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Thread: Accurizing C308 CETME - Process over time

  1. #11
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    THE PLOT THICKENS.

    Went back to the range yesterday with some buddies. We messed around mostly with pistols at first, but the main event was 200 yards with precision rifles.
    Given that I still don't have a good way to attach a bipod, I was shooting off of a Nylon sack full of empty cartridge boxes and a rolled-up jacket.

    Initially I couldn't hit the broad side of the barn with either the wood or resin foregrip on any of the ammunition I brought. This was obviously quite upsetting. Except once the barrel started to get fouled, I noticed my 168gr ammunition went *right* back to shooting 2/3 MOA groups.

    The reason this is interesting is because my 4-5MOA groups with the bipod attached earlier were also done using a clean barrel.

    This weekend's 200 yard group using the very interesting and surprisingly hot new PMC X-TAC 168gr match-grade ammunition:

    aug 16 200y subMOA group.jpg

    I do feel like it's entirely practical to get the gun under 1/2MOA with an actual support of some kind, a little more practice, and maybe a well-developed handload. As of right now I'm confident calling this a reliable 2/3MOA gun though, which is... Surprising, to say the least. This is not what I expected when I paid $569.99 for a Century Arms rifle sight unseen on the internet.

  2. #12
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    Another amusing story.
    I got a hunch that maybe it was dumb to trust that CAI knows to use fresh mainsprings in their guns and ordered a fresh one for like $8 along with parts to do a blind pin pin paddle release installation for another ~12.

    I think it was a good call, as the new spring is *substantially* stiffer. I predict this will reduce the gun battering the buffer, thereby reducing felt recoil. PROBABLY also have a marked effect on how chewed-up the brass is and how far it gets spat out.

    I'm also predicting a similar effect in improving consistency to the 40-degree wedge due to the increased forward pressure on the bolt.

    Every day I thank my lucky stars that whichever chimpanzee at Century assembled this seemed to only goof-up on the stuff that I haven't been able to repair.
    Next stop? Installing the paddle mag release.

  3. #13
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    I did a buffer study recently using both my Cetme pistol (10.5" barrel) and a standard Cetme with an original barrel. I thought I'd posted it here but maybe not. None of my recoil springs are new, just used kit springs. In the test I made buffer posts with many different lengths to see how far back the carrier came after firing and measured the distances. I even used different locking piece angles in the tests.

    I'd have to look up the data again, but I can tell you that if you have a standard 50 degree LP and a good locking lever spring in your carrier, the back of your carrier will never touch the post on your buffer. Using 40 ~45 degree LP's reduces the movement of the carrier even more.

    if you want to do a quick/cheap test for yourself get a sheet of aluminum foil and roll it into a loosely packed ball about 1" in diameter and place it between the recoil spring guide and the buffer post in your stock. Reassemble the rifle and fire one shot. Remove the now flattened aluminum and measure it's thickness, then measure the length of your buffer post. If the foil is thicker than the length of the buffer post, the carrier was that far away from the post. The study was part of investigating the usefulness of heavy buffers. My findings told me that they are a waste of money.

    In short, instead of your rifle recoiling less, you may find that it now has more forward momentum due to the bolt closing faster than before (with the new spring). In my high speed videos you can see how much the rifle moves forward when the bolt slams home. One of the reasons I like this platform is that there are so many things I can learn from studying it. Keep doing what you're doing! It becomes addictive after awhile.
    Last edited by holescreek; 09-04-2020 at 10:20 PM.

  4. #14
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    Y'all might find this interesting!

    Recoil was SIGNIFICANTLY reduced with the new recoil spring, but forward velocity was ALSO increased to the point where it caused an entirely different problem. We had a range day yesterday, the gun no longer pitches brass into low orbit, now it just sends it into the next county, which is a small unexpected bonus, as it is probably no longer lethal all on its own. Recoil is now so sufficiently manageable that when I switched over from steel-cased Wolf Military Classic for warmup over to match-grade, the resulting slam-fires were hitting within about 3" of the original point of aim at 100 yards. So as it turns out, my gun can shoot itself as accurately as most people can shoot their AR15's.

    My theory after looking inside is that the friction of the steel cases on the steel magazine lips were decelerating the forward velocity of the bolt to the point where it wasn't slam-firing, but that the brass was too slick to fix this issue.

    SO.

    My thoughts are that the slamfires I'm getting now are being caused by a weakened firing pin safety spring in combination with the new higher forward bolt velocity. I'm not willing to trade off the new reduced recoil because frankly, it makes followup shots measurably quicker and I like that a lot. I've elected to replace the firing pin spring, and while I'm in there replace the firing pin with a titanium one and update the locking lever spring since that's probably clapped-out as well.

