Quantcast

Need help understanding Bores.

Discussion in 'Rifle Country' started by TheFlynn01, Sep 21, 2020.

  1. TheFlynn01

    TheFlynn01 Member

    Joined:
    Feb 12, 2020
    Messages:
    65
    Location:
    Colorado
    Hello there folks!

    I wanted to ask you all about bores, people toss around alot of terms with them, and I feel I understand just enough to get myself confused.

    What is the difference between a dark and bright bore?

    one with strong or worn rifiling?

    What does the twist do?

    feel free to use small word, sometimes I need things slow haha!

    Thank you!
     
  2. rbernie
    • Contributing Member

    rbernie Contributing Member

    Joined:
    Jan 21, 2004
    Messages:
    23,238
    Location:
    Norra Texas
  3. Rubone

    Rubone Member

    Joined:
    Jul 12, 2020
    Messages:
    211
    Location:
    Beautiful Northern New Mexico
    A dark bore generally signifies surface rust over time which can blacken the surface of the metal, a bright bore is clean and shiny.
    Strong rifling is sharp edged and prominent through the entire bore, worn rifling can be thin to almost nonexistent when viewed along the length of the bore.
    The twist spins the projectile. Faster and slower twist increases or decreases the rate of spin. Every caliber has twist that works best for it depending on the size of the projectile. The twist stabilizes the projectile in flight helping it to fly true and far.
     
  4. P5 Guy

    P5 Guy Member

    Joined:
    Apr 13, 2004
    Messages:
    3,299
    Location:
    Tampa Bay area
     
  5. David Hoback

    David Hoback Member

    Joined:
    Dec 30, 2019
    Messages:
    328
    What Rubone said is dead on. The only thing I would add is Twist Rate is expressed as a ratio... 1:7, 1:8, 1:9, etc. This means the bullet will spin One complete revolution in whatever inches as specified by the second number. So taking a 1:10 twist means the bullet spins 1 turn in 10” of travel down the barrel.
     
    .308 Norma likes this.
  6. g3ops

    g3ops Member

    Joined:
    Sep 3, 2020
    Messages:
    16
    Location:
    Minnesota
    Also related to twist rates, different grain ammo works better at different twists. If you are like me and a marginal marksman, it probably won't make a whole lot of difference, but if you want to try to at least try to use ammo that works better with your twist and barrel length it's worth reading up on this. I admit I went way too deep into this probably, and also that this is all book learning rather than true range experience so your mileage may vary - I like to study before buying so I did this research to decide what twist and length of barrel to buy for a build I'm working on right now. How stable the bullet is does have a large impact on accuracy. If your bullets are tumbling every time you shoot (if you see holes on your targets that aren't circular you know you are having tumbling), you might have an issue with your ammo not working well with your twist. That tumbling can put your shots off by many inches depending on how far out you are shooting.

    Here's an example from one source with a cheat sheet - keep in mind there were a lot of variables to calculate this more precisely. The chart below is referenced in this article: https://www.everydaymarksman.co/equipment/rifling-twist-rate/. He used a ballistics calculator located at this link to make the chart below http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

    The chart, keep in mind that the assumption made is 2800 fps which is dramatically impacted by barrel length - for example, a 10.5 barrel is closer to 2500 fps instead of 2800 so this chart is probably off:
    224-Stabilization-1024x576.png


    For a build I'm working on right now, I wanted to know how close this chart would be to what I could get with my specific ballistics. I went to the site this guy used to calculate and figured out how to use the calculator.

