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

  1. #1
    Senior Veteran bladeworks123's Avatar
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    Delayed Roller Blowback - Understanding Actions, Gap and Ammo

    I put this together to try and cover the three things that seem to be most confusing, even to me a lot of the time. The videos that we have about bolt gap are really excellent, and I urge everybody to review them. I thought maybe this would be a little addition to the stuff we already have. I tried to put it together as a starting point for folks who are new to the sport of the good old delayed roller blowback rifles we all love so much, and cuss about so often when they don't run right. So,,, to some of you old hands it might seem pretty basic and boring, but hopefully it will make newcomers lives a little less stressful.
    If any of you guys see anything I can change to make it easier to grasp, or any errors I have made please give me a shout.

    First, I think it is real helpful to owners and builders alike to understand the basics of how the delayed roller blow back action is designed to work. It does not rely on gas pressure to function the action with the exception of using pressure via chamber flutes to aid in case extraction. The action works on the force exerted by gas pressure against the bolt head together with inertia and kinetic energy with some complicated geometry thrown in....

    Attachment 18186

    For all of this to function correctly, two things have to be accomplished that must work hand in hand together. Headspacing and lock timing.
    There is no true way of checking headspace on a DRB action, because the bolt head can lock up in a variety of positions. For example, most rifles can be checked with headspace guages of known and acceptable lengths. A delayed roller blow back action will lock up on all of them, making them worthless. I know, I've tried it. The key is that the rollers must lock up in a certain spot against the shoulders of the trunnion, and the location of that lock up point is where the proper headspace is determined. The engineers of this amazing piece of equipment calculated that combination of lock up timing and headspace, and established that the measured distance between the bolt head and bolt carrier could be used to determine both headspace and lock timing. The distance between the bolt head and the bolt carrier when the action is locked into battery in the correct position is known simply as bolt gap. It is checked with no round in the chamber. Placing a round in the chamber will interfere with the accuracy of the bolt gap test. It can prevent the bolt head from setting in it's correct position for proper measurement. This diagram gives nomenclature of the working parts of the action together with the known specifications necessary to achieve proper bolt gap. Please note that dimension "A" listed in the diagram below is not an absolute. In all DRB rifles, the barrel breech must be proud of the casting on the inside of the trunnion, so that the bolt head can come to rest, un-obstructed, against the barrel breech. The amount that the barrel sticks up is usually in the range of .005 to .030, the only important thing to know here is that it should be above or proud of the trunnion for the rifle to be set up correctly.

    Attachment 18181

    And no DRB owner can exist without a copy of this page...

    Attachment 18187

    Common problems for all weapons is wear. The delayed roller blowback is actually designed to be extremely user friendly in being able to be somewhat adjustable to compensate for that wear. As the parts of the action wear, changes can be made to correct for worn surfaces. Rollers can be changed to larger ones. Worn locking pieces can be replaced. Worn bolt heads can be replaced. And if needed, the barrel can be removed and pressed in again to a different position to make things right and new again. Here are the common points of wear in the delayed roller blow back action.

    Attachment 18182

    Last but not least, and the most controversial perhaps, is the question about why these rifles are notorious for shredding perfectly good, high quality U.S. made .308 Winchester commercial ammo. The simple answer is that they were not designed to shoot it. Remember that earlier I said that the engineer of these beauties calculated lock timing and headspace together to make them function? Well, that was partially based on the chamber length as part of the formula. The truth is that .308 Winchester ammunition and chambers are shorter than 7.62 x 51 NATO. And .308 Win ammo is thinner walled and not cut out to be fired in these rifles. The combination of the slightly shorter case, the softer and thinner brass, causes three things to happen.....First, the thinner .308 case expands fully and deeply into the flutes, closing them off and stopping the flow of gas between the casing and the chamber necessary to allow the case to be extracted. The case is now stuck, and secondly, the case tries to stretch out to match the length of the chamber, causing the inevitable separation of the case, third and last, the rearward inertia exerted when the rollers unlock, rips the back end of the case off, leaving the front half firmly and tightly stuck in the chamber, requiring the use of a broken shell extractor to get it out. Some DRB rifles will shoot commercial ammo no problem, some won't. It all depends on the hidden and un measureable headspace inside the chamber of each rifle, how soft or hard the .308 brass is, and whether or not it flows into the flutes tightly enough to plug them off for the flow of gas around the outside of the case. This is why you will find that imported steel cased .308 win ammo may work when brass cases won't. The steel cases, depending on the brand, and lot, may or may not be strong enough to keep from expanding fully down into the depths of the flutes. Some chambers are tighter than others, or have a little deeper well defined flutes, and will digest .308 just fine, some are loose and hate you for putting it in there, and will make you sorry, and make you spend money on a broken shell extractor. Some of the later U.S. produced clones with U.S. barrels might have been built with .308 Win. chambers. And if you have one of those you may or may not experience problems shooting military surplus ammo. The best advice I can give is that if you have a 7.62 NATO chambered rifle, shoot 7.62 NATO ammo in it. If you have a rifle constructed with a .308 Win. chamber, shoot .308 Win in it. Any deviation from that advice is your own experiment. Here's a diagram that attempts to explain the broken shell casing adventure....


    Say this outloud with me.....


    There are only three machined points within a DRB trunnion/barrel assembly that come into play for "headspacing". Unlike the conventional bolt or rotating actions which have five. The three critical surfaces in a DRB are the two faces of the locking recesses and the breech of the barrel.

