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Thread: Accurizing C308 CETME - Process over time

  1. #1
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    Accurizing C308 CETME - Process over time

    So, Holescreek probably has the market cornered on this stuff, but I figured I'd throw my two cents in just for fun.
    So over the years I've been gradually working on minor low- or no-cost improvements to my C308 to improve the overall fit and finish, as well as the performance of the firearm. Most of this stuff could be done with very basic hand tools, sandpaper, files, a wad of steel wool on the end of a pair of foreceps, or occasionally something a little more ambitious like a fine grinding stone on a Dremel.

    Mostly this was an experiment to see if my understanding of what accuracy comes from gathered over the years translated into practical results.

    The answer is a sweaty, wide-eyed, voice-cracking "YES".
    IMG_20200702_080608328.jpg
    IMG_20200715_214023905.jpg
    These two were taken at 100 yards on different trips using the same 168gr PPU "match grade" HPBT ammo. This gun is a sow. Both groups were taken with a *filthy* trunnion, barrel, chamber flutes, and bolt face, using ammo that is... Let's be honest, middling at best. It seems to like it better than the 175gr Federal stuff I was doing so well with earlier.

    To clarify, no major structural or mechanical alterations have been made to this rifle. THe gun has its original pencil barrel, non free-floated, military surplus trunnion installed by the chimpanzees at Century Arms, and bolt gap on the low side of "In spec" after about 1,000 rounds.

    Here's the short list so far.

    -Honed inside of cocking tube with coarse steel wool on a set of hook nose forceps for smoother, quieter action. Mostly just for feel.
    -Honed inside of receiver and used sandpaper to "lower" spots beneath scope rail welds where the bolt was rubbing to reduce shot to shot variance from friction.
    -Ground down the rear of the trunnion to ride flush with the receiver rails in order to reduce or eliminate roller predeployment for more consistent lockup and less wear on structural parts.
    -Replaced trigger with German unit because I like the way the sears work better for the purpose of polishing, adjusting, and cleaning.
    -Replaced locking wedge with Holescreek's 40 degree wedge.
    -Used Dremel wire wheel to "polish" the inside of the trunnion to increase consistency of roller engagement and release. Rechecked bolt gap. Satisfied with lack of change on the feeler gauge.
    -Homemade cheek riser for ergonomics and to reduce parallax inconsistencies.


    I recently had the notion that I wanted to have an actual bipod for additional stability, as well as a degree of flexibility in the field or in an emergency situation. The gun has been more reliable on a wider variety of different types of ammunition than my well kept and otherwise totally stock S&W M&P15, if you're wondering why this oddball firearm enters into the equation at all when "emergency" comes into question.
    I opted for the custom ultralight M-lok foregrip from HKParts as it was advertised as compatible with the CETME.
    That ended up being a load of crap. The grip is so short that after the first two shots the whole thing walks forward out of the trunnion and dangles loose from the barrel, requires substantial grinding to fit over the spacer rings behind the front sight post, and the M-lok holes are out of spec and required significant filing to fit any of my accessories. Given that HKParts deleted my review twice for the item, I'm not sure I'll be going back to them in the future unless completely necessary.

    So given that there really aren't any other options for an aluminum M-lok foregrip beside the Midwest Industries unit that is advertised as incompatible with the CETME without significant modification for exactly the same reason I had trouble with this one, I ended up deciding to make it work in my own way.

    So I designed a spacer.
    It had to accomplish the task of securely pressing the grip back into the trunnion, reducing any directional play it possibly could, and putting as much pressure on the front sight block and cocking tube as possible in order to avoid introducing any new and unexpected harmonics to the whippy little pencil barrel. I ended up doing all this *and* making it significantly easier to repeatably install the thing after removal for inspection.

    IMG_20200718_211041282.jpg
    IMG_20200718_213404372.jpg
    IMG_20200718_213540895.jpg

    The curved surface is designed to tension the mounting screw evenly without forcing it downward or putting too much stress on any one part of the threads. The little tapered "wings" force the back of the grip into the trunnion during insertion and provide enough front to back play with the locking bolt removed to uninstall the grip without any more force than I can apply with my bare hands.

    The whole thing is *rigid* now. Planning to test it out today. I feel like removing the extra weirdness and play from the wooden foregrip will at the very least not hurt, but probably enable a slightly greater degree of accuracy. I am also hoping the airy ventilated aluminum grip will vastly improve barrel cooling and significantly increase the effective accurate rate of fire without having to wait for the barrel to cool as long between shots before it heats up enough for my grouping to start spreading out.

    IMG_20200615_104913204.jpg

    I love the look of the wood. At some point I might make a nice, blond-colored wooden handgrip section that fits nice and close over the back end of that aluminum grip to preserve that aesthetic, but for the moment, my priority is balancing performance and weight. It's just convenient that the wooden stock with a homemade cheek riser weighed less than the Magpul PRS since it allowed me to keep the pretty wood mostly intact.

    The accuracy is getting close to where I'd consider scientific testing of the different delay wedges likely to yield meaningful results. I still need to get a better feel for how barrel heat affects grouping size.

  2. #2
    holescreek's Avatar
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    It will be even more interesting to see if wedging the hand guard up against the front sight post has any effect on accuracy. You were blessed with a good barrel from the factory, did you rework the HK trigger too?

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    I did. Wirewheeled the sear surfaces and then used a piece of carbide paper wrapped around a chunk of plate steel as a rigid backer. Used some steel epoxy as an internal overtravel stop to reduce creep rather than cutting the nose off of the sear because I'm not into accidentally machinegun. Polished everything, cleaned it good, then added a few judicious drops of Lucas Extreme Duty in specific locations and holy shit, what a difference.

    The front sight isn't freefloated so I feel like there's minimal chance of barrel deflection, given that it should be pulling forward slightly on both the cocking tube and the barrel due to no uneven loading. But! We'll see this afternoon. If it ever stops raining, that is.

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    Holes:
    If you're wondering still about how the gun shoots with the foregrip braced against the front sight?
    If I were being generous I'd say "Compared to the batch of surplus rifles that recently came in from Ethiopia, it's a remarkably good shooter."

    The firearm is sentient, it has a personality, and it has a soul. And it's not evil. It's worse.

    It's pedantic.

    Infuriating as it is, this is the learning process, and that's exciting in its own way.
    And today I learned that just because the foregrip isn't moving, doesn't mean it's not affecting accuracy negatively. My next test is to try making a different spacer to attach to the bottom of the locking collar on the foregrip to sort of stop the thing up on the little steel tab beneath the barrel directly behind the sight post. Since the gun went immediately from "Can usually hit a milk jug" to "Reliable MOA-ish" the moment I ripped the offending component off and replaced it with the wood foregrip.

    I JUST WANT MY BIPOOOOOD

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    Yeah, that happens. Barrel mechanics don't change much with the personality of the rifle. Long and skinny is light weight but too easily influenced by outside forces and heat. When you get one that shoots good you can't change anything that touches the barrel without affecting it's performance. For example, Mosin Nagants were test fired for accuracy at the factory with their bayonets attached. Remove the bayonet and the barrels wobble around.

    I've chased accuracy in several types of rifles for many years, building several variations of each searching for "the thing" that made them accurate. It's fun when you figure it out but frustrating until then. Spoiler alert, you will probably have issues from bipod use unless you figure out how to attach it without putting additional pressure on the barrel when it's resting on it. You need to match the pressure from the rest you're currently using for a decent result. You'll figure it out, at least you're starting with a rifle that you know is capable of shooting MOA.

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    Senior Veteran Has Been's Avatar
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    It's been my experience that forend fit has the biggest effect on accuracy, with trigger let off being a close second.
    Cetmes are like Garands in that a properly fitted forend should have a little wiggle to it, otherwise as the barrel heats up group size will open up.
    I also found mine to be noticeably more accurate/consistent when wearing a G3 slim forend. I suppose it has all to do with the way it attaches up front with the cross pin that bears against the bottom of the CT and let's the barrel "float" more freely than a Cetme forend that clamps directly to the barrel.
    When shooting at 100yds from a bench rest starting out just in front of the mag I saw no change in group size/poi shift or verticle stringing issues as the rest was moved to the middle of the forend then to very end with the G3 forend. Groups measured a fairly consistent 1.25 m.o.a. with Tula ammo, 5 shot.
    The same experiment with a Cetme forend verticle stringing was an issue right off the bat. With the rest just in front of the mag groups averaged 1.5 x 3", the last group opened up to 4.5" with the rest at the end of the forend.

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    Oh, that's interesting. That's *very* interesting.

    Because I "magically" became a better shot the moment the retention threads on the receiving end of my CETME's foregrip stopped holding the screw perfectly and skipping a thread every time I overtightened it.

    This is *perfectly* consistent what what you're saying, Hasbeen. I have a G3 foregrip to throw on there, I've just been leaving it off because I was more interested in the cooling and the lighter weight of the wood one and later the significantly better cooling of the aluminum one.

    Come to think of it, I could use the spacer I just made in combination with some slight retooling of the front end of the aluminum foregrip to achieve the same effect. In for a penny, in for a pound. Time to go get the dremel!
    Last edited by Redtail; 07-20-2020 at 03:10 PM.

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    Quote Originally Posted by holescreek View Post
    It's fun when you figure it out but frustrating until then. Spoiler alert, you will probably have issues from bipod use unless you figure out how to attach it without putting additional pressure on the barrel when it's resting on it. You need to match the pressure from the rest you're currently using for a decent result. You'll figure it out, at least you're starting with a rifle that you know is capable of shooting MOA.
    This thing is like a puzzle for me.The difference between satisfying and frustrating is knowing that there is potentially some form of solution to be found.

    All this gives me an idea about how to design a second spacer. I just have to find another scrap chunk of 1/4" steel in my garage.

  9. #9
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    Quote Originally Posted by Redtail View Post
    This thing is like a puzzle for me.The difference between satisfying and frustrating is knowing that there is potentially some form of solution to be found.

    All this gives me an idea about how to design a second spacer. I just have to find another scrap chunk of 1/4" steel in my garage.
    Shove a foam earplug in the gap and try it out.

  10. #10
    Senior Veteran slim's Avatar
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    Nothing technical to add, but those are impressive groupings.

    I haven't even had my Cetme out of the safe in a few years. Life got in the way for a time, blah, blah, blah

    This inspires me to get it out, dust it off and start fine tuning mine. Those groups are nice but my "battle trigger" needs alot of work

    Thanks OP, and Holes
    They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety.Ben Franklin.. 1775

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