New guy question about SD & BC

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  • New guy question about SD & BC

    Howdy folks,

    New on the forum. I'm a firearms enthusiast (to put it mildly) and am a big fan of the inherent advantages of a 6.5mm bullet; namely the higher SD's and BC's possible for a given weight. I was hoping someone who is more knowledgeable than I am on this subject could shed some light as to why this is the case. Why is the 6.5mm diameter the "sweet spot" for SD & BC?

    I am currently in the market for a Mini-30 which I plan to build in the following order:

    Trigger job @ ~3#
    Hogue stock, I haven't decided on the color yet
    Accuracy Systems International fluted 20" BBL
    Ultimak scout rail/hand guard

    I was going to ask what length of barrel is actually the best, but I guess I can use the search function for that.

    Regards,
    Colorado
  • bwaites
    Moderator
    • Mar 2011
    • 4445

    #2
    Welcome!

    The 6-6.5 is the sweet spot in bullet weights usable with cartridges that fit the AR15 magwell.

    Why a Mini 30?

    Mags may create an issue, since the Grendel has much less taper than 7.62x39.

    The Grendel likes 16-24 inch barrels for sporting use.
    Last edited by bwaites; 07-13-2011, 02:10 PM.

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    • #3
      Bill said it pretty well!

      Also, the 6.5 was historically used in a number of military firearms in Europe, Japan, and elsewhere. The caliber reached its first maturity in the early days of smokeless powder. Designers, particularly military designers, tend to prefer evolving things unless driven by wartime necessity.

      This manifested in using very heavy bullets by today's standards, e. g. 160 grains for some of the military 6.5 cartridges. Lighter bullets bullets became available as the sporting use of the cartridges became popular in the first half of the 20th century.

      Phil Sharpe's Complete Guide to Handloading, 1941(!!) shows bullets from 100 through 160 grains in 6.5. At this time, the larger diameter .270 had 95 grain offerings. The fact that the 6.5 bore was a lot older than the .277 is reflection of the "evolving" characteristic of a lot of military firearms and bullet design. Also, the fast twist of the military rifles and associated civilian rifle barrels did not favor the lighter bullets. The 44th edition of the Lyman Reloader's Handbook tells us that we could get 85 grain bullets in 6.5 by the mid-'60's while the lightest .277 bullet was still 90 grains.

      The range of bullet weights for both calibers remains more or less the same today (excepting a new 85 grain offering or two in .277). The main difference is that effectively no .277 diameter bullets suitable for target competition until the advent of the 6.8 SPC. The 6.5, however, has long enjoyed a rich target competition history, so a variety suitably accurate target and bullets were readily available along with a good collection of hunting bullets.

      The Grendel shines with 130 grain and lighter bullets, so the ready availability of bullets in more or less the mid-range of the 6.5 lineup gave the cartridge an immediate boost. The popularity of both the 6.5 Grendel and the 6.8 SPC have encouraged the bullet manufacturers to add richness to their line-up.
      Last edited by Guest; 07-13-2011, 04:27 AM.

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      • #4
        Originally posted by bwaites View Post

        Why a Mini 30?

        Mags may create an issue, since the Grendel has much less taper than 7.62x39.
        I like the Mini-30 because of its classic rifle ergonomics. I learned how to shoot with my Opa on a Ruger 10/22, which I still own, (although the bolt, receiver and trigger guard are the only original pieces left on it) and that's what I'm most comfortable with. The Air Force didn't spend a whole lot of time hammering shooting technique with the M16 into my brain, so I'll stick with what I'm most comfortable with.

        Furthermore, it's cheaper to train on a 22, and it's better to train with a weapon that has similar ergonomics to what you plan on using in the field; thus my thought process led me to the conclusion that a Mini platform would be a good match for me. Plus, there is nothing cool about a 22lr AR15 (in my mind at least).

        As far as the taper issue on the x39 vs 6.5G, would a new follower in the mag do the trick, or would it be easier to modify a 223/5.56 mag?

        Regards,
        Colorado

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        • BluntForceTrauma
          Administrator
          • Feb 2011
          • 3897

          #5
          Welcome, Colorado!

          Regarding the "sweet spot," first, any caliber can have long, streamlined, high SD and high BC bullets. My opinion is that 6.5mm became a sweet spot because larger caliber bullets with the same SD and BC start to be heavier and generate more recoil than is desired. Yet smaller caliber bullets with the same SD and BC were traditionally less desirable for big game, four- or two-legged, and were unnecessarily light. Just a hunch. . .

          John
          :: 6.5 GRENDEL Deer and Targets :: 6mmARC Targets and Varmints and Deer :: 22 ARC Varmints and Targets

          :: I Drank the Water :: Revelation 21:6 ::

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          • RangerRick

            #6
            Originally posted by JWH View Post
            Welcome, Colorado!

            Regarding the "sweet spot," first, any caliber can have long, streamlined, high SD and high BC bullets. My opinion is that 6.5mm became a sweet spot because larger caliber bullets with the same SD and BC start to be heavier and generate more recoil than is desired. Yet smaller caliber bullets with the same SD and BC were traditionally less desirable for big game, four- or two-legged, and were unnecessarily light. Just a hunch. . .

            John
            Yes, geometry dictates that. As the radius of the projectile goes up, while keeping the same basic shape, it's volume goes up by the same amount roughly cubed. So the amount of lead goes up tremendously as the caliber goes up. Since the surface area only goes up to the square of the radius, you get less drag per grain as the bullet scales up.

            So, if the bullet gets too small the drag goes up per unit weight, if the bullet gets too big the weight gets to be too much for a man-portable rifle to handle.

            SD is more like a measure of puncturing ability. It takes a lot more force to punch a stick through a piece of cloth than a needle. If you put the same force behind the needle and stick, the needle will punch through easier. So high SD bullets (skinny per unit weight) punch through air and targets easier, other factors being equal.

            There's a limit to SD, too though. To get heavier a bullet of a given caliber has to get longer, or heavier in other ways. That's why anti-tank rounds are made with depleted uranium, it's heavier than lead.

            So all those factors converge around 6.5mm for a rifle with medium recoil.

            Rick

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            • #7
              Welcome to the horde.

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