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  1. #11
    Senior Member djandj's Avatar
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    OK gang hoping for some direction here. Working with an Historic Arms semi MKII in .308. I have a .303 bolt in there with a .308 extractor. (casings are a bit "loose" in the bolt face when clipped in by the extractor. ANYWAY been having trouble with failures to extract/eject. Every few rounds having the empty case get stuck in the receiver and mucking up the loading of the next round. We have tried drilling out the gas port in the regulator to match the .15 of the case port in the barrel = no real change. Tried clipping off the recoil spring in the buttstock down to about 1/2 its original length. = shorter we cut the spring, the worse the problem got. With the original spring we could get 5 to 10 rounds at a time and with the lighter spring it jammed every other round or maybe got 2-3 rounds at a time.

    Now when I hand cycle the rounds, it functions perfectly. But fails when under gas pressure from being fired. Doesn't change with mag (using ZB 39 (curved) mags from when the gun was originally chambered in 54R)
    Also observed that both with hand and operational cycling when fired, some of the rounds "flick" out the bottom with reasonable force while others drop out with very little force. Ideas as to what could be causing this? Ideas to try to fix?

    Thanks, feel free to PM me to chat about it.

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  3. #12
    Senior Member WallyG.'s Avatar
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    I am curious why you have not mentioned the striker spring in your list of diagnostic components. You have reduced the recoil spring in length - rather materially... so it's resistance to rearward bolt travel is now a fraction of what it started as when at full length. The other (and I might add new component in the semi design) spring that is being compressed with bolt rearward movement is the striker spring. Have you considered shortening this spring as well? It would be a trade off as less length would create more bolt rearward travel per gas recoil impulse... but too short and the striker's forward momentum might be reduced to a point of producing light primer strikes at trigger pull. As to your extractor and case wobble... are you certain that it is in fact a .308 extractor? Can you find additional ones to swap out and see if any performance changes with a different one. also.. same idea with the bolt... would a different .303 bolt face provide a less wobbly engagement? Some parts swapping might be revealing... but keeping proper headspace would be necessary in the gun. It sounds from your reporting that the manual action operation produces a positive ejection... human force brings extracted case firmly rearward against the ejector. Have you considered a new ejector in your parts swapping? BUT.. under firing (gas powered recoil cycle) there is not positive ejection??? right? I agree this seems to point to not enough rearward bolt travel... so you're back to less springs or more gas. If the extractor is not pulling the case out of the chamber or is dropping the case while traveling rearward... suspect the extractor in this calculus equation. Can you post some pictures of the component parts? So the barrel's gas port has never been enlarged? Len usually did this to get more impulse power to act on the larger resistance of the semi internals - as well as over sizing one of the ports in the gas nut and the channel that carries the gas rearward to the gas cylinder after going through the nut. Have you compared the gas port sizes and various channel sizes on your Len adapted 7.62x54r barrel to the ones on your SA .308 barrel etc? Smaller ... Same ... larger? If your .308 barrel has smaller gas delivery channels... larger (more gas) would be one possible source of more rearward impulse... just like less springs resistance. Most (if not all) of my HA guns have enlarged gas channels... port in barrel + gas nut + channel through the gas block that feeds the gas cylinder. All done to obtain more rearward force to overcome the larger intern mass of the new semi components. (striker and striker spring). hope some of this rambling thought might be of use in your efforts. Best of luck.

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  5. #13
    Senior Member djandj's Avatar
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    Thanks Wally. Yeah, I thought about chopping the striker spring, but that spring is significantly less stiff than the recoil spring(s) in the buttstock. Since it kept getting worse as I chopped the recoil spring I figured it wouldn't make sense to chop the relatively weak striker spring.

    I can post pics - just let me know specifically what parts might be usefull.

    I checked the extractor spike length with a few other .308 guns and they appear to be the same length. As far as the gas port sizing, I drilled out the gas regulator to the same size as the barrel gasport size (but didn't drill the gasport on the barrel)

    My first thought was that we just aren't getting enough gas pressure to get the spent casing thrown clear of the receiver before the bolt comes home. But after completely removing one of the TWO springs I had in the stock when the gun was in 54R AND chopping the remaining recoil spring in half (and presumably decreasing the resistance faced by the gas and thus magnifying the effect of the same amount of gas I figured maybe we aren't on the right track. Now it DOES work flawlessly when hand operating AND will fire 5-10 rounds as is. I just can't get through a mag. I did notice that the grip achieved by the extractor against the bolt was a bit sloppy because the .303 bolt cut out is a bit larger than the non-rimmed .308 round. Len sent me a .303 bolt so I KNOW it's not a .308 bolt, but because he modifies the bolt to fit his design of bolt carrier, I can't just go and buy a .308 bolt as it won't fit with the carrier. I can write to Len again and see if he has (or can modify a .308 bolt) perhaps. Just not sure where to go with it at this point. Extracters are cheap and easy to get, but changing the extrator won't change the fact that the .303 bolt face cut out is too large to tightly hold the rimless .308 round.