    On a conventional rifle, headspacing is achieved by locking the bolt face in the action at the correct depth in relation to the center line of the chamber shoulder in the barrel, known as the datum line (on bottle necked cases). That of course is done by the use of the locking lugs on the bolt. The bolt head never contacts the barrrel breech. Headspacing is then measured from the "fixed" location of the bolt head to the center line of the shoulder (datum line) of the chamber.

    A DRB has nothing to lock the bolt head into a "fixed" position that will insure that the bolt head locks in the same position every time except the breech of the barrel. The rollers lock against the trunnion recess faces, to prevent rearward movement but there is nothing there to stop the bolt face from coming forward except the breech of the barrel. In order to allow for that, DRB engineers factored headspacing by the measurement from the trunnion locking recess faces to the centerline of the chamber shoulder, (datum line). The chamber of the barrel is cut to 7.62 NATO Spec "cartridge headspacing" depth minus .001" to .005" to help prevent the bolt head from slamming into it each time the action cycles. So each time a round is chambered, the position of the bolt head is purely at the mercy of what the cartridge headspacing length is. Each time the action cycles, the bolt head is coming to rest solely on the cartridge, nothing else, and the rollers lock against, not into, the trunnion recesses.

    Because of that combination of what I call "floating geometry" it is important that you understand that the chamber depth in the barrel is an absolute part of the picture here, and that bolt gap has absolutely no effect on headspacing. Bolt gap is only a measurement which lets us know that the bolt head is back far enough to adequately be locked "against" not "into" the action to safely achieve headspacing. If I've explained it the way I intended to, the light bulb will come on and you will say, "that means that every time I use a different length ammo, my headspacing changes?", and the answer is yes. That is, again, another reason why short .308 winchester cartridges tend to rip in half when fired in these rifles. You put one of those short babies in there and you've automatically created excessive head space. The bolt head comes to rest on the barrel breech and the little old .308 is dangling in there lacking enough length to seat aginst the shoulder of the chamber. If you are lucky enough to have a barrel that was machined a little on the short side, your rifle might shoot .308 all day. It's a grab bag. And also why if you buy a US made barrel chambered for .308 win, it probably is going to only work well with 7.62 NATO cartridges that happen to be sized a little on the short side.

    Barrel pressing on the DRB is done to set the barrel face at a point that allows for safe headspacing, by positioning the bolt head in the correct position to be locked back against the trunnion recesses.

    So, in short, there is no way to "change" the headspacing on a DRB action.

    If you press a DRB barrel too shallow, you will "create" too little headspace, and the bolt will be short of locking. If you press one too deep, you will create excessive headpsace, because the bolt will be locked too far forward. There is no "adjustment", so to speak, because the bolt head floats in relation to where it locks up each time. Replacing or changing worn rollers, bolt heads or locking pieces is only a way to keep the rifle properly headspaced, or to keep headspacing between GO and NO GO, Bolt gap is the measurement of that, not the adjustment.

    There are other glitches that pop up just to throw us all curves, things like cocking tube gap giving us false bolt gap readings, bolts ground off to create "feel good" bolt gap, (a technique used by those who have never hung around this sight much and don't know better). Bottom line is this is just an appetizer for information. The guys on this sight have an extraordinary amount of information on all these topics, if you have a question, ask. If you have some knowledge share it. If you have an adventure, include the rest of us. If you have naked pictures of your DRB post them up.
    Last edited by bladeworks123; 07-24-2010 at 03:48 AM.

  2. #2
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    Thanks Bladeworks123,

    I have been perplexed by this action since I first heard of it. Though I won't say I completely understand it, your tutorial appears to be very complete and worth further study. Definately sticky material.

  3. #3
    Otis61's Avatar
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    Good info bladeworks123!!!
    "Without gunpowder there can be no freedom!"
    -German-American proverb during the War of Independence

    COG# 1961

  4. #4
    Senior Veteran Woodman in MO's Avatar
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    right as...right as...right as...right as...

    This is going in the reference folder...
    "When shoes and clothes and food, when even hope is gone, we'll have the rifle."

  5. #5
    Militant Asshole Big Steve's Avatar
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    There is one way that you could check head space (or at least chamber length). The problem is, no one makes a tool to do it.
    What you would need is a Go Gauge with a rod attached that would protrude out of the end of the barrel. Then you would need like a sliding set collar that would go on your rod.
    Then you could just push the rod/go-Gauge back against the bolt head, lock the collar up tight to the end of the barrel. Then pull the rod out, seating the Gauge into the front of the chamber. Now measure the gap between the collar and the end of the barrel, add it to the go-Gauge length and you would have the chamber length.
    I was going to make such a tool one time but the Gauge was too hard to drill to attach the rod to it and I lost interest.


  6. #6
    holescreek's Avatar
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    Bladeworks, that is great! Your timing is perfect. I can use all of the dimensions provided to back figure the locking piece angle on my 39 build.

  7. #7
    kodiak's Avatar
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    Outstanding info! It's going in my reference notebook! Thanks bladeworks!


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    Senior Veteran The Great 308's Avatar
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    Thumbs up

    Thanks bladeworks that is awesome!!!

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    Bladeworks123: Thank You. I have wondered for a very long time how it really works. Read alot about it, but your pictures make it all come together for me. Excellent info. Will save it, because I've been concerned a long time about shooting Remington 308, thin case and not sure about the internal case head webbing. My smith told me to try some Federal, because the case and internal case head webbing is stronger in Federal 308. I'm holding my Winny White Box in reserve.

    Thanks again for the great intel!


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    Thank you, Bladeworks123 this is the most complete info I have ever seen on the web about these rifles, besides the video on youtube. This definitely the site for HK style rifles.

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