Gunsmithing E. Cohen, Dakota Scimitar 260 Medium Tac Build

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  • Mar 14, 2010
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    Sturgis, S. Dakota
    www.longriflesinc.com
    This is probably one of the rarest/most unique builds we will ever do. The Dakota Scimitar was a receiver I helped design with Glen Harris (Defiance Machine), and Cyle Miller (Emerging Design Group) in 2004. It was a project done for the Royal Jordanian Anti Terrorism unit. Most likely the 71st Battalion forces, but I don't know for sure.

    It lead to my department being tasked with building a pair of presentation grade versions for President G.W. Bush and Vice President R.B. Cheney given by His Majesty, King Abdullah III.

    A little over a year ago Mr. Cohen contacted me about building a one of a kind rifle chambered in 260 Remington for his collection. The Scimitar was my immediate choice because there are very, very, very few in public circulation.

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    It's a controlled round feed, non rotating extractor type receiver featuring a cone breech. Nothing really special other than that the breech has NO extractor cut in it. It's a true cone like you'd find on a single shot custom action. This was done to provide rock solid feeding at any orientation. The non rotating extractor aids in this because it physically latches/retains the cartridge rim.

    The trigger unit is all Dakota Arms model 76. Open architecture with a crisp single stage 2.5lb trigger pull. Bar stock EDM machined trigger components are fully heat treated so that they will last for generations. No castings or case hardened pieces exist anywhere on the action.

    The three position safety with the fire control block means it can never go bang regardless if the trigger breaks in half or not. It also has a very clever anti rotation feature on the shroud so that it can never accidentally rotate when out of battery.

    The magazine retention system is also unique in that its attached to the action. The magazine (Sako TRG) is physically attached to the receiver during lockup. This means the trigger bow can be set to run perfectly flush with the stock with no fear of the magazine creating feeding issues due to insufficient case overlap with the bolt head. This was done so that the unit could run in a variety of stock configurations while still maintaining a fit and finish consistent with other Dakota rifles.

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    The non rotating extractor and shroud designs means we have an effective breech block to mitigate gasses reaching the shooter in the event of a cartridge failure.

    The mechanical ejection setup just ices the cake. If the shooter needs to go full throttle he can do so and cases will resemble an NFL kickoff as they catapult out of the action. If a shooter needs to minimize his signature he can fire a shot, pull the fired case manually, and put it away to preserve a sterile hide upon exit.

    Sounds kind of Hollywood I know, but this was a heavily funded/committed effort with the idea of building the ultimate bolt action precision rifle for killing men.

    The port and tang design makes for a robust receiver that will support a fully floated heavy barrel with no problems and no barrel pad in the channel. This was done to make Level 3 echelon service simple and fast.

    Bolts are one piece design. No soldered handles on this bugger and it had more primary extraction than any other receiver design of its time.

    All in all a very well thought out and robust unit designed for any environment/encounter.

    So it begins. Here's Garrett working on getting the barrel fitted up. We proofed out our barrel tennon program on a piece of stock material before going after the 8.5ROT fluted Kreiger that were fitting it to. 260 Remington Tactical is the cartridge. Just a 260 setup to run hot/cold/dirty/wet with heavy bullets in a magazine length appropriate for the Sako TRG magazine.

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    We'll update this as we move along further with the build. Hope to have the stock inletted this afternoon/evening.

    Won't be long now Mr. C. !



     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Damn...

    If someone were selling those actions, they could make a mint...
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Wow Chad; thanks so much.

    Beautiful piece; so worth the wait!

    I know we need to talk about stock and paint colors; can do by phone or pm.

    Don't forget the second barrel; will make future replacement easier. Also, brakes for both. Want to spot my hits AND coddle my broken clavicle.

    Will get my NF NXS 5.5-22X50 out to you Wednesday (am on call and won't be home until tomorrow night).

    Again, my thanks.

     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    I can get them still.

    Dakota Scimitar 308 version

    $2500.00 Includes one magazine and the floor metal.

    Dakota Scimitar 338 Lapua Magnum version (looks the same, just longer and uses a .750" bolt

    $3100.00

    Be glad to take orders. . .
    smile.gif
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: my human host</div><div class="ubbcode-body">Sweet. What are the specs on the tenon. The base shouldn't come loose!!! </div></div>

    1-1/16X18 pitch 1.250" nominal tennon length
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Mr. "C",

    Were onto getting the stock run. I've got to dig this program out of the rafters! Last one I did was in 2005!

