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Thread: gas leakage

  1. #1
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    Default gas leakage

    I just ran into a new (to me) problem at the range today. Using one bullet in particular I got sooty cases at one powder level and when I moved up one grain, the cases came out with a major crush indentation at the shoulder. (Sorry, no pics right now, but the cases are covered with soot almost down to the base and the shoulders are crushed in on one side.

    I've been testing loads for the new 257 Wby barrel that I recently installed on my Savage 116. I headspaced it carefully using go/no-go guages, and I ran about 40 bullets through it without trouble earlier this week. Here are the details of what happened today.

    I started out loading 100 grain flat base bullets, and ran about 40 bullets though the barrel, using four different powders at various levels. All the brass I've used so far is fresh, unfired factory brass. Since the Wby throat is quite long by design, there's no hope in reaching the lands so I seated the 100s about one bullet diameter into the neck of the casing. All 40 shot great, with several groups well under an inch, and the cases came out clean and showing no signs of pressure.

    Today I wanted to start working up loads using the Berger 115 VLDs. I loaded up four different charges of RL25, also into new factory brass. Since Berger recommends seating their bullets touching the lands I seated them as far out as I could. I was still nowhere near the lands (about .030 or more away) but due to this the bullets were less than one diameter into the neck of the case. They seemed secure, no give or wobble in the case neck, so I went to the range. My first two shots at 100 yards were almost touching, and the third not much further away. I was thrilled, but I noticed that the cases were coming out very sooty. No pressure signs, easy extraction, but sooty cases. I cleaned the chamber before the next set, which had one more grain of powder than the first. The first shot of the second set was normal, nothing odd I could notice. It extracted easily, but the case shoulder was crushed in. Puzzled, I decided to clean the barrel and the chamber thoroughly. After that I tried one more shot and the case was damaged/crushed in almost the exact same way, so I bagged it.

    I went back to shooting some more loads with the 100grain bullets and shot another 30 rounds before coming home. No soot, no dents, just normal everything, so it's not the gun. Since I was starting out fairly conservative with the Bergers, it's not pressure either. I shot some much hotter loads with the 100gr flat base bullets, no problems.

    So why am I getting gas leakage ONLY with the Bergers? I've never heard of or seen this, but maybe some of you have. Could it be the minimal seating depth, or is there something about these sleak boat-tails that's contributing.

    My first test will be to drop back down in charge weight some, and then seat the bullets considerably deeper to see if that solves the problem. I'm all ears to hear from some of you though!

    cheers,
    Archerdaddy

  2. #2

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    I get exactly the same thing when shooting a full case of H4831 and a 130 Sierra btsp in my 270. I get right at 3100ft/sec and shoot sub MOA. These bullets are seated pretty shallow as well.

    I don't worry about the soot on the case neck. I have had that happen on many other rounds in several different guns. I have even had it happen some with factory ammo!

    The dent in the case has me puzzled though. Is your chamber clean? Are you shooting rounds that still have case lube on them?

  3. #3
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    Default

    This is a not too uncommon problem with very large case capacity (low expansion ratio), slow powder and very low pressure loads. When a round is fired, one of the functions of the brass case is to expand and act as a gasket to plug the back of the chamber and also to expand the neck tight against the chamber wall neck and seal off any gas from getting between the case and chamber.

    When the ignition starts it moves the powder charge and bullet forward together. If the bullet leaves the case mouth before the powder charge is fully ignited, bottom to top, it will not have the expanding gas at the neck to expand the neck and close off the gap between case neck and chamber. This allows gas to get between the case and the chamber and sometimes it dents the case and leaves the unburned and partially burned powder residue on the case. When the bullets were seated deeper, the powder was allowed more time to burn to the top. Also an increase in the charge weight of powder will give more force to the process and will probably help too.

    I've have only noticed this with light charges of slow powder. I have experimented with seating depth and charge weight and duplicated this condition in several calibers then corrected it by increasing depth or charge. Also using slightly faster burning powder (like RL-22) helps eliminate it as well.

    The powder weight to bullet weight ratio is one of the problems. With the 257 Weatherby and similar (264 Win, 300 RUM, 7mm STW) we have this situation. We also must be cautious of under charging these cases. Some say the concern is too much air space but it really has nothing to do with that but we want to avoid light charges of slow powder. You should never load a starting load that is less than the recommended starting load in your manuals. Add some powder and seat deeper or, another aid to stopping this is use heavier bullets. I realize you want that bullet but you do need more bullet in the case. You're on the right track with the caliber of bearing surface, I do believe that is an aid to accuracy, giving consistant bullet pull just as a tight fit in the case helps. Good luck with it an dlet us know how it works out.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



  4. #4
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    Default

    You are losing case tension on the bullets by seating them out so far. Seat them deeper and do NOT worry about touching lands or getting closer to them. Your bullet is coming out too soon, most from just primer pressure.

  5. #5

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    I put dents and have sooty cartridges in 8X56R and charges of H4831.
    Sooty cartridges, same powder in 7.5X54 and 45/70 with cast bullets.
    Like Murphy says, low pressures, which do not allow the cartridge case seal the chamber, are the culprit. Thanks!

  6. #6

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    Quote Originally Posted by Murphy View Post
    This is a not too uncommon problem with very large case capacity (low expansion ratio), slow powder and very low pressure loads. When a round is fired, one of the functions of the brass case is to expand and act as a gasket to plug the back of the chamber and also to expand the neck tight against the chamber wall neck and seal off any gas from getting between the case and chamber.

    When the ignition starts it moves the powder charge and bullet forward together. If the bullet leaves the case mouth before the powder charge is fully ignited, bottom to top, it will not have the expanding gas at the neck to expand the neck and close off the gap between case neck and chamber. This allows gas to get between the case and the chamber and sometimes it dents the case and leaves the unburned and partially burned powder residue on the case. When the bullets were seated deeper, the powder was allowed more time to burn to the top. Also an increase in the charge weight of powder will give more force to the process and will probably help too.

    I've have only noticed this with light charges of slow powder. I have experimented with seating depth and charge weight and duplicated this condition in several calibers then corrected it by increasing depth or charge. Also using slightly faster burning powder (like RL-22) helps eliminate it as well.

    The powder weight to bullet weight ratio is one of the problems. With the 257 Weatherby and similar (264 Win, 300 RUM, 7mm STW) we have this situation. We also must be cautious of under charging these cases. Some say the concern is too much air space but it really has nothing to do with that but we want to avoid light charges of slow powder. You should never load a starting load that is less than the recommended starting load in your manuals. Add some powder and seat deeper or, another aid to stopping this is use heavier bullets. I realize you want that bullet but you do need more bullet in the case. You're on the right track with the caliber of bearing surface, I do believe that is an aid to accuracy, giving consistant bullet pull just as a tight fit in the case helps. Good luck with it an dlet us know how it works out.

    IMO, Murphy is 100% right on. I have only seen this happen with loads of very slow burning powders that had too low of pressures to make the case expand and seal off the powder gases from flowing back between the case and rifle chamber. Either that, or maybe some of the powder gases flow back down the case just before the pressure peak gets high enough to seal off the chamber. The problem doesn't seem to occur as much with faster powders.


    Of course (CMA), I'm not saying you should increase your powder charges...


    Marshall/Ak

  7. #7

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    In my case with the 270 I am shooting a case full of H4831 and not a light load. Part of my problem is that these cases are at the end of there case life and have been trimmed to length but I've not turned the outside of the neck. With each firing the brass just naturally flows forward resulting in an increase thickness in the neck and the brass becomes more brittle. Both hamper the neck expanding to seal the chamber! Lazy I guess. The next batch reloaded will start with brand spanking new nickel cases.

    I am going back to Core Locks as opposed to boat tails for perfomance reasons. because of the structure difference in these two bullets there will be more bullet surface for the case to hold onto. I spect the powder residue will go away with these changes! If not all will still be well!

  8. #8
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    Default a few details to add

    I agree with you, although in this case I think, especially after reading your replies, that it has almost entirely to do with the seating depth. I pretty much had it pegged as the neck not sealing the gas off, but there were a couple of complicating details that made me go hmmmm.

    For example the bullets I was having trouble with were 15% heavier, not lighter, than the ones that worked just fine. (hmmmm) Also, I tested RL 25 with the 100 gr bullets too, and didn't have any gas blow-by there. (hmmmm) Light powder charge was definitely not the problem, since I was within a grain of what Berger lists as max for their 115gr bullet. In addition to this, I got only soot with the lighter charges, and then dented cases when I went higher. (hmmmm) It all goes to show how fine a line we sometimes walk, and that there isn't one factor or formula that will always outweigh the others. Fortunately, in this case all it cost me was two outrageously expensive Wby cases. By the way, what do you think: would it be safe to blow the dented shoulders back out with a light charge of fast powder behind a DEEPLY seated bullet, or should I just write them off? (Sorry, I come from a fix-it family, and from a generation that hates to waste anything!)

    By the way, from my preliminary work using the 100 gr bullets, RL22 put the highest numbers on the chronograph, but RL 25 did nearly as well and produced more consistent groups. H4831 is promising as well. From my research beforehand, I expected IMR 7828 shine, but although it did reasonably well, it was unspectacular in my 26" Pac-Nor barrel. I'm far from finished though! I still plan to test the most promising powders at different seating depths...deeper, not further out!

    Once I have finished breaking in the barrel, I'm also going to test some 100 gr Nosler Ballistic tips and Partitions. I'm also interested in the 110 grain Accubonds, which seem like a really nice combination of weight, speed, toughness, and BC. They won't be the highest in any one of those categories but they do well in each, which is nice. The Bergers have the best BC out there and a good reputation as an accurate bullet, but are probably a bit fragile for anything bigger than a deer.

    I expect RL 25 to be a top contentender for the heavier (110 gr and up) bullet weights. It sure was the cat's meow for 160 grainers out of both of my 7mm Rem Mags. In both cases, the moment I tried it speeds went up, and group size and ES went down.

    I'll report back as I go! Isn't this fun?

  9. #9
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    Default

    This is fun and you have a handle on it. It is all a game of experimentation with a long list of minor variations of several aspects of components and assembly technique. The cook book method will get you in the ball park but rarely does that produce the best results out of the gate.

    Also you have a caliber there that will always need special attention, a special needs kind of cartridge. That is more challenging and more rewarding when you arrive at the capabilities of the cartridge.

    Even though the case size vs bore size of the cartridge warrants the use of the ultra slow powders such as IMR7828 and RL-25, I rarely find them to give the best accuracy, but usually the best velocity. So we work around the components until we can accet the compromise. The 257 and 7mm WBY use the same case. I made a custom heavy barrel long gun on an old FN single shot action on that case necked to 264, with a special throat for the 140 grain Sierra Matchking bullet. My intent was to shoot the long line matches but moved away from the local range and never shot it at target beyond 400. I was a real shooter with the Dan Lilja barrel but I had a heck of a time getting the velocity where I wanted it, it is a fine line to walk. Not over pressure and still up to it's capabilities, a narrow envelope for sure. I learned a lot about powder and low expansion ratio cases. I have loaded for the 257 and 7mm but haven't owned a gun in either caliber. I've developed load data for the 7x300 Wby and the 6.5x300 WBY since then and the 8mm Remington case wildcats of 25 and 26 bore. I did own a 240 once and actually got to like it's capabilities. It is the 30-06 size belted case with the Weatherby shoulder.

    By the way the shoulder shape of the Weatherby design is meant to maintain the charge gas vortices inside the case neck, rather than out in the throat. This is supposed to reduce gas errosion in the throat of the rifle. I can't say whether it does but the theory of it makes sense to me.
    I do believe it does make a less effective gas seal at that shoulder area and this seems to contribute to the shoulder callapse from gas leaking around the neck. I've seen it much more on the Weatherby style case. Also you can revive those shoulder collapsed cases, just load as normal but don't seat a bullet. I just take a case of powder and point the rifle up and fire the powder down the barrel, note the recoil with no bullet. This is fireforming and those hard powder kernals do what they do with a bullet on top. Push their way through that skinny neck and press the case out to fit the chamber. The case will be sooty but will fill out and can be reloaded with a bullet this time. If the cases are old and brittle from many firings this may not work or may split but it's worth a try. Don't fire straight up, the kernals sting a little when they hit your head. Sometimes they end up in your underwear.
    Is there nothing so sacred on this earth that you aren't willing to kill or die for?



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