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Thread: 375-6.8 FUW

  1. #1
    MGO Member Moleman-'s Avatar
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    375-6.8 FUW

    Usually make a post deer season gun related project every year. For the 2018-2019 deer season it was an AR15 upper in .375" based off of the 6.8SPC case. Sarco had cheap barrel blanks on sale and Starline makes a 6.8SPC basic case that just needs rough cut to length, resized and a final trim/debur. You can also make cases from 6.8spc brass. I had some once fired 6.8spc cases and the first 50 or so were made from once fired Remingtons. Figured out a cartridge and chamber spec, then made a chamber reamer from O1 drill rod and ordered the blanks from Sarco and a 6.8SPC bolt carrier group from Brownells.
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    MGO Member Moleman-'s Avatar
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    A 38-55 Win die set will not quite size the cases enough to chamber and have good neck tension. Perhaps if they were shortened slightly, but as is the cases needed sized more. Since the barrel was getting cut down to 16"-18" the extra barrel length was threaded 7/8"-14TPI with a knurled band, bored to the same ID as the neck area of the 38-55 die set and then the taper added up to where the neck would start in the die before parting the "die" from the barrel. The barrel was then finished profiled and the muzzle threaded and eventually parkerized. The main bullets I've been shooting through it have been Makers 175gr solid copper (2350 fps), Sierra 200gr SPFN (2250 fps), Speer 235gr SP (1980 fps). The Makers bullet at 2350fps (bullet on right) opens up to about 1.2" when shot into water. If I remember correctly Maker says the threshold is about 1400fps for expansion. Wanting to test realistically if a max range shot would expand or not a bullet was loaded to 1738 fps which works out to 270yds if shot at a 2350fps muzzle velocity. The recovered bullet when shot into water measured roughly 1.015" (bullet on left).
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  3. #3
    I am a Forum User
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    As always, great information.
    Very interesting and understandable.

    Joe

  4. #4
    MGO Member Ol` Joe's Avatar
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    Interesting project.
    How much shoulder do you have? I’m wondering about headspace issues you’ve had if any.
    This one might be a better choice then the 350 you did for deer here in the lower if proper bullets are readily available
    "Saepe errans, numquam dubitans --Frequently in error, never in doubt".

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  5. #5
    MGO Member Moleman-'s Avatar
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    Thanks guys, just something to keep me busy as the snow blows across the yard.

    Ol' Joe, The cartridge ends up straight walled with no shoulder and headspaces on the case mouth. No headspace issues. You do have to keep an eye on the cases better than you would if it were a rimmed/belted/bottleneck case. The big one is, NO ROLL CRIMPS or the very short taper crimps that act like roll crimps that come labeled by some die makers as a taper crimp. A heavy roll crimp or modified roll crimp can make the case mouth dia small enough to slip into the freebore which will cause excessive headspace and increased pressures as the case mouth cannot expand anywhere when the cartridge is fired, forcing the bullet behind the crimp to swage down to fit through the crimped area. The 357AR was a few years before the regulations changed here in the limited zone so I hunted with it in a handgun before the rule change and was my main rifle for the first year the DNR allowed straight walled cartridges. Made the freebore longer than usual on it so the bullets could be seated with the cannelure fully exposed and use a true taper crimp into the shank of the bullet. Don't know why winchester went with 9mm other than most 9mm's don't have a cannelure and they had to change it enough to not be bringing out a +P rimless version of a 357 Remington Maximum. There have been many instances of issues with 350L and headspace many caused by improper chambers or excessive crimps or a combination of both. Going on 9 years with the 357AR and have yet to have a case separate. There are several off the shelf bullets that will work for the 375FUW. The easiest is the Sierra 200gr SPFN 2900. Any bullet meant for 38-55 or 375 Win should work.

    Season after the rule change I used a 44x1.8" in a 1891 Mauser, followed by an AR10. Then an ar15 in 44x1.6" off of 308/30-06 brass(same case capacity as a 445SM) using standard bolt and extension parts, then finally a 800 series bolt/extension became available and was able to shoehorn a high pressure 44x1.8" into a AR15. Also have a 40x1.8" that was this past winters project. Bottom row of cartridges L-R 357AR, 375FUW, 40x1.8",44x1.6", 44x1.8" and 450BM for comparison. Top row is 40x1.8", Barnes 195 on the left and 200gr XTP on the right.
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    Last edited by Moleman-; 08-25-2020 at 12:46 PM.

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