    Nothing like a blast from the past.
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    Here's the parts laid out in preparation for me to inlet. I've got some puter work to do now.

    More to come. Were gonna document the snot out of this one.

    C.

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    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    I wrote this initial inlet with software thats now about 10 years old. Had to wash it through a couple "filters" to get it cleaned up and imported into my latest stuff.

    This was one of the first surfacing programs I ever wrote for gun work. Pretty neat seeing things evolve over time.

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    Here's the line geometry straight from the computer. Much cleaner now and ready for me to stitch in the barrel contour. Back then we did that after we bedded them. Just a different kind of process Glen had us doing. Now I just run it all at the same time as its easier I think.

    For those unfamiliar with surfacing and what its for/does, its a very powerful tool for inletting a stock as it allows you to mirror the footprint of the receiver very precisely. This means all the features of the action (radius, length, tang taper geometry) gets matched in the stock. Bedding compound fills a predetermined gap (in this case .05") and is uniform in film thickness at every contact point. This delivers a really nice presentation when its all done and finish machined.

    Just another way to peel skin off a kitty cat. (as a general rule, I'm not a cat person btw)
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    And for how the code looks for those unfamiliar with CNC language. Just numbers and letters for the most part. This is only the first few lines. This inlet uses probably around 30,000 lines of code as the surfacing and helical hole boring is quite "busy" and uses up a lot of memory resources.

    You may notice that it has 2024 AL as the material. That's just a default text setting that I'm too lazy to go change. The software has some parameters you can set for speeds/feeds for various materials. 2024 is the default and I've just never taken the time to go set it up properly. The stock is a Manner's composite. There's no AL in this bugger.

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    Helical boring is what I use instead of a drill for poking the holes where the pillars go. This is advantageous for us as it minimizes the chipout to almost nothing when the endmill exits out the bottom of the stock. Also ensures the holes are laser straight and don't wander due to hard spots in the filler material.

    I'll patch in the barrel inletting portion and get on with it.
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    What is the major difference in that action as compared to a 76 action? The stiffer reciever body with the shroud? It looks stout.
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Hey man, great talking with you. Don't do too much runnin 'n gunnin these days (too old with too many broken parts) but I can still drill small holes AND appreciate a great work of art.

    Can't tell you how much I appreciate this. Say Hi to Callie and the "new hire."

    Boy, I can't wait to shoot this.

    Later, E
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: steve123</div><div class="ubbcode-body">Nice build Speeddog!

    Mind if I come down and see it this winter? </div></div>

    Sure. It's always fun to share. Give me a shout.
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Onto machining the stock inlet:

    I ran this last night, but it was getting late so I sluffed off uploading the pics.

    First is finding out where the trigger locates in relation to the rear tang. This action has the root of the shoe 1.1" from the rear of the tang.

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    Now we position the action on the stock based on where we want the trigger location to end up.

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    Now its the usual ritual of getting the setup plate glued to the stock's showline. First I block sand the top of the stock to get it nice and flat. This way I don't have to worry about the bar popping loose when I clamp it up in the mill.

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    Once it's set at the appropriate height is time to make chips.

    First tool has a special cutter geometry to break the internal edges without grenading the gel coat. It's a R/H cut, L/H flute endmill. This pushes the chip down instead of evacuating it out. Easier for the tool to load up, but it makes the edge razor crisp because the material isn't being torn away from the bulk of the stock. Instead it's sheared against itself, kind of like a pair of scissors.

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    Next is punching out the pocket feature for the recoil lug. Nothing special, just a long winded 1/4" EM for this.

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    I write a rough surfacing tool path next. A 3/4" lollypop EM comes in and blasts out the bulk of the material, leaving .100" behind for the finishing tool. Forgot to snap a pic on that part so you'll just have to imagine it.

    Here's the finishing em. A carbide 3/16 ball is used to surface the inlet on a .03" stepover.

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    That about wraps it up. Now its just boring the pillar pockets and surfacing the barrel channel.
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    As we can see, the ass end of the receiver is kind of hanging out a bit from the stock. I anticipated this as the Scimitar trigger/tang/screw geometry isn't the same as more conventional receivers. Not to worry, we'll do a "butt implant" on the stock and get its rear end covered up when we go to bed it. The important thing here is trigger location and ensuring the bolt handle is positioned in the right spot to ensure easy manipulation.

