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Thread: FTF Open Bolt Reweld

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    FTF Open Bolt Reweld

    First off, this is the most informative forum I have stumbled across, so it goes without saying that after scratching my head for a while at my current problem, I'd post here and look for assistance.

    I have completed an open bolt postie using a repair section and a parts kit. I am also using an SMG feed ramp. Though I am not a welder by trade, I am confident in my TIG welding abilities. After assembling for this first time, I noticed the bolt seemed to be rubbing significantly on the top cover. I reviewed the maintenance guide and using feeler gauges, got the top cover back into spec. With that out of the way, it seemed the bolt would go home and close with plenty of force. Inserting the magazine, not so much. I adjusted the magazine catch slightly which was a significant improvement. Using dummy rounds, they seated just fine. I inserted a magazine with live ammo which would chamber but not fire. Light primer strikes, if any at all were occurring.

    Leaving a round CHAMBERED with the magazine OUT firing from open bolt is successful every time.

    I'm not really sure what this can be attributed to at this point. It almost seems as if the bolt is not under enough tension with loading a round being a friction point. I began to think maybe to just try a new recoil spring, but am not sure to what extent that would help.

    I'm hopeful that one of the many experts come upon my plea for assistance, and would appreciate any input in the diagnosis.

    Thank you!

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    Welcome!
    Try a different Mag before any modifications

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    Take the recoil spring out, re-assemble, tip to and fro while holding the trigger and grip safety. If the bolt does not slide full to and fro with the tiling due to gravity then that drag is your problem. Do it wilh empty magazine in, if draggy that is your problem.
    Class 3 shooters blow thier loads faster and with only 1 pull of the trigger

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    Im getting no drag sans magazine, but with the mag there is the slightest hang up. If i adjust the mag catch any lower, the bolt wont strip a round off the magazine upon closing.

    If trying a new magazine is the next step, would a factory IMI be the preferred choice?

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    I would lock the bolt back, drop a round in the chamber, then try to fire it without a magazine in place. If it doesn't fire, the mag probably isn't the issue. If it does fire, try firing a chambered round with the mag in it, if it doesn't fire the mag is probably the issue.

    Easy way to tell where it is dragging is to coat the top of the mag with Dykem or other metal ink, even ink from a magic marker will work. Insert mag and cycle gun a few times, pull the mag and look for worn away ink. A bit of filing on that spot and retry.

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    Quote Originally Posted by slimshady View Post
    I would lock the bolt back, drop a round in the chamber, then try to fire it without a magazine in place. If it doesn't fire, the mag probably isn't the issue. If it does fire, try firing a chambered round with the mag in it, if it doesn't fire the mag is probably the issue.

    Easy way to tell where it is dragging is to coat the top of the mag with Dykem or other metal ink, even ink from a magic marker will work. Insert mag and cycle gun a few times, pull the mag and look for worn away ink. A bit of filing on that spot and retry.
    Dropping a round in the chamber, it will fire without the magazine. It will not fire if i chamber a round, and then attempt to fire with a magazine in, leading me more towards the magazine being the issue. I purchased a few IMI surplus mags, but I will try to work out this one. Good call on using ink to find the friction point!

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    Good, missed the part of the OP where you had already tried it. If the new mags have a similar issue and rub in the same spot, apparently something is misaligned and you will likely need to tweak it or file away on the bolt a bit to clear.

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    Replaced the top cover and a fresh mag. It will successfully strip off and fire a round if there's only one round in the mag. Otherwise, it seems to be having difficulty stripping off a round causing a fail to feed. If it does make it into the chamber on a trigger pull, there is again little to no primer strike. Hand cycling the bolt without the top cover, and recoil spring I don't feel or see any drag or resistance. I'm starting to wonder if a new recoil spring assembly will work?

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    You may want to check the underside of the ejector sometimes it can sit to low or have a to large of radius and keep the round from fully getting into proper position for feeding.
    Last edited by 124NATO; 07-03-2021 at 08:39 PM.

