - Exploded View
Throat Erosion Gauge Assembled
M1, M1A, 308 & 30-06 Throat Erosion Gauge Kit

 

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  • SAMMI & Custom
    Throat Gauge
  • Throat Erosion
    Gauge Kits

 


SAAMI & Custom Throat Gauge

For all Standards and Wild Cats

This gauge is made to the minimum throat length of a SAAMI chamber but also can be used to determine bolt face gradual erosion depth.


Gauges are manufactured on order.
Available for Pistol, Revolver and Long Rifle

 Pistol #SCUSTTG01
Revolver #SCUSTTG02
$57.00

Long Rifle $75.00
#SCUSTTG03

SAAMI & CUSTOM THROAT GAUGE




 

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Throat Erosion Gauge Kits

Description Price

M1, M1A, 308 & 30-06 Throat Erosion Gauge (ARA-MOR Style) Kit

$69.95
Due to miniscule variations; Throat Erosion Gauges can be made to customer supplied prints. Click Here to email us a print for quote, or fax us a print to 541-826-8403.

Calibers Available

  • 224
  • 40 S&W
  • 458
  • 6.8
  • 7-08
  • 7.62
  • 9MM
$88.00 <-- Starting Price

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How To Use

 

Caution: Before attempting to work on or clean any rifle, make absolutely sure it is unloaded! Put the safety on, remove the magazine, open the action, and inspect the rifle to make sure the chamber is empty. Also make sure that the removed magazine and any other magazines in the work area are unloaded, and that any ammunition has been properly and safely stored away. Support the rifle upright in a rest or cleaning cradle, if available, to make the work safer and easier.

Begin by assembling the tool. Screw the appropriate spacer to the gauge body (spacers are laser marked for rifle make, model and cartridge) and screw the correct tapered plug (tapered plugs are laser marked for caliber) to the spacer, hand tighten only. Before use, wipe the tool clean of any oils, dust or dirt. For M1s, make sure the action is empty and completely locked open, for M14/M1As, insert an empty magazine and lock the action open. This will prevent an accidental bolt slam on the tool or on any fingers. For bolt rifles, remove and set aside the bolt.

After assembling the tool, you will notice that the gauge body has a series of 11 rings machined on the body, corresponding to readings from 0 to 10. The rings are numbered from 1-9, 0 and 10 are not numbered. Above the 10 ring there is a "reject" region which is used to identify excessive throat erosion. When the tool is inserted into the chamber throat, the reading is taken by the ring number and lines up with the front of the ejection port in the receiver. The reference for the tool is the land diameter of the caliber and is based on the nominal chamber. For example, if the gauge reads a "3", that means the gauge enters the throat to 0.303" (for a .30 cal taper plug). Due to barrel and chamber dimensional tolerance and throat variations, the gauge provides a relative indication of erosion and should not be used for an exact reading. New chambered barrels can typically measure from less than 1 to as much as 2. Match barrels typically perform properly up to a reading of 4 or 5, some perform to a reading of 5 or 6. If a used rifle exhibits a high number, it might have a high number of rounds through it or it may have been throated longer than nominal. A chamber case will help to determine if excess throat erosion exists or if longer throating is present. What is important is the change in the reading over time, and a new barrel chambered and properly headspaced is the most desired chamber to gauge and track with records. For normal National Courses of fire an M1 or M14/M1A typically will exhibit a throat erosion 0.001" (a change of one unit on the gauge body) per 1000 to 1500 rounds fired. Longer sustained rates of fire, along with high performance magnum calibers, can cause faster rates of erosion.

Hold the tool by the handle on the gauge body and carefully insert the tapered plug end of the gauge into the chamber through the ejection port or opening in the receiver. For M1s, the clip follower will have to be pushed down slightly to allow the tool to enter the chamber. For M14/M1s carefully insert the tool either through the chamber clip guide or from the operating rod side of the receiver while pulling and holding the op-rod handle slightly rearward. Insert the tool carefully, without a lot of forward pressure. A lot of force isn't required and excessive force could damage either the tool or the chamber throat. A "feel" will be developed quickly as to the proper use of the tool. Read and record the gauge body ring/groove reading by looking straight down past the front of the ejection port or opening in the receiver to the gauge body. A reading taken once or twice a year will determine the rate of throat erosion in a rifle. After use, wipe down the entire gauge with a quality light oil and store it in the poly-bag to protect against rusting.