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    Running the barrel channel now. Then its out of the machine and off to the bedding table.

    More to come later.

    C.
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Every time I see one of these builds, it makes me want to send you another pile of parts....

    AWESOME STUFF Longrifles crew!
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: steve123</div><div class="ubbcode-body">Nice build Speeddog!

    Mind if I come down and see it this winter? </div></div>
    Very nice build! I would also like to check it out!
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    That action oughtta be called a Winchestington.

    Just add a floating bolthead and barrel nut, and it could be a Savchestington.

    Nice looking work!
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Speeddog</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: steve123</div><div class="ubbcode-body">Nice build Speeddog!

    Mind if I come down and see it this winter? </div></div>

    Sure. It's always fun to share. Give me a shout. </div></div>

    Looking forward to it!
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Doc,

    Garrett bedded her up last night. I get in before him (banker's hours. . .) so I went ahead an popped it out.

    Were good to go. Setting up the mill for 2ndaries right now.

    More pics to come. I've got to go back into the 2ndary programming and tune it up a bit. Last time I did one of these was 2004-ish so we've come a long ways since then.

    This was a sort of two part evolution to get this thing bedded yesterday. As the earlier photos showed we had a problem with the tang sticking out in space. To resolve it what I did was scuff the stock, add some pins to give glass something to latch onto, then build up the rear portion. Once cured we gave it a rough shape to blend to the rest of the stock. This provided the needed bodywork and gave us a dam to prevent bedding from running out the back of the action inlet as we bedded it.

    Came out really well. There's pin holes and surface inclusions, but we'll save that for the final bodywork portion. A little milled fiberglass and resin will patch those up nice.

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    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    <div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: C. Dixon</div><div class="ubbcode-body">I can get them still.

    Dakota Scimitar 308 version

    $2500.00 Includes one magazine and the floor metal.

    Dakota Scimitar 338 Lapua Magnum version (looks the same, just longer and uses a .750" bolt

    $3100.00

    Be glad to take orders. . .
    smile.gif


    </div></div>

    Forgive my ignorance but is that for the action, barrel, stock and trigger...all minus optics? B/C that would be sooo reasonable....

    Thanks for sharing your work, I enjoy your posts greatly.
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    It buys you:

    The action
    The recoil lug
    The trigger (with trigger job and assembled)
    The floor metal
    The scope base
    One magazine

    Not bad when you consider the alternative.

    Rem clone: 800-1250
    Recoil lug: $50
    Trigger: $100-$225
    DBM floor metal: $100-$300
    1913 Base: $50-$200
    Magazine: 100+
    M700 3pos safety conversion: $150-$200
    Control round feed-Not available in a 700
    Non rotating extractor-Not available in a 700


    Total: $1350-$2375
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Top side inlets are almost done.

    Just a bolt handle and the cocking piece notch remain.

    Here's the receiver with the magazine guides installed. Just had to check the fit in the stock. Rather unconventional, but very effective in positioning the magazine.

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    Bedding:

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    Bodyline along the tang now dramatically changed so that everything flows together again. Added quite a bit of beef to the thumbhole bridge.

    Before:

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    After:
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    C.
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    I'm waiting with bated breath. When I saw my build reappear I had hoped there would be progress and new pictures.

    Obviously I'm really looking forward to receiving this rifle; it's been a focus of my attention ever since Chad and I worked out details for the build
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Back at it!

    Doc I'm loading the program to profile the magazine girdle right now. Using a chunk of 4140 Prehard that I've had kicking around for about 10 years. Started to make myself a 10-22 receiver with it years ago and ended up scrapping it cause I decided to make some changes in the design. My son has the gun now.

    Anyways, here we go!

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    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build


    First I profile the general shape with a "corn cob" rougher. These kinds of tools just bulldoze there way through material to get the unwanted stuff out of the way and shape the part for the finishing tool later.

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    Next is the finish profiling. Normal rules dictate using a larger diameter tool however because of the tight inside radius along the nose feature I can't. So, it's slow and steady with a 5 flute, high helix, coated carbide endmill. leaves a great finish but you have to watch your feedrates. There's .700" worth of stock contacting the tool. Go too aggressive and it'll snap that bugger quick when it runs on the backside of the part and enters into the radius. You go from very little tool contact to almost 25% of its diameter very quickly. That puts a great deal of load on the cutting surfaces and it's easy to break it. I'm finishing this in 2 passes with a .01" stepover.

