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Old 12-06-2009, 11:16 PM
MonkTx

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CM-2 sight questions



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Hello all, I am new to the forum and stumbled across it on my quest to find a rifle to allow me to practice for Highpower competition. I got a couple of the old CM-2's from CDNN and love them but I have a couple of questions that weren't really answered in the stickies.

Has a consensus been reached on what diameter Slimline inserts fit the CM-2 rifles' front sights? Also, is their an adapter available to allow the use of Gehman irises on the rear sights?

Thanks, Monte.
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Old 12-07-2009, 07:27 AM
Kevin6q

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Not sure about the front inserts but for the rear I can address. The rear Izhmash sight uses a 9.0 mm hole and 1 mm thread pitch and most others are 9.5 mm by 1 mm pitch. The 9.5 X 1 is almost 3/8 X 24 and they will work but are not precisely interchangeable. The two threads are off a few thou. and don't start binding until quite a few threads have been inserted. For a sight, things work. The headache is the hole diameter.

I have made up some sight extensions to hold blinders from rotating or binding the sights during adjustment. The extension moves the aperture back about 3/4" and gives clearance for the blinder and gloved fingers. These are meant to be used for biathlon racing where time is important. I'm making a few up with an Izhmash thread on the male end and the 9.5 on the female end. They should be finished in about two weeks. Finding a 9.5 x 1 tap was very difficult and having one ground was going to be expensive. The parts are being cut on a CNC lathe so the threads and where they are cut can be manipulated.

I had these made to open up the availability of apertures available to Izhmash rifle owners. The rear apertures from the 7-4 are just okay and I wanted the ability to fit almost anything to the 7-4.
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Old 12-07-2009, 09:47 AM
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The rear iris issue has not been addressed by anyone other than Larsen Biathlon and perhaps Altius who have adjustable irises for the Izhmash. The 9.5X1m thread size is a custom size and seems very unlikely to find a tap to open the existing threads from 9.0 to 9.5 unless you have it custom made and at that point you might as well buy another rear sight. The centra economy rear sight is under $200 and has 9.5X1m thread size for the iris.
Although this makes those with machinist backgrounds reel and gnash their teeth the 18mm plastic inserts are oversized by 1mm which means that you only have to remove half that much in a swipe around the circumference with a nail file. Once the insert is placed in the front globe the edge of the outer perimeter is not visible (by a margin of about a tenth of an inch) and the clamp does not allow the insert to move so, all anguishing aside, any minor variation from perfection won't show up on your target. I know. I've proved it. Anything else is unfounded speculation.
Certainly if you were to sit and sand flat spot after flat spot you'd have a nasty looking and grossly undersized insert but by rotating the insert on its edge across the nail file and checking for fit with each revolution it only takes a few rotations to get it to fit.
BTW, the plastic inserts are the bomb! Great sight picture, just make sure you block the top of the globe off with tape to keep light from reaching the edge of the plastic insert.

Last edited by Charlotte; 12-07-2009 at 09:49 AM.
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Old 12-07-2009, 11:28 AM
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Reducing Outside Diameter of Slimline Insert

Quote:
Originally Posted by MonkTx View Post
... I got a couple of the old CM-2's from CDNN ....

Has a consensus been reached on what diameter Slimline inserts fit the CM-2 rifles' front sights...?
The Stickies:
The 17mm diameter inserts are the KRAG. - 17mm Slimline Inserts work in CM-2
CM-2 Front Sight Inserts - CM-2 ... insert size of 16.85mm
refer to new contemporary CM-2's being currently manufactured and sold.
This information may not apply to vintage/surplussed VOSTOK CM-2's.

Quote:
Originally Posted by Charlotte View Post
Although this makes those with machinist backgrounds reel and gnash their teeth
the 18mm plastic inserts are oversized by 1mm which means that you only have to
remove half that much in a swipe around the circumference with a nail file....
If you're more of a pragmatist than machinist ... Charlotte has advanced a solution that works in the field.
Her method involves a simple tool and finesse - a fingernail file and hobbyist skills.

For the machinist types (that be me ), I'm proposing a more mechanized method.
Using a jewelers file, an electric drill (poor man's lathe) and machine screw with nut.
Rig a a simple mandrel using a machine screw and nut.
The screw diameter should be slightly smaller than the aperture size of the insert.

Sandwich the insert between the screw head and nut.
Synthetic washers maybe placed on both sides of the insert to prevent marring.
The complete assembly should vaguely reassemble a cut-off wheel apparatus.

Then mount the assembly in the chuck of an electric hand drill.
This is done in preparation to uniformly file the circumference of the insert.
A vise would be a handy way of mounting the drill in a steady position for this procedure.

Power the drill to rotate at high rpm.
Then very, very lightly touch the file to the outside diameter (OD) of the insert while it is spinning in the drill.
Stop the drill and measure the OD of the insert to guage the amount of material that has been removed.
Use a micrometer or caliper to measure the outside diameter frequently until the proper diameter is reached.

Quote:
Originally Posted by Charlotte View Post
... the 18mm plastic inserts are oversized by 1mm which means
that you only have to remove half that much in a swipe around the circumference ....
I'd stop at an indicated 17mm to check the fit of the insert (still mounted on the mandrel) within the globe.
If the fit is too tight, re-chuck the mandrel and further remove a minute amount of stock from the OD.
Then, check the fit again and again until you are satisfied.
Remember: "measure twice" ... "before cutting once."

This is a semi-precision process that may yield acceptable results.
(I'm cringing a little as I write.
But, my teeth are not gnashing.
However, I am wringing my hands.)
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Old 12-07-2009, 12:46 PM
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Modifying Slimline Inserts

Charlotte,
I've Stickied your post:
Reducing Outside Diameter of Slimline Insert - by Charlotte
I had to give it a title ... hope it's OK with you.
Let me know if you would prefer your own title - aom22.
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Old 12-07-2009, 09:22 PM
MonkTx

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Quote:
Originally Posted by sport507 View Post
Do you have a Vostok CM-2 or an Izhmash? On the Vostok there can be two different sizes?

Sport


I got two of the Vostoks.
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Old 12-07-2009, 10:30 PM
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I don't know if this will fit the CM-2 globe but this is the ticket for blocking the light from hitting the edge of your plastic insert. My rifle sports a couple of layers of camo tape that works as well but doesn't look as good. I'd guess it will be a close fit and it's pretty cheap so I could still use it on my anschutz front sight.
http://www.championshooters.com/stor...cat=329&page=1

For anyone too frustrated with finding an adapter for an adjustable rear iris here is an alternative rear sight that has 9.5X1m iris threads. http://www.champchoice.com/prod-CENT...IGHT-2995.aspx
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