    Since the barrel I selected is a 1/8 twist and 10.5 long, I found another site that helped me guess at what the fps would be, around 2500 instead of 2800 used for the picture above. Also, I wanted to know how much impact temperature swings might make so I calculated at 2500 fps, for temps ranging from -9 to 90f. For 62 grain 5.56 ammo this came to a range between 1.4 and 1.8 on my barrel - you'll note on the chart above that 1.8 was the listed value for 62 grain at 59 degrees. At 59 degrees, my lower fps actually got me a 1.698 (better) value. This number is called the "Miller factor" and the formula originated with Don Miller and Dave Brennan. They say that you want a range between 1.3 and 2.0. There was an article in Precision Shooting from 2005 that had the formulas that were implemented in this calculator, you can read it at this link if you really want to get into the math http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_1.pdf

    The thing I found difficult to do the calculations correctly was related more to getting the right bullet lengths for 55 grain vs. 62 grain, etc. It's not something easy to Google, I found the length for 55 grain somewhere here on these forums (.739) and for 62 grain I used US government specs for their standard 5.56 62 grain (.907), in reality each manufacturer has their own range of lengths for the bullets they use. Also, these lengths vary even within one manufacturer, if you bought bullets to reload and measured them you'd find variation enough to affect the calculations in a minor way, probably not enough to be off enough for any concern.
     
    Ru4real likes this.
  7. TheFlynn01

    TheFlynn01 Member

    Joined:
    Feb 12, 2020
    Messages:
    65
    Location:
    Colorado
    Wow thank you all for the info. I really appreciate it! So with bores, is there a time when it is simply too old or worn? As in it is simply shot out?

    I have a old Krag and I love shooting it. However I can’t seem to get good grouping, I feel it’s a user (me) problem. However my dad feels the thing is shot out. As the bore is dark, but the rifling looks strong to me.

    is there a way to tell if it’s the barrel?
     
    P5 Guy likes this.
  8. David Hoback

    David Hoback Member

    Joined:
    Dec 30, 2019
    Messages:
    328
    Shooting. If your accuracy will not improve with it or if you can have someone you know is a very competent shoot it, and unable to get consistent accuracy, than yes the barrel has come to it’s accuracy end.
     
    Demi-human likes this.
  9. TheFlynn01

    TheFlynn01 Member

    Joined:
    Feb 12, 2020
    Messages:
    65
    Location:
    Colorado
    so what’s my option then? I don’t want to make it a wall hanger.
     
  10. David Hoback

    David Hoback Member

    Joined:
    Dec 30, 2019
    Messages:
    328
    Take it to a Gunsmith, have it rebarreled. Or if the bore & rifling are still ok, perhaps it may just need the throat chamber re-cut. Is it a 30-40?

    But on your own, no there is nothing you can do. As the Krags were never competition level accuracy guns to begin with, your other option is simply enjoying it as a piece of history. Shoot it for the best IT can do, nothing more.
     
  11. TheFlynn01

    TheFlynn01 Member

    Joined:
    Feb 12, 2020
    Messages:
    65
    Location:
    Colorado
    Yeah it is a 30-40. I might have to look into it. I still think it is a me thing. Is there any good pictures of a good bore and ones that have been shot out? I am sure I have seen them, but I’m green enough to doubt what I am looking at.
     
  12. Dave DeLaurant

    Dave DeLaurant Member

    Joined:
    Nov 12, 2018
    Messages:
    1,325
    Location:
    People's Republic of California, Central Valley
    One other thing to bear in mind about evaluating a bore are the numerous forms of polygonal rifling that do not present sharp edged lands/grooves. Best know are the rounded lands of the Metford system, oval rifling such as the Lancaster system and the hexagonal Whitworth rifling, but there are many, many others. The Wikipedia article referenced above mentions polygonal rifling forms.

    My point is that it can be helpful to know in advance which kind of rifling you are inspecting -- a sound Metford-rifled bore appears badly worn if you are expecting sharp edges.

    A borescope is also a handy, if expensive, tool for evaluating the condition of an old rifle bore. Assuming the bore doesn't have obvious, deep pits when viewed from the muzzle, the next thing to do is check the breech end for erosion just forward of the chamber. From an accuracy standpoint, a properly maintained rifle bore eventually wears out from the rear, where the internal temperatures and pressures are high and initial bullet stability is determined.

    The final proof of course is to shoot the thing. You would be amazed how well some corroded bores can still group with the right bullet selection.
     
    Demi-human likes this.
  13. Jim Watson

    Jim Watson Member

    Joined:
    Dec 24, 2002
    Messages:
    28,609
    Location:
    Florence, Alabama
    First, some details. What do you consider "good grouping?" What are you seeing on target?
    What ammo, what sights, what position?
     