    Would probably be fine just refreshing the springs, but since titanium is so inert I figured it would be a nice maintenance upgrade as well.

  5. #15
    holescreek's Avatar
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    Slamfires will be cured by replacing the firing pin and spring with new HK parts. Titanium is an unnecessary expense. Launching brass further should be cured with a real HK locking lever spring unless there is damage on the lever or bolt head already.

  6. #16
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    Yeah, that's what I was thinking, thanks! I tentatively diagnosed the issue as a combination of firing pin mass and a worn firing pin spring yesterday at the range and just went back to shooting steel cased stuff for the safety factor. Confirmed with research this morning while pooping. Hooray for multitasking.

    Got the parts ordered before posting this because they were cheap enough and I figured that the $50 and a little wasted time was worth whatever amusement I can generate for anyone reading this and laughing at the stupid things I'm fumbling with to try to make a $560 Century Arms parts kit build perform like a real gun.
    The steel firing pin was $20 and the titanium one was like $26, and $6 is kinda not a big deal to me because it'll just be cool to have, let alone for the extra margin of safety. I like safety.

    I probably wasn't sufficiently clear when I was talking about the ejection profile, what I was trying to say was that it's actually *not* spitting it as far. As in, it's not going into space anymore, it's only flying into the next county.
    Instead of going about 60 feet in a graceful arcing pinwheel across the sky, they're flying about 30 feet a little lower and slower which I'm 100% into.

    I am still unable to push the grouping any tighter than about 0.67MOA. And yeah, that's super respectable by any reasonable standards, but MFFFF.
    It is so close to 1/2MOA that it's almost physically painful.
    Last edited by Redtail; 10-04-2020 at 10:33 AM.

  7. #17
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    I did a thing today.
    IMG_20201011_192038800.jpg
    H&K paddle mag release. Seems this has been a topic of interest on and off as of late.
    Parts were like $25 shipped.

    IMG_20201011_180709077.jpg
    I didn't get too technical with the positioning. I imagined there was a possibility I'd regret it, but I kinda like to shoot from the hip some times to validate the fact that I'm not totally a moron. Makes me feel alive. Basically just used the gripframe to to give me a rough location, eyeballed the fine position based on the dimensions of the lever and the position of the existing mag release, then dove in once it looked close enough. Started with a drill but quiockly realized the semiauto shelf doesn't actually extend all the way back into its own length.

    IMG_20201011_180719200.jpg
    This is what I mean by that. The actual path the bore takes doesn't exist totally inside the shelf block. This meant a drill bit was not an option. I ended up jerryrigging it with a Dremel end-mill bit that did a far better job than it had any right to, at the cost of taking about five full minutes to create the hole. It did nicely enough that I wouldn't use anything else in the future though. Start with the dimple, make the first millimeter or so of bore with the regular old drill bit, then go the entire rest of the way with the end mill bit chucked up in my drill press.

    IMG_20201011_192057660.jpg
    Obviously you have to be REALLY careful not to drill through the back side of the receiver otherwise you have an accidentally machinegun per the NFA. No matter how nonsense that is. ANyway. I used a carbide side-mill bit to remove the inside of the semiauto shelf. It did, again, WAY better of a job than it had any right to. However, the milling bits don't generate a metal dust like the regular old grinding stones do, they generate these needle sharp steel slivers that make your entire body feel like it's on fire. I will probably be regretting this tomorrow but for now I'm on cloud 9.

  8. #18
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    IMG_20201011_215358573.jpg

    Finished product. Will probably redo the weld at some point because it's kind of hot garbage, but it is what it is for now.
    Was ROUGH initially, but I snapped some vise-grips onto the lever and worked it 1,000 times and it's a while different animal now.

  9. #19
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    weld.jpg

    You guys are never gonna believe this, but I discovered that welding is easier when you can see what you're doing.
    FInally replaced my autodim hood after discovering that it was at my parents' house in Florida. Really didn't want to spend that $100 but here we are. It's no Reddit karma-farming TIG stitch weld, but I trust it a lot more than the last booger weld I had on there going in completely blind.
    Ground it down and it appears to still be solid and sturdy, wirewheeled it, accidentally took a chunk out of the receiver with my bench grinder removing the booger. Is what it is. Gives her some character.

    mag release is stiff and occasionally seems to bind a little bit, but it always works. I'm thinking I did this one a little imperfectly. Not like I mind that much. I'm just happy it worked out in the end.

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