    Drilling out to get MORE power is possible I suppose, but barrels are scarce (and expensive) so I don't want to kill the barrel. I checked with one of our members with a full auto Bren MKII who says his works perfectly in .308 and is 5/32. Mine is currently 5/32 from the barrel port through the gas regulator. I have heard of others drilling out further.

    Can others report back as to what they have drilled their MKII's to in .308?
    Last edited by djandj; 03-03-2021 at 10:32 PM.

  6. #14
    Really Senior Member tankhunter's Avatar
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    I wouldnt worry too much about the bolt face being 'sloppy' with a 7.62mm Round. Remember, the SADF converted guns NEVER changed thier bolts from the .303" Versions.
    They did obviously change the extractors, due to needing a longer claw to gript the cartridge canilure.

    if at all possible. get a standard full auto gun at a range. Put a spring balance on the cocking handle & JUST begin to pull the springs rearwards.
    Then put the balance on the cocking handle of your own gun & repeat the above procedure.
    Check the spring pressures. & that should give you an indication of how much you need to
    reduce the spring pressure on your gun. It would be a simple matter to clip off one coil at a time, to reduce & check. Reduce & check.
    A bit of a ball ache & slow. BUT, you are doing it under a controlled manner! Better to slowly reduce, than cut off too much. & if you dont have a spare spring. You will be in the Shi.....Er, Soft stuff up to your neck!

    EASY to do in this manner & you can see your work creeping forward. To a positive result!
    Last edited by tankhunter; 03-05-2021 at 11:13 AM.

  7. #15
    Senior Member djandj's Avatar
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    Quote Originally Posted by tankhunter View Post
    I wouldnt worry too much about the bolt face being 'sloppy' with a 7.62mm Round. Remember, the SADF converted guns NEVER changed thier bolts from the .303" Versions.
    They did obviously change the extractors, due to needing a longer claw to gript the cartridge canilure.

    if at all possible. get a standard full auto gun at a range. Put a spring balance on the cocking handle & JUST begin to pull the springs rearwards.
    Then put the balance on the cocking handle of your own gun & repeat the above procedure.
    Check the spring pressures. & that should give you an indication of how much you need to
    reduce the spring pressure on your gun. It would be a simple matter to clip off one coil at a time, to reduce & check. Reduce & check.
    A bit of a ball ache & slow. BUT, you are doing it under a controlled manner! Better to slowly reduce, than cut off too much. & if you dont have a spare spring. You will be in the Shi.....Er, Soft stuff up to your neck!

    EASY to do in this manner & you can see your work creeping forward. To a positive result!
    Well, after having NO luck with the recoil springs, I am really beginning to suspect the bolt. I THINK it is dropping the spent cartidge into the receiver on extraction. Take a look at the vid. My extractor is holding the case, but there is nothing above or either side to hold this round in. Pics of a friend's bolt face clearly shows the bolt surrounding the casing much more closely.

    https://vimeo.com/520220695

    here is a pic of my Friend's (operational) FA Bren.
    Last edited by djandj; 03-05-2021 at 10:51 PM. Reason: new pic

  8. #16
    Really Senior Member tr63's Avatar
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    Check your extractor is it marked 7.62 on it's face ? compare it to the .303 extractor ,if they are the same in up and down length ,the one marked 7.62 is not high enough to extract .308 ammo . See if you can find a 8mm extractor and see if that cures the failure to extract the shell and ripping the rim off the case . My SADF Bren barrel in .308 installed in my .303 MkII will not work with the extractor marked 7.62 , works 100% with the taller 8mm one . BRP had the 8mm extractors see if they still stock them .

