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Thread: Delayed Roller Blowback - Understanding Actions, Gap and Ammo

  1. #21
    Militant Asshole Big Steve's Avatar
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    Quote Originally Posted by bladeworks123 View Post
    BTT ....Today I added a little more about headspacing at the end of the original post....
    Great job Blade!
    You might want to explain Lock Timing and how a tight bolt gap increases pressure, to make this complete.

    -Steve

  2. #22
    Senior Veteran Evilblackgunsrfun's Avatar
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    Great info Blade. its nice being a member here and getting real info and help and not bull
    COG#99999

  3. #23
    Senior Veteran bladeworks123's Avatar
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    Quote Originally Posted by Big Steve View Post
    Great job Blade!
    You might want to explain Lock Timing and how a tight bolt gap increases pressure, to make this complete.

    -Steve
    Perhaps I wasn't real definitive about lock timing, but primarily and to put it into perspective with DRB, lock timing refers to the locked position the bolt head is in when the rifle is ready to fire or in battery, with a round in the chamber.

    Correct lock timing on the DRB occurs when the rollers are fully extended and are resting snug against the trunnion faces. Incorrect lock timing can occur two ways. Either the bolt falls short of coming far enough forward for the rollers to properly extend and lock into the trunnion recesses (out of battery), or the bolt head travels too far forward, the rollers are fully extended but are not held tight back against the trunnion recess faces.

    On a conventional rotary style locking bolt, timing is critical to the full lock up of the rifle at preset headspace. Headspacing can be correct but lock timing off and the bolt is not fully locked.

    On the DRB, the position of the bolt head is controlled by the depth of the barrel breech and the position of the bolt head in relation to the trunnion, not a fixed position, So on the DRB if lock timing is not correct, neither is headspace. The bolt being out of time by being too far forward is caused by the same number of things that cause gap to decrease. Improperly pressed barrel (too deep), worn rollers, worn locking piece, worn trunnion faces, worn breech face, worn bolt face, worn bolt head windows, etc.

    So, when building it is important to know that if you are pressing the barrel and you end up going too deep(low/tight gap), you are not only increasing headspace, you are creating an out of time situation on the bolt head. Both of those conditions create excessive pressure inside the chamber, because the cartridge is not being held tight against the chamber, nor is the bolt head tight against the locking recesses. The loose cartridge casing will expand and swell forcing the bolt head to slam back against the trunnion faces with tremendous force. The casing will split or pull apart and will fail in it's role to help contain pressure. In essence, the cartridge case becomes a gas piston/bomb. Low gap due to normal wear will create the same problem. That is why is imperative to check gap on a fairly regular basis to watch for that.

    To sum up, low or tight gap (below .004") means excessive headspacing, improper lock timing and excessive pressure inside the chamber. High or loose gap (above .020") means too little headspace and bolt being out of battery so the rollers do not properly delay the blow back of the bolt, and can lead to a premature release of high pressures outside the chamber into the trunnion.
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  4. #24
    Senior Veteran bladeworks123's Avatar
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    Today I got an interesting e-mail concerning the subject of my post here on this subject, and so I will quantify what I said above and add this....
    I am not bashing the .308 win cartridge, it is a wonderful round. I am not bashing people who chamber DRB rifles in .308 win.

    This is not a "I like NATO ammo and therefore .308 win is worthless" thing. If you think it is, then you need to spend some time learning to read and comprehend the english language. Bottom line is that in the original G3's and CETME rifles, the chambers and flutes were cut to a specific depth and configuration to use the ammo they were going to be firing on a freaking battlefield. I'm relatively certain that when the rifles that we now build as de-milled kits were being designed, the engineers and craftsmen did not set around the workbench and say "You know Fritz, we should build this rifle so it will also shoot .308 win ammo so that when some guy 40 years from now wants to re-build this gun, he can shoot .308 win. in it, without being inconvienienced by seperated cartridge cases on a safe and peaceful firing range."

    The flutes and chambers in these military designed and built rifles were not designed to have thin walled brass fired in them, they didn't even think about that being a problem... They cut the chambers and flutes to a depth that would work with what they were building at the time....

    Yes, some of them will fire it but not all,,,,, why not? They are just lucky I guess. Why do some birds crap on your windshield and others don't? Who cares!! The guys with the problem are the ones who need answers. If your gun shoots it OK then good for you,,,, enjoy it,,,, bask in it,,,, but don't minimize the problem for guys who cannot, it is a "real problem".

