USMC MST-100 & MST-100M Glass etched reticle retrofit – Sentinel Tactical

Available only through Sentinel Tactical is a glass etched reticle retrofit for the USMC UNERTL MST-100 & MST-100M.

If you own a USMC MST-100 from an M40a1 or the Canadian C3A1 MST-100M scope you will understand why the glass etched reticle is a great idea. All of these old scopes are just waiting for the tungsten reticle wire to fail rendering the scope useless. It is amazing what John Unertl was able to accomplish when he designed and built these scopes creating the first Mildot scope, with the resin dots on the tungsten wire, perfectly spaced to create the Mildot reticle, amazing craftsmanship that is extremely difficult to replicate.

The glass etched reticle takes this to the modern era with no discernable optical difference with the exception of perfection in spacing of the bars and dots to a true mildot. The dots are a cleaner round shape rather than the inconsistent football-ish shapes made by dripping resin onto the wire. If there was enough interest we could produce etched football reticles as well as the true mildot.

The MST-100 optics by design, are not Nitrogen purged and sealed. The eyepiece focus adjustment is not and cannot be sealed effectively thus making the optic unsealed. And change in temperature or adjustment of the eyepiece both create an atmospheric pressure change inside the optic creating either pressure or a slight vacuum. Anybody who has used these optics in less than acceptable climate conditions know they can and do fog, this is why. With that said, if you are doing this retrofit do it in the cleanest area with warm dry air you have available to you.

The replacement of a blown reticle is well within the capabilities of the untrained, as long as they are very finicky. Any optic repair outfit will be easily capable of installing the reticle cell on the back of the reticle cell holder with a very fine ring of epoxy such as the epoxy shown in the picture that can be purchased at most big box stores or Amazon. The key to the installation is to ensure the reticle is true to the old reticle grooves and it is concentric to the centerline, that and ensure you don’t get dust inside the optic.

For about 5% the value of your optic you can have the cell needed to repair your piece of history.

Reticle cell available for $149.99 shipped to CONUS and Canada





Additional items needed to complete this work, click the pic and it will take you to the link.

USMC MST-100 & MST-100M Glass etched reticle retrofitUSMC MST-100 & MST-100M Glass etched reticle retrofitUSMC MST-100 & MST-100M Glass etched reticle retrofitUSMC MST-100 & MST-100M Glass etched reticle retrofitUSMC MST-100 & MST-100M Glass etched reticle retrofitUSMC MST-100 & MST-100M Glass etched reticle retrofit

USMC MST-100 & MST-100M Glass etched reticle retrofit instruction

Step 1 – Thoroughly clean the exterior of the optic
It is important the optic is clean and free of dust, oils and debris that could potentially get inside the optic when doing the conversion. Use a tooth brush and thoroughly clean the exterior except the lenses, then use glass cleaner and thoroughly clean and blow off the body with compressed air.

Step 2 – Remove the rear eyepiece tube from the optic.
The eyepiece tube threads into the erector assembly with right hand thread. Unthread it and set it aside for reassembly later. You may need to mount a scope ring to the tube to give you leverage to unthread the tube.

Step 3 – Reticle cell and tension spring removal
The reticle cell will lift free from the turret body, it is not threaded, only slide fit. The reticle cell body has an index pin at the 12 o’clock position. The two tension leaf springs will lift free from the turret body at the same time.

Step 4 – Preparation for reticle installation
The old wire and bars need to be removed from the brass reticle cell as shown above, in preparation for glass etched cell installation. Clean the solder from the grooves as this is used as your alignment indicators when installing the new cell. Thoroughly clean the brass reticle cell holder with alcohol, clean the lens with eye glass cleaner and dry with a lens cloth. Both pieces must be cleaned thoroughly as it affects the final product.

Step 5 – Cell installation
The next steps must be done in the best lit area you have access to. Mix the Permatex 2 Ton epoxy on a clean, oil and grease free surface. With a fine instrument such as a tooth pick you are going to put a very, very fine bead of epoxy on the outer diameter of the top face of the brass reticle cell holder. You need very little to sufficiently retain the reticle, do not get carried away as the epoxy will bleed into the line of sight if too much is used.

The reticle is applied with the etched side facing the reticle cell holder, this keeps the reticle on the same plane as the original design. At this point the critical part comes, you must ensure the crosshair lines are centered on the grooves in the reticle cell holder and that the outer diameter is concentric to the cell. Once this is good, let the epoxy cure for at least 2 hours before touching anything.


Step 6 – Assembly
An item to check when the optic is apart is the friction between the erector cell pivot tube and the ball. This is the little tube you can see inside the erector assembly with the lens in it. This should be very free and not bound up. If it is you will get sticky elevation and windage adjustments. Do not get any dust, debris or fingerprints on the lens.

The next step in the process is to thoroughly clean again and inspect the reticle by shining a flashlight through it to ensure no dust or debris is on it. This will show up in the scope, any debris, clean well. Once it is clean, you will install the tension leaf springs in the reticle cell holder and feed the springs on the bottom and side of the erector cell until the reticle cell bottoms and is aligned with the 12 o’clock indexing pin.

If you live in a very humid climate you can purge the scope prior to installing the eyepiece tube. Stand both the main section of the scope on its objective end and the eyepiece end face down. Use the Bloxygen to purge the interior of the scope as the Oxygen will rise and flow out of the tubes. Once this is done, the rear tube can be flipped and gently threaded together.

Once assembled, look through the optic and confirm the reticle is clean and clear. If it is clean and good to go, unthread the eyepiece tube until the last thread. We put a very fine ring of Permatex Aviation Form a gasket #3 on the thread when it threads together to keep water out of the threaded joint and prevent rust in there.

Original USMC MST-100 Football Mildot reticle


True Mildot reticle

Congratulations, you are now back in business! Time to go to the range!

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