Powder Bridging for Kiwi Nate & Tang et al

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by royinidaho, Feb 17, 2010.

  1. royinidaho

    royinidaho Writers Guild

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    I read and responded (PM) to Tang of my powder bridging experience. I didn't know the cause but an email to Kirby brought me up to date. (He made a side comment that the 721 Rem held up to that test.)

    I also read kiwi's post with the purdy pics.

    Not to highjack the other thread I have some questions.

    I understand the post regarding 'detonation' (I saw it occur once with a 22-243 and 4831 or it may have been an untimely reflected pressure wave. Big boom and rifle in pieces. Impressive! No one hurt)

    Bridging may be a bit different than detonation. At least to this simple mind (mine).

    My problem in this case seemed to be too much powder trying to follow the bullet down the barrel and "log jamming" at the base of the neck. It would seem that this would/could cause 'detonation' of the bridged powder as described by Kiwi's pics.

    From my use of the RSI pressure lab I would estimate the pressures (didn't use the RSI for these shots:rolleyes:) to be in the very high 70s to lower 80s (k psi) Thus no come aparts or recoil lug problems.

    I do shoot about 84 grains of 7828 ssc behind a 150 Nosler with great results. But for heavier bullets I stick with the ball powder.

    Additionally I'm confident that all problems would be taken care of with a forward ignition system (flash tube which I've experimented with a bit but the small RUM cases are too small with too high of operating pressures in extreme magnum loads to be of much benefit)
     
  2. Kiwi Nate

    Kiwi Nate Well-Known Member

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    Your description of 'log jamming' sounds feasible. I have loaded plenty of cartridges to the brim followed by the big crunch and never had a problem but it would be ignorant to assume that such a problem doesn't or couldn't exist. All I can go on is my own experiences. I could only guess that it is a problem that is more likely to occur in .224-6.5" high power cartridges due to the physical size of slow burning powder kernals versus the inside neck diameter of the case.

    The problem I was trying to address was relative to the underloading of fat case/ small bore ratio cartridges. The 7mm RUM produces huge problems below 90% capacity. This is definitely going to catch out a few people over the next few years and is something that has not been addressed by Remington.

    As another example of directional ignition or detonation as it has been called, Australian gun writer Nick Harvey built a .25WSM a few years ago and had similar problems. The powders/ loads that filled the case to 95-100% full were safe, anything less was dangerous.

    I have twice had clients bring locked up rifles to me from one other mistake pertaining to powder. In both instances, the wrong powder was used, a fast burning powder, loaded to 90% capacity in (by coincidence) two seperate .30-06 rifles. The bolts were locked up, brass ruined but otherwise, the rifles survived- just.

    Its an nteresting topic, I hope someone else can chime in with their own experiences with pressure problems.

    Nate.
     

  3. royinidaho

    royinidaho Writers Guild

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    Nate,

    You are spot on with your observations.

    I also think there are some conditions/experiences not yet mentioned and hope other chime in.

    I've also crunched powder into a case even to the extent that the cartridge was difficult to chamber from expansion during bullet seating. It took a lot of neck tension to hold the bullet. All went well but it was better to select a more proper powder.

    Roy
     
  4. MontanaRifleman

    MontanaRifleman Well-Known Member

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    I dont know if this relates but I'll chime in. I decided to experiment with RL17 in my 300 RUM since I had got some very good results in a couple of other cartridges. I wanted to see how fast it might push the 180 E-Tips. There was absolutly no load data so I looked at H4350 for comaprison and decided that 80 gr would be a good consrvative starting point. I loaded up 11 cases in 1 gr increments up to 90 gr and started firng, hoping I would get to 90 because that should give me about 90% capacity. When I got to 87 the chrony read about 3440 (26" barrel) and I had a slight sticky bolt. I wasn't sure if this was a pressure sign or just sticky brass because some of my brass had been a little tight chmbering after being fired a few times. So I went up to 88 and let it go. The chrony showed a whopping 3500 fps. Then I treid lifting the bolt - no go. I happened to have a hammer with a plastic handle in the car and used the handle to tap up the bolt. With some effort, I was able to pull the bolt back and both the primer and ejector fell out. It cost $25 to get the ejector reinstalled and fortunately that was the only damage. It seemed like a very sudden spike of pressure from the previous light bolt lift, but I was going up a full grain and the E- Tips are said to overpressure quickly when they do overpressure.

    -Mark

    Correction: I went up to 88 gr from 87 gr
     
    Last edited: Feb 18, 2010
  5. Kiwi Nate

    Kiwi Nate Well-Known Member

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    Thats a tough one because it probably involved both directional ignition yet max pressures had the load burned uniformly. In other words, with that powder burn rate, you wouldn't be able to step up a few grains.

    The 87 grain charge is right on that low point I was talking about, even if you had been using Retumbo.

    I should mention, in my 9.3-300RUM and .375RUM, I can download all bullet weights to 2400fps with H1000 (Optimum burn rate for these medium bores is H4350). It would seem that wider the bore in the RUM's, the lower the danger of directional ignition at charge weights below 90 grains.
     
  6. royinidaho

    royinidaho Writers Guild

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    It sounds as if I may inadvertently experience some severely adverse results by downloading to simulate 1K impacts at 100 yds.