 

Additional Information

 

As stated, the gauge indicates a relative reading of the amount of throat eroision and provides a means to track the rate of throat erosion of a rifle. When accuracy of a known good rifle/ammunition combination begins to deteriorate, throat erosion can be the cause. However, it isn't necessarily the only cause as other factors such as the rifle's bedding, barrel muzzle wear and rear sight wear also can effect accuracy. If accuracy has dropped off and the throat has deteriorated to a high number of 6 to 7 or greater, the barrel needs replaced. If accuracy suddenly falls off and the throat measures as high as 5, have the rifle checked by a competent gunsmith or armorer, particularily the bedding and rear sights, before rebarrelling. If accuracy suddenly falls off with a low throat reading then definitely re-bed the rifle and/or have the rifle checked over by a competent gunsmith or armorer.

Another factor that can effect accuracy and usually isn't taken into account is the amount of muzzle wear that a barrel has, which can be due to two factors. The first is muzzle wear from the cleaning rod hitting or rubbing the muzzle area when cleaning the rifle. The second is muzzle wear or erosion from high pressure gases due to normal firing. As the bullet exits the muzzle, there is a hot high pressure gas present that can slowly check or wear the muzzle over time. As the muzzle wears, uneven gas venting can result which effects accuracy. The Throat Erosion gauge however, can also be used to check the barrel for muzzle wear. While observing all previously stated cautions, for bolt rifles, M1s and M14/M1As with the flash suppressor removed, insert the tapered plug into the muzzle until it stops, and mark it with a pencil at the muzzle. Withdraw the tapered plug and measure with a scale how far it entered the muzzle. If the tapered plug entered the muzzle to a depth of 1/2 to 9/16 inch or more, the barrel is exhibiting muzzle wear which may effect accuracy. (The tapered plug can enter a new barrel's muzzle up to a depth of approximately 1/8 to 1/4 of an inch, depending on the barrel's land diameter and type of crown.) For M14/M1As with a GI style flash suppressor in place, assemble the tool with M1 30/06 spacer, and while observing all previously stated cautions, insert the tool in the flash suppressor until the tapered plug stops in the muzzle and take a reading on the gauge body at the end of the flash suppressor. (The gauge body will indicate between 1 and 2 at the face of the barrel muzzle and can indicate up to approximately 4+ where the tool stops in the muzzle for a newbarrel, again depending on the barrel's land diameter and type of crown.) If the reading is approximately 6 1/2 or higher, then the muzzle is exhibiting wear which may effect accuracy. Again, one item that will effect the muzzle reading is the depth of the crown on the muzzle. Used barrels will sometimes be crowned deeply to try to overcome any muzzle wear, which sometimes can help to restore some of the lost accuracy. The muzzles on bolt rifles and M1s can be refaced or trimmed back and recrowned to help overcome any muzzle wear. M14 barrels can be slightly refaced and recrowned with some limitations to note. At the muzzle end of the M14 barrel there is a reduced diameter section (.4775 to .4795 inches in diameter) that should be a minumum of .125 inches long to provide for positive seating of the muzzle face in the counterbore section of the flash suppressor counterbore. Also, the muzzle face has to be maintained at a 90 degree angle to the bore to help prevent flash suppressor misalignment when the flash suppressor is installed on the barrel.

Bolt rifles should be cleaned from the receiver, not allowing the cleaning rod jag to slap against the barrel muzzle when it exits the barrel. However, in a bolt rifle, the cleaning rod can also wear the chamber throat. A chamber rod guide for bolt rifles can minimize cleaning rod throat wear. M1 and M14/M1A service rifles, by necessity, are cleaned from the muzzle and cleaning rod wear at the muzzle can be minimized by the use of a cleaning rod guide for the muzzle. Several excellent chamber and muzzle guides are available from various manufacturers.