Case capacity vs. accuracy

Discussion in 'Reloading Forum (All Calibers)' started by RonS, Dec 5, 2018.

  1. RonS

    RonS

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    Gentlemen, I was asked a question yesterday about changing from one brand of brass to another by a fellow shooter and beginner reloader. His question to me was, why the big change in his MOA and/or group size when he switched from new Lapua to new Hornady brass, using the same lots of bullet, primer and powder in his 6.5 Creedmoor and 308 Win? He loaded the Hornady brass the exact smae as he did the Lapua cases. I did not want to give him any wrong info so I thought I'd ask a more knowledgeable community. I understand the difference between case capacity and pressure but I myself don't understand why the change in group size. He is somewhat of a fanatic when it comes to his style of shooting, since its a hunting rifle he waits ten minutes in between shots. He cleans the barrel after every 75 shots and does not use any type of metal brush in the bore. He says it takes 5 shots after cleaning to get his gun to shoot to point of aim on the first cold bore shot. Thanks in advance for your comments
     
  2. SPJ

    SPJ Silver $$ Contributor

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    when I changed brass the first thing I noticed was the neck wall thickness was different, bullet hold can effect group size.
    Any component change leads me to revisiting tune, starting with a short ladder then seating followed by neck tension and possibly even primer test to confirm optimum load.
    You have already mentioned case capacity difference i.e. Pressure difference.
    Just my 2 cents
     
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  3. Bc'z

    Bc'z Gold $$ Contributor

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    What Jim said.
    I did a small sample test last year
    "Inconclusive to most here"
    2 different die makes 1/2 crimped 1/2 just seated. Every thing else stayed the same.
    Weighed all charges, same lot of primers, same brass, and bullets, all cases measured and trimmed to length.
    4 completely different groups.
    What I learned is if change even 1 part of original load recipe "no matter how small"
    You've changed your load.
    You'd be surprised even .001 neck tension has an effect on target.
     
  4. KevinThomas

    KevinThomas

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    Offhand, I’d say you’re more than likely looking at some differences in the brass itself, and not just a capacity issue. Other factors such as brass hardness, neck tension and neck wall concentricity (major factor there) are the probable answers.

    Bottom line as always, you’ve changed a component, which means you’re essentially starting from ground zero.
     
  5. savagedasher

    savagedasher

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    You won’t have good Es without having the same capacity
     
  6. dedogs

    dedogs Gold $$ Contributor

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    Did the fellow shooter chronograph loads with different brass? Therein might lie the answer. Case capacity differences between different manufacturers is common. My belief is that these differences can cause significant changes in velocity. If there is a significant change in velocity it can, at times move the load off it's accuracy node. dedogs
     
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  7. ShootDots

    ShootDots Gold $$ Contributor

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    I believe that dedogs is on the right track. I would add something by asking a question and offering a comment. QUESTION: Was the Lapua brass with SMALL rifle primer pockets? I know that the Hornady only has LARGE rifle primers. COMMENT: The difference can be vast! I went from LARGE rifle primer Lapua brass in a .260A.I. to the Lapua "Palma" SMALL rifle brass. The differences were astounding! That could easily account for any variations..
     
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  8. BoydAllen

    BoydAllen

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    Any time that you change a component, expect to have to make some sort of load adjustment. One of the characteristics of less experienced reloaders is that they tend to think of a load as a fixed thing.

    Reading about his cleaning procedure, you made it sound as if he thinks that not using a bronze brush is a good idea, a virtue of some sort. I think that this is a phenomenon that has mostly come to be because of the internet. I do not know of any short range benchrest shooter that does not brush, and I can only think of one top shooter who prefers nylon to bronze. Additionally, I have been to a number of matches, and spoken with a lot of top shooter, and cannot remember any of them taking their brush off before drawing the rod back down the barrel. It would be interesting to take a look at the inside of his barrel, with a good bore scope.
     
  9. RonS

    RonS

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    Guys, thanks very much for the lessons. I think both he and I will benefit from all replies. I don't think he has a chronograph but I do and I had not considered neck wall thickness as well as neck tension. I don't think he changed the type primers he was originally using in either cartridge when he made the change. Again thanks for all your comments and if someone else has something to add, I'm all ears...

    Ron
     
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  10. Laurie

    Laurie

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    That is almost certainly the cause of the big change in the Creedmoor's behaviour. There can be as much as 1.5gn difference between charges in small v large primer brass in Creedmoor / 308 size cartridges needed to attain any given MV. It's not just an effect of the less vigorous primer in the SP brass, but the 0.5mm reduction in the flash-hole diameter.

    This raises another issue, potentially with safety implications, of the move from SP brass and loads to large primer cases for the inexperienced / unwary. Many of the load combinations bandied around for SP 308 and Creedmoor (and no doubt in due course for 260 Rem and 243 Win as new SP varieties gain currency) are already 'hot' and in some instances likely exceed SAAMI / CIP max pressures significantly thanks to the extraordinarily hard / strong case-heads in Lapua SP brass. Then put them unchanged into a weaker LP case where the primer / flash-hole changes may in themselves additionally increase pressures by 5,000 psi or more is a potential recipe for disaster. In such substitutions, users dropping charges by 'a grain or two' in the belief that this will be sufficient to guard against over-pressure starting loads may still get a very nasty shock.
     
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  11. KevinThomas

    KevinThomas

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    Just to expand a bit on something Boyd touched upon, the OP mentioned not using metal brushes, and Boyd gave a short rundown on the vast majority of competitive shooters who used bronze brushes, and the fact that he only knew one who used nylon. A very good friend of mine is a barrel maker, and coincidentally probably the most knowledgeable individual in this field in regards to nylon. Before entering the barrel business, he spent a career as a tool and die maker for one of the top producers of fishing gear; including nylon fishing line. During his time with them he made literally thousands of dies and other parts used in creating nylon line of all types and sizes. He’s flatly stated that nylon is brutally hard on steel, very abrasive and wore out steel tooling in a remarkably short time. He’s an absolute advocate of bronze brushes, but won’t let a nylon brush anywhere near one of his own barrels.

    I have no idea where the notion came from that bronze brushes are detrimental to a good barrel, but having cleaned many, many hundreds of barrels (tens of thousands of times), I’ve never seen a thing that gave me pause in using these brushes. Used properly, I don’t see anything that would give me pause in recommending, or using them regularly. Used improperly, I doubt there’s a cleaning tool or process out there that won’t damage a barrel. I’ve seen guys using patches (no brush) that had them so tight they bent the rods badly in trying to shove them down the bore, going at them like they were trying to run a sword through an attacker. That, will damage a barrel. Failing to use a bore guide, and allowing (read:causing) the rod to bend enough to slam the tops of the rifling, that, will damage a barrel. Sloppy work at the muzzle, and so on, will damage a barrel. But a properly used bronze brush . . . have at it with no worries.
     
  12. Laurie

    Laurie

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    Now that brings back memories from my long-lost youth of something I'd completely forgotten. (Thank you, Kevin.) As a mad keen angler as a kid doing a lot of what we in Britain call 'spinning' ('lure-fishing' is I think the US term) where you cast and retrieve the lure, cast and retrieve the lure all day, I'd quite forgotten how quickly nylon monofilament lines wore the rod-rings, especially the small diameter one on the rod tip. The answer was to replace the steel tip ring with IIRC what was described as an 'agate ring' which had an inner ring of some form of stone or maybe ceramic material that wore much less than steel.

    (Oh, happy days fishing on the Rivers Tay and Earn near Perth in what was called 'spring fishing', the season opening on January 15th for the former river and usually very unspring-like back in those colder 1950s and 60s winters! It makes winter F-Class in northern England quite civililised by comparison - at least I can feel my fingers at the end of match and we don't have to keep breaking off shooting to thaw our hands out on cigarette lighter fuelled pocket warmers! :) )
     
  13. KevinThomas

    KevinThomas

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    Always happy to bring back a fond memory, Laurie! Cheers!
     
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  14. Bart B.

    Bart B.

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    There is not enough paper on earth to list all the myths about rifles and their ammo.
     
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  15. 22BRGUY

    22BRGUY Silver $$ Contributor

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    I did the same thing a couple of years ago with my 260AI and had similar results. The problem in Minnesota when the temperature reached about 20 degrees and I started having some "failure to fire" events but in temps above that they are the way to go for me also.
     
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  16. Uncle Ed

    Uncle Ed

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    This is slightly off topic about myths but the radio broadcaster Paul Harvey out of Chicago once said the following below to wake up his listeners.

    "Scientists at the Mayo Clinic today announced that orange juice causes cancer in gay rats."

    I always wondered if Anita Bryant sent Paul Harvey a nasty letter.
     
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  17. KevinThomas

    KevinThomas

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    That, I don’t know. But I know he DID once say in a broadcast that the top shooter in the USMC was a woman. No surprise there, as Julia Watson (ne Julia Carlson now) was knocking down national records faster than they could set them up. He then announced that the Marine’s number two shooter was also a woman, and they were sisters; Michelle and Sherri Gallagher. Now you’d be hard-pressed to find two better shooters, anywhere, but at the time, I think Sherri was still about 16. And neither of them had (or has since) ever been a Marine. Sherri later joined the Army and shot (decisively) for the USAMU, setting a NM-HP aggregate that still stands as the Camp Perry record; 2396/2400.

    And that, as Paul Harvey used to say, is the rest of the story.
     
  18. rr2030

    rr2030

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    Do we even want to know how they decide the rats are gay ?
     
  19. Bart B.

    Bart B.

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    Michelle and Sherri's Mommy and Step Daddy put a lot of their scores in the record books, too.
     
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