  14. JDinFbg

    JDinFbg Member

    Joined:
    Dec 15, 2018
    Messages:
    195
    Location:
    Fredericksburg, TX
    This thread is germane to some of the questions I had about rifling. I have a 94 Winchester with a 26" octagon barrel that, based on the serial number, was manufactured in 1925. I've been trying to develop some new loads for it and have tried 3 different bullets and 2 different powders. I can get no better than 5 inch, 5-shot groups at 100 yds. with the rifle fired from a machine rifle rest. I just bought a Teslong rifle borescope camera with side-view mirror so I could see what the inside of the barrel looked like. I definitely see a lot of pitting and copper deposits, and this should probably not be surprising for a 95 year old rifle. But, don't really know what a 'normal' barrel should really look like and expect that even a new barrel might have some tool marks or other imperfections. What I found interesting is that the copper deposits start just ahead of the chamber and extend most of the length of the barrel, but the copper deposits seem to abruptly stop about 3" from the end of the muzzle. I'm trying to understand what this might mean. I also observed copper deposits on the surface of the lands and in the middle of the grooves. However, the areas of the grooves adjacent to the lands have no copper deposition. I don't know if this is normal for how copper deposits inside a barrel or not. The rifling is not pristine, but I can see a clear delineation between the lands and the grooves for the entire length of the barrel and I could see no place where the lands were totally worn away. My first task will be to try to get the copper deposits removed, but I am unclear whether there is anything I can do about the pitting. If someone has some borescope pictures of what rifling should look like - say, the good, the bad, and the ugly - that would be helpful.
     
  15. dave951

    dave951 Member

    Joined:
    Feb 20, 2018
    Messages:
    248
    Being a fan of old rifles, I'll add that often an old rifle has been abused through it's life and sometimes the bore is suffering from lead deposits. Before getting really crazy and relining or rebarreling a rifle, I'd make darn sure there were no lead deposits. Case in point, the 50/70 has been around for a long time although it's not as popular as the 45/70. Many of the old guns were shot with bullets that were way undersized for their nominal bore and as such, it's not uncommon to see the old guns look like there's hardly any rifling, but after deleading, there's good rifling under all that lead.
     
  16. highlander 5

    highlander 5 Member

    Joined:
    Sep 15, 2006
    Messages:
    2,557
    Location:
    lynn,ma
    Bore may be worn also the rifle might be a candidate for cast bullets. Though the Krag bore diameter is supposed to be .308 a cast bullet of say .310 with a gas check might shoot better plus cast bullet loads would be a lot less stressful on the action. Give the bore a good scrubbing,get a 32 cal lead bullet or equipment sized lead shot lightly lube the bore then drive the bullet thru the barrel measure the bullet with a micrometer do not use a caliper. Once you know the bore's diameter then you'll know if jacketed or cast bullets are your best option for accuracy.
     
  17. 3Crows

    3Crows Member

    Joined:
    Apr 22, 2018
    Messages:
    974
    Location:
    Wichita, Kansas
    The first thing to do is to actually clean the bore.
     
  18. Jim Watson

    Jim Watson Member

    Joined:
    Dec 24, 2002
    Messages:
    28,609
    Location:
    Florence, Alabama
    C.E. "Ed" Harris did articles on the management of less than perfect barrels.
    He said, as best I recall, that you can clean one thoroughly and get out the powder and jacket/lead fouling, and the rust; then you might have to shoot it some to get back to a "condition" with just enough fouling to smooth over the pits.
    He was also realistic on results. Grandpa's bear gun or Sergeant Schultz's Krag are not going to shoot like a target rifle.
     
    Demi-human and gotboostvr like this.
  19. Archie

    Archie Member

    Joined:
    Dec 31, 2002
    Messages:
    2,895
    Location:
    Hastings, Nebraska - the Heartland!
    What 3Crows said. A somewhat overlooked 'cleaning' problem is that of copper fouling. This comes from the copper jackets and is difficult to see. There is at least one cleaning solution made to remove it. For older rifles (especially military rifles) this is worth the effort.
     
  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice