How to Put Steel on Target - Boresighting the M1A1

Eric Daniel

Step Six: Dot on

To align the cannon, a crewmember, usually the TC, inserts a device called, surprisingly enough, the muzzle boresight device (MBD) into the muzzle of the cannon, and while looking through the lenspiece of the device, talks the gunner "onto" the boresight target. This procedure is the actual heart of the boresight process - up until now everything was just preparation. While anything can serve as a reference target for boresighting (I've boresighted off of flag poles, power poles, buildings, vehicles, and rocks) the best targets to use have well defined right angles or points. 

Once the TC has talked the cannon onto the target, the gunner uses a toggle switch on the CCP to align the GPS reticle with the boresight target (to prevent myself from second-guessing where the TC wanted the cannon laid I would not look out either the GPS or GAS while laying the gun and would only look through the GPS once the TC told me what he was looking at.)

After aligning the GPS reticle with the boresight target, the gunner performs a "G pattern" check by taking the cannon off target and returning it to the same point of lay, using the GPS reticle as the aiming point. This check performs two functions. First, it is a means of determining if the gunner (looking through the GPS reticle) and the TC (looking through the reticle on the MBD) are looking at the same thing, and two, it checks for physical "slop" in the system. Without input from either the cadillacs/overide or the gunner's manual turret controls, the cannon/reticle should not move. If, once the gunner has re-aligned the cannon, the reticle moves off target, there is a mechanical fault in the system which must be corrected before boresighting can continue.

To give you an idea of the degree of precision involved, the aiming dot in the MBD and in the GPS reticle is 1/10th of 1 miliradian across. For gunnery purposes, 1 miliradian describes an arc 1 meter wide at 1,000 meters. Therefore the GPS aiming dot represents a circle 10cm in diameter at 1,000 meters. In relaying the gun, if the dot was more than one diameter off, the toggled data was dumped and the entire laying procedure was repeated.

Step Seven: Align the TIS and GAS

Once the cannon and GPS are boreshghted, and that alignment verified by a successful G-pattern check, the next step is aligning the Thermal Imaging System (TIS) – the tank's thermal sight and GAS (Gunner's Auxiliary Sight). While part of the primary sight assembly, the TIS uses a completely different reticle, and consequentially, requires its own alignment step. Aligning the TIS is similar to boresighting the GPS; you use dials to physically move the TIS reticle until it's on the same boresight point you used with the GPS and you perform a G-pattern check to verify lay. The biggest difference though, is that to correctly align the TIS reticle, not only do you need a well defined point or right angle, but there needs to be enough of a thermal gradient in the target to make that aiming point visible in the thermal sight. On gunnery ranges this is solved by heating specific portions of the boresight panel, but in the field you may have to get creative (one we frequently used was a "fire box" – an ammo can filled with dirt and burning JP-8 fuel.)

The Gunner's Auxiliary Sight (GAS) is a no-frills 8x telescopic sight mounted co-axially on the right side of the cannon. Intended to be used in the event the GPS/TIS becomes non-operational, the GAS is a daylight only (nighttime use requires external battlefield illumination) emergency sight with two interchangeable reticles.  Aligning the GAS is accomplished by using reticle adjustment knobs located at the base of the GAS housing to center the boresight cross on the sight on the boresight target.  As with the GPS and TIS, a G-pattern is done to verify proper alignment.

Step Eight: Reference the MRS

The MRS is the Muzzle Reference System, specifically the little doohickey located at the end of the gun tube. The purpose of the MRS is to be able to provide the gunner with a quick and dirty means of evaluating the continued accuracy of a boresight over time. Because the barrel is made of metal, it is subject to thermal flex which means that, as the barrel heats up, it will flex and bend, and not always in a specific or predictable direction. To help mitigate some of the effects of thermal bending, the gun tube is encased in a thermal sleeve that equally distributes absorbed thermal radiation (most often coming from the sun) so that the barrel heats evenly, but the primary method is by doing MRS updates. 

In referencing the MRS, what you are doing is establishing a reference or starting point to evaluate changes in your boresight over time. For example, if, after several hours, you check your MRS and determine that the muzzle is "high" (meaning that the MRS crosshair located at the end of the cannon is above your GPS aiming reticle) then you can assume that your round point of impact will also be high, because, the barrel is now "looking" in a different spot than it was when you first boresighted the tank.

Exactly when to do an MRS update is open to discussion and debate and of all the components of the FCS, none is more subject to the baleful influence of "voodoo gunnery" as is the MRS - Folk will either swear by, or swear at, the MRS. As a rule of thumb though, you should only refer to your MRS if, over the course of several rounds you notice a distinct trend. If you see your shots trending right, then you should do an MRS update and see if there's been a right hand shift in the muzzle's alignment. If the MRS confirms this then go ahead and make the correction. If, however, the MRS does not support this trend (by pointing in a completely different direction) then something else is going on and you need to do some troubleshooting before you waste any more caps. Something else to keep in mind is what were the conditions like at the time of your last MRS update. If you did an update while the tube was hot, for example (either from the sun or from having fired a number of rounds) but now the barrel has cooled off, any MRS displacement is probably a result of that cooling. As a result, you may not want to do another update until after you've reheated the barrel (by firing) since any adjustments you make on the cold tube will be negated once the cannon heats up again (this is known as "chasing the MRS.")

Step Nine: Punch in Your Data

Once the GPS/TIS, and GAS have been boresighted the next step is entering ammunition specific data which the fire control system will use to make solution corrections. In addition to firing a variety of ammunition types (SABOT, HEAT, MPAT, etc) the M1A1 is capable of firing different varieties of each ammunition type. To tell the ammunition varieties apart, the FCS relies on a system of ammunition sub-designators. The ammunition SUBDES is important because each round flies differently. It's not enough to just tell the system that you're firing SABOT; the computer needs to know which type of SABOT you're shooting. Generally speaking, the SUBDES values are average values for that particular ammunition type (referred to as Fleet Correction Factors or FCFs) based on accumulated round flight data, so the theory goes, when firing off of a "fleet" CF, your rounds should be impacting close to the point of aim. During gunnery ramp up, each tank will be screened to verify the validity of this data. Any tank with a shot pattern outside tolerance will have a discrete correction factor created for it right there on the range, which it will use throughout the remainder of the gunnery rotation.

Step Ten: Write it Down

At this point, boresighting is complete. All that's left is a couple of "housekeeping" procedures. Foremost among these is recording your data, specifically your boresight and zero data, as well as the air temperature and barometric pressure at the time you did the boresight. Generally speaking, the trends in your boresight data should never change. If, for example, you need to "toggle" down and to the left to properly align the GPS reticle with the boresight target, then every time you bore sight, you should be toggling down and to the left. You night not get the exact same values but they should be within a couple hundredths of a miliradian. Any gross deviation in this trend indicates that a physical change has occurred in one of the FCS components. Any time the GPS assembly itself (which weighs several hundred pounds) is removed or replaced, your boresight data will change, as it will if the LRF (laser rangefinder) or the TIS are removed or replaced. This is because replacing any one of these components will cause a very minute change in the equipments physical location (when you're dealing with variations of hundredths of a miliradian, it is virtually impossible to "reseat" the component in exactly the same spot.) If, however, you know that nothing has been removed or replaced in the FCS, you'll need to do some additional troubleshooting; there's a reason your boresight data radically different, and before you start flinging rounds down range, you need to figure out what it is.

As demonstrated by its performance in the '91 Gulf War, the M1A1 is a high speed killing machine. But for all its capabilities, it is still just a machine. What makes the Abrams so good at what it does is the attention to detail the crew invests in its operation, and no one procedure is more critical to the tank's lethality than is boresighting. To be sure, there are any number of reasons why a gunner might miss the target (improper range evaluation, improper ammunition selection (SABOT loaded but HEAT designated on the GPS panel), poor target tracking, improper retical lay on the target, radical changes in atmospheric conditions or ammunition temperatures) but none will have such a total and systemic effect on target destruction as boresighting; a tank with a bad boresight isn't a tank, it's just a high priced firecracker.

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Eric Daniel is an associate editor at Military.com. He has 16 years of combined service in both Active Component (1st Armored Division (D/1-37AR) as an M1A1 gunner during the 1991 Gulf War, where he was awarded the Bronze Star Medal with "V" device and Oak Leaf Cluster) and the National Guard (HHC (IA trainer),1-103 Armor, Pennsylvania Army National Guard, OIF III.) He is currently serving as a Cavalry Scout in the California Army National Guard.

Copyright 2012 by Eric Daniel