Purpose ????

forgemonkeyforgemonkey Member Posts: 20,459 ✭✭✭✭
I've decided to try and maximize the accuracy from my .300 Win. Mag.

However, I ran into unfamiliar territory when prepping for reloading.

The bolt will close on a 'no go' gauge but will not close on a 'field gauge',,,,,,,so it's within safe specs. The gun has been shot very little (Browning A-Bolt) purchased new.

The COAL is 3.340" for 180gr. Berger SPBT bullets per manual. By using a Hornady OAL gauge (same bullet) the OAL, with the bullet touching the lands, measures 3.620",,,,,,,a difference of .280",,,,,,,,,thus, attempting to have a bullet 'jump' to lands of .020" leaves very little of the bullet within the case neck. In addition, the magazine max. length is 3.375",,,,,,a moot point trying to have the ogive .020" off the lands.

Is the long throat common in production guns,,,,,,,,,I understand they are chambered for all the various bullets shapes/weights but the above seems excessive.

Your thoughts ????


  • MIKE WISKEYMIKE WISKEY Member Posts: 9,301 ✭✭✭
    edited November -1
    as to headspace; I've seen a lot of difference in brass on the belted mags, as much as .010" between brands.
    as to cart. oal/jump to lands' a lot depends on bullet shape, the burgers are exc. bullets ment for high b.c. and have a long 'point'. the original weatherby cals. had a 1" jump to lands.
  • charliemeyer007charliemeyer007 Member Posts: 7,348 ✭✭✭
    edited November -1
    Welcome to trial and error. Production stuff is made to fit into a range vs exact (much smaller range) in a custom built unit. Having the barrel set back one thread then re-chambered would likely fix the head space and remove perhaps half the free bore. I checked my Carl Gustav vs the P.O. Ackley rifle and was surprised that the CG was tighter head spaced the Ackley.

    I would try heavier bullets first like 190, 200, 220, or maybe some all copper ones. You might consider making it a 2 shot repeater with longer ammo by dropping a round in the chamber and pressing the second round down into the magazine and slipping the bolt over it to close the action. A buddy had an "Bubba" custom Sako rifle "built" in 7mm STW. Yea well the guy built it on 7mm x 61 S&H so it had a way short magazine box for the STW. It did however shoot good as a 2 round repeater.

    Not all firearm can be optimized completely by reloading for them. I have sent many back to the gun show table.

    added The 120mm Rheinmetall cannon on the M1 Abrams is a smoothbore, which means 100% free bore to me. I think the muzzle velocity is 5200 - 5600 feet per second. Theoretical IIRC for burning gunpowder is around 6400 fps. I once read that around 20% of the energy is consumed by imparting spin on bullets. I think they get good accuracy compared to the Rusky version which suffers dispersion at longer ranges. Better bullets or sabots?

    Back in 70's in Germany I had an interesting discussion with an old soldier that sniped on the Russian front with a ground mount 88. Solid bullets not HE's.
  • nononsensenononsense Member, Moderator Posts: 10,652 ******
    edited November -1

    Factory rifles have been notorious for having longer free bore than we can judiciously reload for. Magazine length is the limiting factor here. Weatherby and Remington have always taken a beating from the general public when it comes to too much free bore. But, in the case of Weatherby, you cannot get the velocity advertised for factory ammunition unless you have that free bore to reduce the pressure to a safe amount.

    Fast forward to your Browning A-Bolt. When SAAMI and their not necessarily bright advisory board set the specifications for the .300 Win. Mag., all 'standard' belted mags fell into the OAL of 3.340" until you moved into the older classics like the .300 H&H. The difference that we run into is that the .300 Win. Mag. case is 0.120" longer than the standard at 2.500", so the bullet essentially winds up deeper in the case.

    What are the choices? In all likelihood, none. The magazine system is the limiter. At 3.375", they have given you that tiny little bit of additional length to allow cartridges with an OAL of 3.340" some wiggle room for feeding. Loading your bullet choice to 3.340" gives you that longer jump but allows the pressure to reduce in the process. You can as pointed out, load for a single shot but that's a little silly when you have the magazine which should work for you, not against you. Your choice.