  9. #17
    Senior Member WallyG.'s Avatar
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    A few more observations to kick around... Comparison of the internal spring resistance between a full auto Bren and a Semi would be a useful benchmark, but remember that the Semi auto spring setup and new internals are not an apples to apples comparison. The Semi design must recoil more mass and to do that will need more gas impulse. Comparing a Full auto gun to Semi gun both using a SA Barrel and the barrel gas port dimension is still the same - so no additional gas is being used to power the more massive semi recoil parts... this could be a big part of the problem... along with the amount of spring resistance in the semi design. Yes, reaming out the gas port in the SA barrel to get more gas is a "risky" step as it is very hard to put metal back once removed. I concur with sourcing a replacement .303 bolt and a 7.92 (8mm) extractor and see if the case handling improves before drilling the barrel port larger. True 7.62 Bren barrels and bolts are dimensionally unique and only work together. The SA barrel you have was fielded using a standard .303 bolt and their SA barrel which has a custom Barrel Face geometry (dimensions) designed to work with the standard .303 bolt. I have a handful of spare striker springs from Len if you need one. I can also dig out one of my .308 SA barreled semis for comparison if needs be. You can get more "power" by having the gas port in the barrel enlarged... I have a belt fed converted 7.62 Bren (by Len at HA) that runs like a champ and that design is moving a lot more mass and overcoming much more resistance to cycle the top cover internals and belt advancement pawls. There is a power level that will cycle your gun... you just have not found it yet. And trying some replacement parts to eliminate potential slop/error before drilling on you barrel's gas port is a sound conservative approach - but I'm of the mind that you should consider a larger gas port opening if all else fails. Ammo choice could also come into play here too... hot pressure rounds (military surplus) would conceptually provide more gas impulse than more recently produced "commercial" ammo that has lower gas pressures. So some variability in performance would be related to the food being fed to the beast - hence the range of gas port sizes on the gas nut to allow more or less gas impulse... but this adjustment all hinges on the restricting size of the actual gas port opening in the barrel. If the gas port hole in the barrel is still in its original form (size) then you are utilizing a gas impulse designed to run the full auto Bren system and not a Semi. Let me know if I can be of further assistance. Best of luck!

  10. #18
    Really Senior Member Hal O'Peridol's Avatar
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    Quote Originally Posted by djandj View Post
    Now when I hand cycle the rounds, it functions perfectly. But fails when under gas pressure from being fired. Doesn't change with mag (using ZB 39 (curved) mags from when the gun was originally chambered in 54R)
    If the bolt is the sme one for when it was in 7.62 Russian, the bolt face is larger in diameter than one in .303.n

  11. #19
    Really Senior Member AmEngRifles's Avatar
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    Djandj....

    WallyG got my .308 L4 running with the suggestion of cutting the striker coil only. Now, ours are a little different animal, but the L4 and the SA .308 converted MkII are pretty close. Most of the recoil and gas forces should be the same. The big differences for you are a confirmed, proper extractor and striker spring adjustments ( Not recoil spring).

    If you can, get the replacement RECOIL spring and leave it full length. That will be the right driving and recoiling pressures for the bolt alone.

    Then, if you need to start with a fresh one, WallyG might help you with a striker spring. Cut 2 coils off to start. Actually, run the gun first with the fresh springs ( full length) to confirm you are still having extraction issues, with possible short stroking ( of the bolt) leading to jammed cartridges on the insertion stroke. That was MY issue with my L4, NOT an extraction issue, as mine has a match .308 L4 barrel and L4 bolt. Cutting two coils from the striker spring brought it to pretty much 100% functioning, depending on magazines ( that's another story for another time)

    With that mod, I had no more short bolt travel on gas setting #3. You have to "tune" these semi autos and then, you may have to stick with the same ammo and same magazine, as introduction of new components can change the mathmatical forces. In your case, hopefully an 8 mm extractor will solve 95% of your issue.

    It has not been mentioned, but the difference between you hand cycling the ammo thru the gun and firing the ammo is cartridge case adhesion on the chamber wall. If there was none, you gun would probably be working 100%...

    Good luck.

  12. #20
    Senior Member djandj's Avatar
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    Bren Fix update

    OK just for closure. So it seems the .303 bolt that was supposed to be sent back to me when I converted from 54R was NOT a .303 bolt but the same 54R bolt as before. So... the spent casing was just rattling around on the face of the breach block and was getting dropped into the receiver during cycling and jamming it up. I replaced the other block with a (correct) .303 block and it ran perfectly without missing a beat. Thanks to all to helped me trouble shoot this. I very much appreciate all your time and effort.


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