    Me thinks I'll just pay the price of safety and shoot the full load at the 1k mark.

    Starting to get some valuable information.

    Kiwi, keep it coming.
     
  7. brianwinzor

    brianwinzor Well-Known Member

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    I believe that I have experienced one instance of powder bridging. It occured in my custom .257Weatherby when using Viht 20N29 powder with the Wildcat 156 grain ULD.

    I had been using H870 and AR 2218 (H50BMG) with the 156 grain Wildcat, but was reaching maximimum pressure at 70.0 grains. This represented a loading density of just 82% with H870, and 86% with AR2218, which is way less than the 95% plus that I prefer, especially when using large cases and very slow buring powders.

    After some research in various burning rate charts and the ADI & Vihtavouri loading manuals, it appeared that Viht20N29 was slower burning than H870 & AR2218, so I decided it was worth a try. I started with 69.0 grains of Viht 20N29, and my first and only shot produced smoke and a jammed bolt, that required my rubber mallet to open it. I immediately wondered whether I had a batch of 20N29 that was much faster burning than normal, or whether it was my first experience of powder bridging.

    I contacted Kirby Allen, and he believed it was an instance of powder bridging, but I was not completely convinced as I had already fired over 200 shots with AR 2218 with the 156WC with not a hint of spiking pressures. I also used AR 2218 in my .224 Clark with the 100 grain WC without any hint of spiking pressures.

    After measuring the granule size of both AR2218 and Viht 20N29 powders, I found that Viht 20N20 was slight longer but thinner. However, there did not seem to be a dramatic difference between the two, as AR 2218 measured about .080 in length and .060 in diameter, while Viht measured about .093 in length and .052 in diameter.

    About a fortnight ago I tried AR 2218 and Viht 20N29 in my newly acquired 300 RUM and there was not a hint of spiking pressures when using 100% loading density loads with both the 180 grain Nosler BT, and Hornady 208 grain Amax.

    I will now try and find some 240 grain Sierra MK's to see what difference there is in burning rate between the two powders, and hopefully experience no powder bridging problems.

    It is certainly interesting to hear the circumstances of Kiwi Nate, royinIdaho, and others who have experienced possible powder bridging. Regards, Brian.
     
  8. Kiwi Nate

    Kiwi Nate Well-Known Member

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    Well, I can qualify my 7mm RUM experiences as being consistant if thats any help. Thing is, this was a .200" freebore chambering rather than the factory .400" freebore. I forgot to mention this in the beginning and I am very sorry about this, I had forgotten. I am not one to BS people so- my bad.

    The range testing went like this (velocities are rounded to nearest 50fps):

    162gr SST 86gr Retumbo 3100fps
    162gr SST 86gr Retumbo 3300fps bolt jammed.
    I thought that I had stuffed up my reloading so fired another shot:
    162gr SST 86gr Retumbo 3100fps
    Then another:
    162gr SST 86gr Retumbo 3300fps bolt jammed.
    Went back to the bench, cleaned the scales (thinking that they must be sticking). Loaded another few rounds:
    First shot 162gr SST 86gr Retumbo 3300fps bolt locked up. No further testing done at 86gr- lesson (finally) learned.

    Increased load to 90 grains Retumbo 3200fps. Max pressure, poor case life.

    Freebore was increased to .400" as per factory specs and I started again with 91 grains Retumbo. Muzzle velocity was 3235fps.

    Current pet load: 91gr Retumbo 180grVLD 3184fps. Groups .5MOA without brake, .3MOA with brake. I shoot it without the brake as I can't stand the noise and gas/ground debris blow back (spiral ports).

    As for H50BMG. 94 grains behind the 162 grain bullet yeilds 2800fps, safe load. Can only guess that it could be reduced to 91 grains for 2700fps with regard to testing 162-180 grain bullets at reduced velocities. Would still have to take a 300-400 yard shot to duplicate 1000 yard shots.

    It takes 102-104 grains of H50BMG to get the 7RUM up to pressure/ accuracy. Unfortunately, at these charge weights, its no different than hitting the bore with a gas cutting torch.
     
  9. Tang

    Tang Well-Known Member

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    The lightest charge I've run in my 300 RUM is 83gr of RL22 behind a 180. That is around 89% load density.

    I think Im going to try 97gr of Retumbo behind a 180 Interlock, and see where accuracy goes.
     
  10. MontanaRifleman

    MontanaRifleman Well-Known Member

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    I would work up to that. 97.5 gr of Retumbo is max behind my 180 E-Tips.

    -Mark
     
  11. royinidaho

    royinidaho Writers Guild

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    3 seems to be the magic number for attempts.:D I think that's all the good Lord gives us.......
     
  12. RockZ

    RockZ Well-Known Member

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    Try US869 for the 7mmRUM.
    You won't have a bridging problem and will get good velocity as well.
    It fills the case well and is very consistent.
    My buddy has a 7mmRUM and we have been working up loads with his rifle.
     
  13. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    What do you think about using an inert material on top of the reduced load for short range long range testing. Like corn meal, to keep the powder column from laying down in the case.

    Just a thought.

    Steve
     
  14. MontanaRifleman

    MontanaRifleman Well-Known Member

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    I'm glad it's just a thought :)