    When loading for limited length rifles, the best practice is to create the very straightest ammunition you can based on the fact that the bullet needs to jump a substantial distance before contacting the lands. There is a bunch of information (opinions) on the internet as to just exactly how to create straight ammunition. About 98% of it needs to be deleted. While it sound a little flippant, I don't care how it's accomplished but you need straight necks on straight cases with bullets seated in a straight relationship to those points. Crafting ammunition so it's straight and concentric takes a tiny bit more time but more important is attention to each movement you use to make that case and loaded ammunition.

    One detail often overlooked by hunters is neck tension. In your situation with the long jump, consistent neck tension can be an accuracy enhancement for your shooting.

    Setting the barrel back to get to the lands requires 4+ threads to be removed. This is not going to happen on your rifle barrel.

    Here's an observation for thought.

    The A-Bolt used to be available for cartridges such as the .375 H&H and the .300 RUM, both of which have SAAMI OALs of 3.600" and magazines to match, which if you had now would allow you to seat perfectly for your free bore. Oh well...

    Make straight cartridges and be sure to check you neck tension.

    The reason for all this harangue is that I'm currently testing extra long 'free bore' which is now called bore rider because of the bullet shapes required to function in these chambers. For 5 cartridges, I have free bore lengths starting at 1.5" going out to 4", so a couple of hundred thousandths is no problem in one of my set ups.

    Fire away!



    That concept of smooth bore tank barrels is what has started a small chain of events connected with longer free bore in rifles. However, the difference is that in the cannon ammunition there are guidance fins and a drive band:

    quote:"DM11 consists of warhead with a programmable fuse as well as a ballistic cowl, tailfin assembly, drive band, combustible casing with propelling charge, and a newly designed case base containing the primer and an integrated data cable for programming."

    Hawk Carse,

    I have been trying to help some of the wandering souls realize the same idea, you do not have to create a benchrest system for hunting rifles. Some ideas that we all espouse are very helpful while other, slightly more arcane ideas seem to cause more problems than they're worth. Given the current beliefs of so many 'experts', most short action cartridges should be moved into long action in order accommodate the now prevalent heavy for caliber bullets and the extra long VLD match bullets.

    Oh well...


    quote:I would like to hear what nononsense gets with 4 inches of freebore. Accuracy and velocity vs standard chambers.

    Me too! Unfortunately some health issues and a backlog of work will keep that on hold until probably after the first of year. I have access to two ranges new to me, which go out past 1,000 yards with poured concrete benches. One is good to 2,500 yards, flat, but it's over desert 'turf', what I call kitty litter. The other is over deep green grass 12 months out of the year with some topographical variations, it isn't necessarily really flat and it's humid. Two totally diverse situations for extreme testing of gear and ballistics.

    I will get to it as time allows and I will report back.

  • 243winxb243winxb Member Posts: 258 ✭✭
    edited November -1
    quote:very little of the bullet within the case neck. Not good.

    Bullets that jump can be very accurate.

    Look up the 300 wm chamber drawing here. http://www.saami.org/specifications_and_information/index.cfm
    [url] https://saami.org [/url]
  • 243winxb243winxb Member Posts: 258 ✭✭
    edited November -1
    To measure the distance to the rifling, take a fired case, pinch the neck with plier to make neck oval. Seat a flat base bullet nose first. Chamber a few times and measure. Should not be longer than SAAMI 2.970"


    Sort of like this 223 round.
    [url] https://saami.org [/url]
  • Hawk CarseHawk Carse Member Posts: 4,296 ✭✭✭
    edited November -1
    The internet has led people to think they can and should apply target rifle methods to mass produced hunting and military rifles.

    Sometimes you can and sometimes you can't.

    I would like to hear what nononsense gets with 4 inches of freebore. Accuracy and velocity vs standard chambers.
  • forgemonkeyforgemonkey Member Posts: 20,459 ✭✭✭✭
    edited November -1
    Thanks gentlemen,,,,,,,,,,good info !!!! ,,,,,,,
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