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Author Topic: Looks Can Be Deceiving  (Read 4203 times)

SuperNaut

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Looks Can Be Deceiving
« on: September 19, 2010, 06:11:48 pm »
While also being right on.

So I just finished loading a bunch of 308 with Hornady Nickel cases and 168 gr Match BTHP @ 2.8000 COAL, and I moved on to some Hornady brass cases and 165 gr SST w/cannelure @ 2.7280 COAL.  As I was finishing up the first cartridge of the SST it looked and felt WAY too short.  So I pulled out my calipers and measured them and they were exactly as they should be.  The shape of the SST and the .072 difference in COAL makes the SST look really short.  But when you put them side-by-side they aren't that far off. 

It is amazing to me that we humans can sense a .072 difference in length.


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    xsquidgator

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    Re: Looks Can Be Deceiving
    « Reply #1 on: September 19, 2010, 06:24:05 pm »
    While also being right on.

    So I just finished loading a bunch of 308 with Hornady Nickel cases and 168 gr Match BTHP @ 2.8000 COAL, and I moved on to some Hornady brass cases and 165 gr SST w/cannelure @ 2.7280 COAL.  As I was finishing up the first cartridge of the SST it looked and felt WAY too short.  So I pulled out my calipers and measured them and they were exactly as they should be.  The shape of the SST and the .072 difference in COAL makes the SST look really short.  But when you put them side-by-side they aren't that far off. 

    It is amazing to me that we humans can sense a .072 difference in length.

    Yet another example of how the proper tools, including calipers, are needed! 

    72 thousandths of an inch is much larger than the minimum difference someone with good eyesight can make out.  I worked in a machine shop for several years, and once you get used to things you can easily see differences down to a couple of a thousandths, like 0.003", at least if the two things are right next to each other.  But of course you always use instruments, as you say the eye can be fooled!

    RandySBreth

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    Re: Looks Can Be Deceiving
    « Reply #2 on: September 19, 2010, 09:37:09 pm »
    It all depends on the design of the bullet, and if I remember correctly, Hornady uses the "Secant Ogive" in most of their rifle bullets. That makes a shorter bullet for it's weight. Usually.

    It also makes for a shorter bearing surface on said bullet, so sometimes you can push it faster than a competing one per given caliber and weight, i.e. like one of my pet 7mm/08 loads that drives a 139 Hornady BTSP to about 2950 fps from a 22" barrel.

    Another thing- the BTHP was developed before the "tipped" bullets, it was all they had back in the day. Now we can have a more aerodynamic bullet (like the SST) with a higher B.C. that will still expand way out there, unlike the BTHP design, which usually doesn't at lower velocity. Better.
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    Re: Looks Can Be Deceiving
    « Reply #3 on: September 20, 2010, 01:07:30 am »
    Another thing to note that RSB's picture points out extremely well is why there can be such a drastic difference in powder charges between slugs with little weight difference. 

    The difference in a particular projectiles bearing surface has a pronounced affect on the pressures generated when you fire the gun because of the varying level of friction in the barrel caused by the length of the bearing surface.
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    cpaspr

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    Re: Looks Can Be Deceiving
    « Reply #4 on: September 20, 2010, 04:22:04 pm »
    72 thousandths of an inch is much larger than the minimum difference someone with good eyesight can make out.  I worked in a machine shop for several years, and once you get used to things you can easily see differences down to a couple of a thousandths, like 0.003", at least if the two things are right next to each other.  But of course you always use instruments, as you say the eye can be fooled!

    25 years ago I worked in the office/shipping room of a screw-machine shop which made millions of parts a year from just a few diameters of steel and aluminum.  I did all the inventory counts, both in, and out, so I was viewing the products twice.  It got to the point where I could easily tell what the diameter of a bar was just by looking, and without having another bar to compare to:  9/32", 5/16", 21/64", 11/32", 23/64", 3/8".   Under 16/1000" between all but the smallest two.  I can still do it most of the time all these years later. 