    There is nothing "wrong" with the .308 win brass, it is not "inferior" to 7.62 NATO, nor is 7.62 NATO "superior" to .308 win. And, no, it is not an issue in other military rifles.....The post specifically says that the problem does not occur in other military rifles. "Apples and oranges"

    You can try to fire 12 guage shells in there if you think you can get them to fit. I am only trying here to explain why some hundreds, maybe thousands of people constantly experience seperated .308 win cases in a "CETME" or "Hk" delayed roller blowback rifle.

    Why did Century mark their guns .308 win.? I don't know the answer to that,,,, call them and ask them that question and then ask them if they re-chambered the barrels after they cut them out of receivers stamped 7.62mm NATO.

    And don't send me any message on behalf "of a friend" who told you about this post, in an effort to "straighten" me out, or convince me I am spreading "propaganda" You will go in the recycle bin just like the guy today did.

    It's as simple as this, I am way too old, fat, short, slow and busted up to play basketball....I know I can't do it without experiencing a possible physical malfunction, In fact, I was never able to play basketball, I wasn't born built to do it. Now if you want to have a whiskey drinking contest or a friendly shooting match, I am relatively sure I will be able to compete and function fairly well doing so.

    As far as the .308 win chambered US barrels issue is concerned,,,,,, gee, I wonder why they have tried different depth flutes, more flutes, etc.??? Guess what, they are trying to accomplish the same thing the old boys at Santa Barbara and Hk were trying to do back in the day.... Build the rifle so it will shoot what they want it to, not what I or anybody else want it to or think it should. I have no doubt they may very well be able to do that, In fact I think PTR probably has it pretty well in hand. If they can figure out how to make it function with both, then good for them. I will be glad. I would probably buy one, that would be just so convienient for me, I would sleep better.

    I have straightened this out with the author of todays e-mail, by inviting him to enjoy his brief visit to my recycle bin..... and now offer up my sincere apologies to "his friend" or any other sensitive over thinkers, and guys who just want to argue, if I happened to get on the only nerve you might have left. And I appologize for being so scary that you can't e-mail me yourself.......

    Speaking for myself, I enjoy trying to help others who are having troubles, by explaining what I know and believe to be the truth. There are lots of other guys on this site that do the same. No, none of us know everything, but we pass along what we do know, in an effort to try and help others out, not to create debate. If you want to debate this issue, don't waste your time sending me an e-mail, a PM or a telegram, unless you also want to take a brief trip through my recycle basket.

    Oh, and spend some money getting a real e-mail address instead of Hotmail, it will keep you out of the middle of the rest of the junk mail in my in-box.
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  5. #25
    Senior Veteran HKILLER's Avatar
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    i guess some people dont have any thing better to do than send you junk mail lol.
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  6. #26
    Senior Veteran bladeworks123's Avatar
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    There is absolutely no need to, when there is a perfectly good forum here and over at Weapons Guild to "discuss" matters on. I've posted the same explanation both places, and my one man fan club and his "friend" have apparently read them both. He was "compelled" in the interest of me passing on bad information to e-mail me and not embarrass me publicly????,,,, I was compelled to tell him..........I'm better now....

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  7. #27
    Imperial Marine Stormtrooper
    Perro Del Diablo's Avatar
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    The flutes and chambers in these military designed and built rifles were not designed to have thin walled brass fired in them, they didn't even think about that being a problem... They cut the chambers and flutes to a depth that would work with what they were building at the time....
    i have posted pictures from a book describing this, and i have posted the text as well too many times over the years to count - if one truly wanted to, they could simply utilize the search, and i would have to believe they would find tons of stuff i have posted regarding this.

    case head seperations with commercial ammo have ALWAYS been a problem for some of these firearms ever since i first acquired my first one sept 12, 2001, and instead of retyping the information over and over again i save the text to a .TXT file so i can just repost it - OVER and OVER again.


    i have learned over the years that it doesnt matter how much proof you give of this problem, some people refuse to accept it. Some of the stuff i have provided is written by the actual people who DEVELOPED THE RIFLE FROM THE INITIAL STOLEN POLISH DESIGN - doesnt matter, there are always those who know better.

    The internet is a scary thing - you can do a google search and find something written by a complete stranger and all of a sudden youre an expert on the subject.
    Some people try to show how much they know on there new forum community and they repeat something they read on the internet (regardless of whether its true or not), and after a while, so much of the same false crap is posted that it almost becomes "true" since so many people are saying it.
    for YEARS i read all about CETME C rifles being unsafe to shoot NATO ammo in because the spanish developed a lighter powered "CETME round" to shoot in it. Didnt matter how much proof you provided against it, this rumor lingered for at least 3 SOLID years - eventually it started to disappear as more and more people bought CETME rifles. When they first hit the market from Century back around 2000/2001 this was something i read about ALOT.

    then came the FR7, FR8, and m1916 guardia civil being chambered for the "cetme round" and couldnt take the pressures of the NATO cartridge crap!!
    i STILL read stuff about this. Doesnt matter that you can show them official spanish literature for there rifle showing that they were indeed converted for use with NATO ammo, there will ALWAYS be those who know better.

    i quit caring - it's too much work - if you would like they pictures to help you back up your post though, i will find them and send them to you so you can use them - straight out of the books

  8. #28
    Imperial Marine Stormtrooper
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    Quote Originally Posted by bladeworks123 View Post
    High or loose gap (above .020") means too little headspace.
    we need to sit down and have a discussion on this some time, cause im either misunderstanding you, or we have differing opinions on this.
    I would love to sit down and brainstorm with you over a beer sometime.

    it is MY UNDERSTANDING (and i need to prequal that with "i have been wrong before" and "i dont claim to be an expert")
    that gap under .004 affects headspace, and timing

    gap OVER .004 affects only timing

    HK armorers manual states that as long as you have a gap of .004 then your headspace is set correctly - this means that your carrier is FIRMLY forcing the rollers into the trunnion - there is a safety factor built into this, and i believe that the .004 number is more for timing issues, and as long as you have ANY gap, then headspace is set correctly.

    it is my understanding that headspace is correctly set between the chamber and the well on the bolt face during manufacturing and has nothing to do with bolt gap as long as there is some pressure forcing the bolt rollers tightly into the trunnion. You can test this with a loose barrel, a go headspace gauge, a new bolt head, and a quality depth micrometer.
    As long as there is at least .001 inches TRUE bolt gap (not artificial), then there is sufficient pressure to keep the rollers firmly forced into the trunnion which means that headspace is correct.

    TIMING - this system is a half roller locked delayed blowback action, and timing is affected by the delay in "delayed blowback"

    during the firing sequence, what is the bolt system fighting to overcome to unlock?? one could say the rollers need to be forced into the bolt so it can unlock itself from the trunnion - that is correct to an extent, but what forces are the rollers trying to overcome??
    The angle on the bolt locking lever mated to the shoulder on the bolt head, and the strength of the bolt locking lever spring is what is providing resistance against the rollers being able to fully go inside of the bolt head.

    when your bolt is in battery on a 100% new firearm with all new factory made components, the bolt locking lever is mated with the shoulder on the bolt head at a certain point. As the rollers are forced into the bolt head, which forces the locking piece rearwards, which also forces the bolt carrier rearwards, the bolt locking lever must be forced over the top of the bolt shoulder before the rollers can enter the bolt head all the way.

    Confusing, but in laymans terms - the more gap you have, the less time the bolt is delayed because the ramp on the bolt locking lever is sitting at a different spot on the bolt shoulder than it is when you have less gap and has less work to do to get over the top of the bolt shoulder.

    the less gap you have, the more time the bolt is delayed because the ramp on the bolt locking lever is sitting at a different spot on the locking lever ramp and has to work harder to overcome the bolt shoulder than one with higher bolt gap


    this is why HK publishes the "sweet spot" for bolt gap which is from .010 - .018 inches

    its the correct spot for the locking lever to sit on the bolt shoulder so that its able to overcome the strength of the bolt locking lever spring at just the right time so that there is just enough pressure left over to overcome the weight of the bolt and move it the distance required to eject the spent casing, and then the spring pressure from the main spring.

    throw in a monkey with a peanut grinder, and ALL bets are off


    not an expert - this is the way i have come to understand how the half roller locked delayed blowback action works
    i will enjoy buying you the beer if you are ever out my way

  9. #29
    Senior Veteran bladeworks123's Avatar
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    Quote Originally Posted by Perro Del Diablo View Post
    i have posted pictures from a book describing this, and i have posted the text as well too many times over the years to count - if one truly wanted to, they could simply utilize the search, and i would have to believe they would find tons of stuff i have posted regarding this.

    case head seperations with commercial ammo have ALWAYS been a problem for some of these firearms ever since i first acquired my first one sept 12, 2001, and instead of retyping the information over and over again i save the text to a .TXT file so i can just repost it - OVER and OVER again.


    i have learned over the years that it doesnt matter how much proof you give of this problem, some people refuse to accept it. Some of the stuff i have provided is written by the actual people who DEVELOPED THE RIFLE FROM THE INITIAL STOLEN POLISH DESIGN - doesnt matter, there are always those who know better.