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    Quote Originally Posted by 124NATO View Post
    You may want to check the underside of the ejector sometimes it can sit to low or have a to large of radius and keep the round from fully getting into proper position for feeding.
    This could be a possibility. I welded my ejector in as opposed to riveting.

    I tried a new recoil spring assembly as well as a barrel swap just because. I’m still getting mixed results. There are times the cycle will feed a round successfully but leave no primer strike. The other times I’ll get a fail to feed. Everything seems to be in spec as far as dimensions on the front and rear welded pieces go, but I’m stumped.

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    Last edited by noimpact; 07-17-2021 at 10:01 PM.

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    At this point, I've tried a new top cover, new mags, different barrel, new recoil assembly spring and I just tried different ammo, to no avail. It's far and few in between that a cycle results successfully, but more often than not I'm having a fail to feed, or a successful chambering with no primer strike. Any advice on where to go from here would be much appreciated. This project seems very rewarding when the desired results are achieved.

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    Your feed ramp looks like it needs to be polished.
    I talked to another guy who did this one a new build
    and it would feed a lot better.
    On a oem smg stamped feed ramp the bullet almost skips
    Over it. This looks like one of the new modified semi ramps that are being made. I've seem some that are so rough they wouldn't feed.
    Based on your mag rubbing it doesn't sound like this is the problem, but couldn't hurt to try.
    I would also buy a new lower and try that.

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    Quote Originally Posted by ADAM12 View Post
    Your feed ramp looks like it needs to be polished.
    I talked to another guy who did this one a new build
    and it would feed a lot better.
    On a oem smg stamped feed ramp the bullet almost skips
    Over it. This looks like one of the new modified semi ramps that are being made. I've seem some that are so rough they wouldn't feed.
    Based on your mag rubbing it doesn't sound like this is the problem, but couldn't hurt to try.
    I would also buy a new lower and try that.
    Good suggestion, thank you. This is a machined SMG ramp without the restrictor ring, and it could very possibly be the source of my problems. I didn’t think they were so finicky, but it would make sense. Are stamped receivers available as a repair kit? I would have preferred one, but I wasn’t aware they were being made aside from OEM receivers.

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    Quote Originally Posted by ADAM12 View Post
    Your feed ramp looks like it needs to be polished.
    I talked to another guy who did this one a new build
    and it would feed a lot better.

    On a oem smg stamped feed ramp the bullet almost skips
    Over it. This looks like one of the new modified semi ramps that are being made. I've seem some that are so rough they wouldn't feed.
    Based on your mag rubbing it doesn't sound like this is the problem, but couldn't hurt to try.
    I would also buy a new lower and try that.
    Yep.....
    If everything is fine with a single round in the mag, but you get a FTF issue with a 2nd round, you're probably getting some sort of interference with the ejector as well.
    I have a bolt "host gun" that had similar issues...
    The 2 fixes were to contour the underside of the ejector to ensure it didn't depress the ass end of the round in the mag under it.....changing the angle it was presented at. If it only pushed the round forward slightly under mag spring pressure, that wasn't an issue...
    2nd thing I did was to polish the feed ramp, and "break" the sharp corners (circled in red in the pic of your gun) .... especially important when using a factory SMG bbl. Not so much of an issue with commercial SMG bbls with enlarged/polished throats/chambers.

    Mine....

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    Quote Originally Posted by Villafuego View Post
    Yep.....
    If everything is fine with a single round in the mag, but you get a FTF issue with a 2nd round, you're probably getting some sort of interference with the ejector as well.
    I have a bolt "host gun" that had similar issues...
    The 2 fixes were to contour the underside of the ejector to ensure it didn't depress the ass end of the round in the mag under it.....changing the angle it was presented at. If it only pushed the round forward slightly under mag spring pressure, that wasn't an issue...
    2nd thing I did was to polish the feed ramp, and "break" the sharp corners (circled in red in the pic of your gun) .... especially important when using a factory SMG bbl. Not so much of an issue with commercial SMG bbls with enlarged/polished throats/chambers.