    Get me beyond 1/2" though, and I'm not quite as good at estimating.
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    xsquidgator

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    Re: Looks Can Be Deceiving
    « Reply #5 on: September 20, 2010, 09:45:10 pm »
    25 years ago I worked in the office/shipping room of a screw-machine shop which made millions of parts a year from just a few diameters of steel and aluminum.  I did all the inventory counts, both in, and out, so I was viewing the products twice.  It got to the point where I could easily tell what the diameter of a bar was just by looking, and without having another bar to compare to:  9/32", 5/16", 21/64", 11/32", 23/64", 3/8".   Under 16/1000" between all but the smallest two.  I can still do it most of the time all these years later. 

    Get me beyond 1/2" though, and I'm not quite as good at estimating.

    Yes, your experience is kind of what I meant.

    Another lesser example is picking empty brass of the ground at the range.  When I started they almost all looked the same.  Enough "training" of the neural pathways and pattern recognition, and bam!  Instant recognition and differentiation between 40SW and 9mm (a 0.045" or so difference in case diameter).  Not  based entirely on size but on the different shapes to the case heads, but still, kind of neat how with enough practice and "training", one can immediately home in on small differences in appearance.

    cpaspr

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    Re: Looks Can Be Deceiving
    « Reply #6 on: September 21, 2010, 01:20:34 am »
    Yes, your experience is kind of what I meant.

    Another lesser example is picking empty brass of the ground at the range.  When I started they almost all looked the same.  Enough "training" of the neural pathways and pattern recognition, and bam!  Instant recognition and differentiation between 40SW and 9mm (a 0.045" or so difference in case diameter).  Not  based entirely on size but on the different shapes to the case heads, but still, kind of neat how with enough practice and "training", one can immediately home in on small differences in appearance.

    Yep.  Exactly.  You're getting real good when you can spot .380s in a bunch of 9mms.  Now if only we could figure out how to spot berdan primed brass vs boxer primed brass without having to look inside.   :hmm
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    Outbreak

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    Re: Looks Can Be Deceiving
    « Reply #7 on: September 24, 2010, 08:16:49 pm »
    Yes, your experience is kind of what I meant.

    Another lesser example is picking empty brass of the ground at the range.  When I started they almost all looked the same.  Enough "training" of the neural pathways and pattern recognition, and bam!  Instant recognition and differentiation between 40SW and 9mm (a 0.045" or so difference in case diameter).  Not  based entirely on size but on the different shapes to the case heads, but still, kind of neat how with enough practice and "training", one can immediately home in on small differences in appearance.

    I still have trouble with that. If it's all in my brass can and I'm sorting in my garage, I can separate them pretty easily. On IPSC night at the local indoor range, by the time my targets are scored and I get back to the line, someone has swept my brass into the pile-o-brass out of the way, and I'm stuck trying to sort my .40 brass from everyone's 9mm, .40, .38 super, 10mm....I'm checking headstamps. Luckily, I'm the only guy who shoots .40 and doesn't reload it yet, so mines the shiny brass.
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    cpaspr

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    Re: Looks Can Be Deceiving
    « Reply #8 on: September 30, 2010, 01:27:25 am »
    Outbreak, once you do start reloading, you might try what I do to keep my brass mine.  Once I get a box reloaded, I'll run a sharpie both directions across the primers on all the rows before I close it up.  Makes a nice X on the bottom of your brass.

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    Re: Looks Can Be Deceiving
    « Reply #9 on: September 30, 2010, 08:02:02 pm »
    Working on machine brakes and couplings, I can usually judge gaps or alignment within a few thousandths.  I still use the tools to make sure.  Inspecting beams and rails I can usually spot an alignment deviation as small as 1/4" in 40 feet.  Not enough to be certain, but enough to be suspicious, and usually verified by laser or string.  Lighting and reflection can make a huge difference.

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