    The internet is a scary thing - you can do a google search and find something written by a complete stranger and all of a sudden youre an expert on the subject.
    Some people try to show how much they know on there new forum community and they repeat something they read on the internet (regardless of whether its true or not), and after a while, so much of the same false crap is posted that it almost becomes "true" since so many people are saying it.
    for YEARS i read all about CETME C rifles being unsafe to shoot NATO ammo in because the spanish developed a lighter powered "CETME round" to shoot in it. Didnt matter how much proof you provided against it, this rumor lingered for at least 3 SOLID years - eventually it started to disappear as more and more people bought CETME rifles. When they first hit the market from Century back around 2000/2001 this was something i read about ALOT.

    then came the FR7, FR8, and m1916 guardia civil being chambered for the "cetme round" and couldnt take the pressures of the NATO cartridge crap!!
    i STILL read stuff about this. Doesnt matter that you can show them official spanish literature for there rifle showing that they were indeed converted for use with NATO ammo, there will ALWAYS be those who know better.

    i quit caring - it's too much work - if you would like they pictures to help you back up your post though, i will find them and send them to you so you can use them - straight out of the books
    Nah, I'm fine now, you know I don't mind if people argue or even tell me right out that they think black is white,,,,just don't be a little girl and waste my time being stupid about it, have the courtesy of telling me who you are. I respect that, I don't respect anybody that intentionally tries to push my button from behind a stupid made up e-mail adress they just came up with today.... What really dissapoints me now is that I even responded to it, or sullied this site by even mentioning it, please forgive my drivel.
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  10. #30
    Senior Veteran bladeworks123's Avatar
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    Quote Originally Posted by Perro Del Diablo View Post
    we need to sit down and have a discussion on this some time, cause im either misunderstanding you, or we have differing opinions on this.

    I would love to sit down and brainstorm with you over a beer sometime.

    throw in a monkey with a peanut grinder, and ALL bets are off


    not an expert - this is the way i have come to understand how the half roller locked delayed blowback action works
    i will enjoy buying you the beer if you are ever out my way
    Yes, I would enjoy that sometime. I am not as well armed with info as you are. I was learning Hk's, with emphasis on straight wall chambers, (not much difference really except case lengths, load pressures and barrel lengths seem to add more complication there) back in the late 70's and early 80's. With absolutely no thought in mind that I would someday actually make a past time out of building DRB's. Back then we learned theory but the important thing was, "Stay in the gap range"..... theory wasn't important when it was a maintenance job. When I left in 1994, all my reference material stayed. It wasn't until I got a wild hair to build a CETME that I even gave much thought to DRB's again and found this site, and began trying to collect information again.
    But, I can tell you that I have attended training where even Hk boys are bouncing on the issue of headspace and lock timing. My understanding has always been that at .004" on fresh parts with bolt resting on breech, that headspace, (bolt face to datum line) will be at go+ with a spec cartidge resting against the shoulder, and timing is at the beginning of it's accepted span. The difference between .001 and .004 is to allow for wear of mating parts, heat expansion, and varying ammunition specs and still insure that timing would never fall to a point that would ever allow the rollers to be slack of the trunnion faces.
    That is why the minimum spec of .004 rather than .001 on gap. The way I understand it is, on new clean parts, you will still have positive contact of rollers against trunnion at .001" but throw in wear at that low a level and you would drop below go on headpsace and the rollers could fall slack of staying mated with the trunnion, and the locking lever is slack. Also, .001" rearward movement of the bolt head rearward against the trunnion faces translates to .004" gap.

    The sweet spot as I understand it is slightly above mid range between go and no go, (bolt head forced back away from the datum line) and is in the center of the acceptable lock timing spectrum, to achieve the "desired amount" of delay. If you set gap above that point you are decreasing delay by putting preloading pressure on your locking lever. The angles on the locking piece relate to the increase or decrease in leverage needed based on available pressures. And as rollers, locking piece an bolt head windows wear, you are increasing headpspace and lock timing is moving away from positive pressure on the locking lever, for lack of the correct word there, which escapes me at this late hour.

    Anyway, I would enjoy pouring beer on this difference of "geometrical" views, and getting this thread as absolutely correct as possible. My whole intent here from the beginning was to use information from folks such as yourself, and the other knowledgeable members here, to add reference material so as to make correct and useful information available to guys,,,,, since it has always been closely guarded by the oracles at Hk. I certainly don't know it all but if we all collect info together, verify it and present it so its' understandable, we all benefit, and it's a whole lot cheaper than attending Hk school for sure. I've slowly been editing parts of it with information and questions provided by members to try and make it as correct and the least confusing as possible.
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