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September 26, 2021, 12:56 PM | #26 |
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Are you saying that a barrel that is looser will not get a bullet moving faster at lower pressures?
I can think of friction difference between a hammer forged and a button rifled barrel (cut rifle as well but those are much rarer) If so, substantially different? I need to look at my data and see what different there was between a 270 Finnbear barrel and a Shilen barrel firing the same ammo (length difference of a couple of inches)
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Science and Facts are True whether you believe it or not Last edited by RC20; September 26, 2021 at 01:03 PM. |
September 26, 2021, 02:06 PM | #27 |
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Back when moly-coated bullets first became a craze, everyone noticed the lower friction caused about -20 to -50 fps loss in velocity with their favorite loads, and that bumping powder charge up half a grain in the 30 cal match rifles was commonly needed to get the velocity back again. It's because the powder needs confinement to build pressure against to advance its burning rate, and the effort to start the bullet moving and get it into the lands provides some of the resistance that pressure builds against (the chamber size and bullet mass inertia providing the rest). If the bullet goes down the tube too easily, expansion behind it happens at lower pressure and earlier in the powder pressure curve. Then the powder doesn't quite burn fast enough to make gas fast enough to keep up with that rate of volume growth. As a result, the average pressure is lower so the average bullet kinetic energy is lower, and velocity will drop as the square root of the drop in kinetic energy. You can achieve a similar pressure curve effect by reducing bullet weight without changing the charge. It still shoots, but the pressures and the resulting bullet kinetic energy are lower.
A favorite quotation of mine: "First contemplation of the problems of Interior Ballistics gives one the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity."
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September 26, 2021, 02:58 PM | #28 |
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Ok, phew.
The hypothesis is that a Hammer forged barrel has more friction as there is more surface in contact with the bullet. So, a button rifled barrel would have less (rougher but less contact) and variations depending on how much lapping was done of course (I had an XC barrel that clearly was rougher than the Shilen barrels I use for the rest ). XC smoothing out over a lot of shooting. So if you started with a Button Rifled barrel and load then the Hammer forged should have a higher pressure? Somewhere in there length plays a part in velocity as well (or so I think regardless of the initial pressure peak which should take place early in the barrel ?) I think I am hurting my bran now.
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September 28, 2021, 01:26 AM | #29 |
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You may relax by contemplating how many angels can dance on the head of a pin.
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September 28, 2021, 02:10 AM | #30 |
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I read the OP’s comments of how quickly does sticky bolt progress as asking how quickly does sticky ball go from slightly sticky to kaboom . It’s always been my understanding once you start getting sticky bolts you’re on your way to Kaboom So never accept sticky bolts as a place to indicate elevated pressures but you could keep going . Short answer sticky bolts bad haha
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September 28, 2021, 12:46 PM | #31 | |
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There's really no way to answer this because "Sticky" bolt lift is going to be different in every different gun and ammo component combination.
You simply cannot say "it happens at X.x level" because it does not. Likewise, you can't say it is ok at this level and gets sticky at .3gr more, or anything like that, UNLESS you are referring only to one specific rifle and load combination. Change even just one factor and the point where bolt lift gets sticky can change. And, that's assuming you and I both agree on what is, and isn't "sticky". Quote:
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September 28, 2021, 01:45 PM | #32 | |
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Quote:
Last edited by robinny; September 28, 2021 at 01:58 PM. |
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September 28, 2021, 02:40 PM | #33 | |
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Quote:
I'd say you are not likely going to miss sticky bolt and go straight to Kaboom unless you are going up in very large charge increments for the cartridge . I like 1% increments , example if the likely charge weight will be in the 30gr area then test in .3gr increments , 40 grain area .4gr increments 50gr in .5gr increments . Another way is 223 in .25 to .3gr increments , 308 in .4gr , 30-06 in .5gr etc etc When there are several grains between min/max I like to start low but only load a couple in the first charge increments . I do this so I am starting at min charge to be safe but I also know it very unlikely those low charges are where I will end up as my best load . So as the charges progress I start loading more rounds per increment . Looks something like this
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September 29, 2021, 08:07 PM | #34 |
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I shoot a pressure ladder. I shoot 1 round at each charge weight at separate bullseyes. I do this to see a sweet spot where multiple charge weights shoot to a similar poi. I separate charges into 1% of max increments.
I find there is a pretty fat range of similar poi’s around max or just under. Then as I go over max, I get into flat primers, primer flow, high velocity and heavy bolt lift. You do need to size cases to fit into your action with small clearance to see proper expansion and spring back. I find checking mulitiple check items keeps me from missing over max and dialing back to a sweet spot. |
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