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    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Now we'll work on setting the draft angle of the part. Most stocks seem to run in the 2.5-3.5 degree range for this. Since were making this one from scratch it only seemed right to go measure the stock and see exactly where its at.

    For this build we chose a Manners T5A. A good choice. I like Tom's stuff. Laying the stock on the surface plate and "dinging it" showed a 3.5* draft angle.

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    Perfect. Now its the simple matter of replicating this in the computer. A 3.5* angle works out to a taper of .061 per inch. (.0612" for you math Nazi's)
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    Now its just a matter of setting up the part to match this. We could just surface machine it, but that would take a long time to do. Instead I'll just rotate the part in the vice and blaze through it with the rougher, then go back and clean it up with a shell mill.

    More to come. . .
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Setup:

    "Back in the day" (does being 41 years old now qualify me to use this term?)
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    we'd use a sine bar for a job like this. Problem is, sine bars are a pain in the ass and you lose some rigidity because you have a contraption between to your mill vice and your part.

    So, instead I've decided to just "dink" with the positioning till the numbers match up. The "distance to go" callout on the positioning allows me to zero it anywhere/anytime I want very easily. Set it to zero and then zero out the indicator. From here its just a matter of moving the part in the vice until the numbers match. Move one inch in X and it should show a .061" change on the indicator. We check our work by repeating, but this time watching the Z axis on the position screen and reseting the indicator to zero at each location. So long as were within a few .0001's were in good shape. In this case it looks like were within a tenth. I don't really believe that, but it's certainly close enough for a dumb floor metal going into a stock that gets sanded on and shot with paint/ceracoat later.

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    Now its onto putting this biche on a diet! Time to eat some steel!

    This build has been draggin on a bit. Normally we'd of been done by now. Work like this demands my attention which can be hard to do during the week with phones ringing and walkins. Saturday's are typically pretty quiet though so it makes it easier.

     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Hey Chad;

    Nice to see this underway again. Greatly appreciate it.

    This'll be one to cherish: it's all kinds of fun getting a work of art AND getting to know the guy doing it.

    Doc
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Got pulled off of it for a little bit.

    Kalli's car had a fuel leak. Ended up pulling the tank. 09 Cobalt. Supply line was pissing right were it inserts into the sending unit.

    $390 bucks for a new fuel pump assy.

    I don't effin think so!

    Ended up making a fitting from 300SS, tapping the hole, splicing a piece of HP fuel line, two clamps later, and she's off to the races again.

    Ok, back to work!

    C.
     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Kalli's a moped kinda girl.
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    I bought here a 1981 Honda Express this summer and if its nice out you can't get her off the damn thing.

    This winter we'll be tearing it down and restoring it. If she lets me I'm going to build a water cooled big bore cylinder for it, bump the compression a bit, find a tuned pipe I can "Frankenpipe" onto it, and maybe put a 5-10hp shot of silly gas to boot. I've got a 19mm Delordo carb for it already.

    What a fun way to end up in the ER right??

    Back to programming. . .

     
    Re: E. Cohen, Dakota Scimitar 260 Medium Tac Build

    Oddly shaped pieces like this are oftentimes difficult to fixture by conventional means. it's so easy to distort the thin walls and have a part come out ruined. A Kurt vice is more than capable of generating several thousand pounds of clamping pressure.

    So, the solution here is to make a set of custom jaws that'll captivate the entire frame of the floor metal.

    Programming is pretty simple. Just take the existing drawing and create a pocketing tool path for the whole thing. Then it just plops down and gets squished. This way I can buzz off the unwanted stuff and have a finished part.

    I'm using a 3F, 1/4" OD high helix endmill. I'm being a bit of a puss and stepping down .1625" per pass to a final depth of -.650". Reasons are its pretty easy to snap a 1/4" tool because if its cutting deep its that much more difficult to evacuate the chips. If you load up the tool its all over and at $35 bucks an endmill its a great way to throw money into the scrap bin. So, nice and easy is the word for Monday morning.

    I could have roughed it out with a bigger tool, but then I gotta get more involved with the programming. I'd rather sit and drink coffee while it runs.
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