    Mine....
    This was immensely helpful and mitigated most of the FTF’s. Though I can’t credit one solution over the other as I polished and broke the edges of the feed ramp as well as contouring the bottom of the ejector before reinstalling and test firing, I was able to get a good string of rounds off before running into an issue. I went back and contoured some more and repolished the feed ramp, and I very rarely run into a fail to feed but with enough frequency that seems excessive, I am getting a light or no primer strike. I should note that when I get the fail to fire, the bolt seems to lock up as it is substantially more difficult to cock the weapon back to the open bolt position. I am incredibly thankful for the answers thus far as they have been tremendously helpful in making quite a bit of progress.

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    Quote Originally Posted by noimpact View Post
    This was immensely helpful and mitigated most of the FTF’s. Though I can’t credit one solution over the other as I polished and broke the edges of the feed ramp as well as contouring the bottom of the ejector before reinstalling and test firing, I was able to get a good string of rounds off before running into an issue. I went back and contoured some more and repolished the feed ramp, and I very rarely run into a fail to feed but with enough frequency that seems excessive, I am getting a light or no primer strike. I should note that when I get the fail to fire, the bolt seems to lock up as it is substantially more difficult to cock the weapon back to the open bolt position. I am incredibly thankful for the answers thus far as they have been tremendously helpful in making quite a bit of progress.
    In my experience a light primer strike and a difficult to extract live round is the results of the barrel chamber being cut to deep so the round is sitting to far into the barrel, they tend to jam in the chamber and are different to remove. You can fix this by cutting the chamber end of the barrel and the trunnion interface of the barrel the same amount . Or just buy a different barrel

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    Quote Originally Posted by 124NATO View Post
    In my experience a light primer strike and a difficult to extract live round is the results of the barrel chamber being cut to deep so the round is sitting to far into the barrel, they tend to jam in the chamber and are different to remove. You can fix this by cutting the chamber end of the barrel and the trunnion interface of the barrel the same amount . Or just buy a different barrel
    I thought I had responded to this but I guess my post never went through. I am using a pike arms threaded 8-5/8” smg barrel as well as an OEM SMG barrel. Both barrels exhibit the same.

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    Quote Originally Posted by noimpact View Post
    I thought I had responded to this but I guess my post never went through. I am using a pike arms threaded 8-5/8” smg barrel as well as an OEM SMG barrel. Both barrels exhibit the same.
    Well shucks that's interesting maybe someone more educated than me can chime in.

    Can you try the plunk and push test and see if the rounds get stuck?

    If you are using 124 or heaver rounds they may have a longer OAL ? If you have the ability to reload you can make up some dummy rounds with no primer and send them into the chamber with the gun assembled and maybe gain some insight, pay attention since it's a double stack mag if the jams are consistently feeding from one side .

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    Quote Originally Posted by 124NATO View Post
    Well shucks that's interesting maybe someone more educated than me can chime in.

    Can you try the plunk and push test and see if the rounds get stuck?

    If you are using 124 or heaver rounds they may have a longer OAL ? If you have the ability to reload you can make up some dummy rounds with no primer and send them into the chamber with the gun assembled and maybe gain some insight, pay attention since it's a double stack mag if the jams are consistently feeding from one side .
    I was using subs, and they did have a longer OAL, but I tried 115 supers as well. Both suppressed and unsuppressed in both barrels, my issue is repetitive. Further, the test yielded results consistent with a satisfactory chambering.

    I should have from the start used dummy rounds, but I have tried brand new 9mm A-Z snap caps, and it has shed a little bit of light on the issue. Some will cycle with a very satisfactory sound, some will produce a more sluggish cycle which I firmly believe would be the cause of the FTF. 1/5 will not chamber. I think working the ejector and feed ramp some more might be the answer, but I am afraid to go too far on the ejector. Additionally, here are some more visible markings on the snap caps that might provide someone with more insight than what I can deduce from it.




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