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Thread: why do shots wander from hot barrels?

  1. #1

    why do shots wander from hot barrels?

    It seems quite common that shots in a group will start to wander as a barrel heats up.

    I have one rifle that does it - not horribly, but they will start to string vertically up to about an inch and a half if I shoot more than about 3 in a row. If I let it cool for 5 mins, the next shot is always perfect again. Neither of my other centrefires does it.

    I've come across a confusing array of explanations for why it happens. I'd be curious to see what people's opinions are here. I should add that I'm not looking to correct it, since I've never taken more than 2 shots in a row when actually stalking. I'm just curious - especially since this is a Heym, so comes glass bedded from the factory.

    Thoughts?

  2. #2
    Hi Mungo.
    I would say that the barrel is expanding and touching the stock causing deflection.

  3. #3
    For the same reason that there have to be gaps between the rail sections on railway lines - when metal gets hot it expands and the thinner the barrel tube the less predictable the pattern of expansion and thus the greater the potential for shots to wander on the target.

  4. #4
    It can be a stocking / bedding issue. However it's usually because the heating releases major stresses imparted in manufacture when the 'blank' was bored through and even more so when it was rifled. Most major manufacturers use cold hammer forging nowadays in which multiple hammers swage the bored blank onto a carbide mandrel belting the components thousands of times. It creates enormous stresses in the component and even a degree of straightening and destressing afterwards often won't remove them entirely. Also, barrels don't heat up evenly throughout their length affecting the expansion / contraction issue already mentioned. The lighter (thinner) the barrel, the faster it heats and causes problems if prone to them.

    http://firearmshistory.blogspot.co.u...er-forged.html

    and here's an example in this country, Armalon Limited which uses the one-time Parker-Hale barrel forging machines in updated form

    http://www.bing.com/videos/search?q=...784A972233E8E4

    Heavy (fat) match quality barrels made by people like Krieger, Broughton, Benchmark, Border Barrels, True-Flite, Lilja and others don't suffer these problems partly through being fatter and stiffer, but also because they are made in small production output machine shops that use methods which don't stress the barrel blank as much and/or are fully destressed after the rifling is imparted and before final work. The least stressful way of rifling a rifling blank after boring is the cut-rifling method that takes a tiny sliver out of a single groove, then indexes the barrel to the next groove before the cutter makes another pass and so on several hundred times to get 0.008" deep grooves. Most match barrelmakers now use the button rifling method which incurs more stress and needs destressing afterwards.
    Last edited by Laurie; 11-11-2014 at 01:22.

  5. #5
    My A bolt in .308 is far more accurate than I can ever be. At my best, from a bench rest I can shoot three bullets touching at a 150 yards or so (you call this cloverleaf or something?). No matter what I do, after shooting about six or seven bullets in quick succession, the 'group' resembles a shotgun pattern. We are talking feet apart. The barrel gets very hot (extremely hot after ten shots or so). Too hot to touch, actually. I have come to accept it, as the A bolt is a very lightweight rifle, designed for stalking, hence you are not expected to shoot more than a bullet or two a time anyway. I am not worried about it, and so I do not shoot more than three bullets in a short period of time, as I feel it is pointless and counter productive (since the 'shotgun pattern' is a certainty).

  6. #6
    I have always assumed that a hot barrel expands slightly, thereby increasing its contact on the bullet and slightly increasing pressure, which results in vertical stringing.
    Last edited by gunner269; 11-11-2014 at 09:18.

  7. #7
    Quote Originally Posted by gunner269 View Post
    I have always assumed that a hot barrel expands slightly, thereby increasing its grip in the bullet and slightly increasing pressure, which results in vertical stringing.
    Being a cylinder wouldn't the expansion be outwards? Therefore decreasing grip on the bullet?

  8. #8
    Quote Originally Posted by palmer_mike View Post
    Being a cylinder wouldn't the expansion be outwards? Therefore decreasing grip on the bullet?
    In theory the barrel would expand to wherever there is a void. This would be inside the barrel as well as outside so should be slightly tighter on the bullet if anything.

  9. #9
    Quote Originally Posted by ChrisWill184 View Post
    In theory the barrel would expand to wherever there is a void. This would be inside the barrel as well as outside so should be slightly tighter on the bullet if anything.
    Not convinced that this is correct.

    one be example that comes to mind is heating up metal tyres for cart wheels to get them on, then cooling them down to tighten them up.......
    Last edited by palmer_mike; 11-11-2014 at 08:48.

  10. #10
    Blacksmiths & heating cart wheel steel tyres spring to mind. In that case the hole gets bigger. The same applies to bearings. Heat is applied to fit them to shafts etc. Go figure.
    Ian

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