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Thread: Rechambering a .308

  1. #1
    Boolit Man
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    Rechambering a .308

    How hard is it to rechamber a .308 Win barrel to a .308 Norma mag? I am interested in doing this to an H&R Handi rifle. The first thing I checked was pressure, and the 308Win has a higher pressure, so I don't see any problems? No boltface to deal with...slight modifications to the extractor...I don't see any reason it wouldn't work.

    Any thoughts or comments?

    Yeah I know it *may* recoil slightly more than what I would like, but...
    Last edited by sukivel; 11-12-2017 at 03:41 PM.

  2. #2
    Boolit Master Texas by God's Avatar
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    If H&R/NEF chambered Handi rifles in 7mm or .300 mag;I would do it. If they didn't- There is a reason. The extra width of the belted mag case may amount to a "bigger hammer" effect on the frame even though the pressures are similar.
    Best, Thomas

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  3. #3
    Quote Originally Posted by Texas by God View Post
    If H&R/NEF chambered Handi rifles in 7mm or .300 mag;I would do it. If they didn't- There is a reason. The extra width of the belted mag case may amount to a "bigger hammer" effect on the frame even though the pressures are similar.
    Best, Thomas
    Yes, that is a thought worth thinking. It would have had big attractions for H&R - not necessarily the Norma, but a large magnum case - if there wasn't some reason against it.

    An Ackley Improved round on the .308-sized head would have had a lot going for it, though, and no magazine follower or lips to worry about.
    Last edited by Ballistics in Scotland; 11-13-2017 at 10:11 AM.

  4. #4
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    308 Norma mag is as dead as Hillary's presidential hopes.

    A handi rifle in any magnum is a bad idea. It may come apart and it will beat the snot out of you.

    If you want a 30 caliber magnum buy a TC Compass or a Ruger American. It won't blow up and should you ever want to sell it you'll get back most of your money.

    The price difference is very small as well.

  5. #5
    Boolit Master
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    You need to calculate bolt thrust prior any conversion. I have a 500 mag H&R wich is considered the limit of the action in regards to bolt thrust. Calcute, compare then decide.
    "It is better to die on your feet than to live on your knees" Looking for an RCBS Ammomaster and H&R shotgun barrels regardless of condition

  6. #6
    Boolit Master Texas by God's Avatar
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    Bolt thrust. That's why the Contender would handle the high pressure .223 with its small case head but not the 22-250 with it's wider head. The 30-30 but not the .308. The 45-70 but not the .444 Marlin.
    Ballistics, you lost me on that last passage.
    Thomas

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  7. #7
    Boolit Master
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    Bolt thrust - actual pressure pushing against the bolt/locking mechanism. 3.1416 (pi) x Head Diameter /2(squared) X Pressure(PSI). Example 30-30 Head Diameter .420 Pressure-38,000 psi
    3.1416 x( .420 /2) x (.420/2) x 38,000 = 3.1416 x .210 X .210 x 38,000 = 3.1416 x .0441 x 38,000 = 3.1416 x 1,675.8 = 5,264 pounds pushing against the boltface. There are other factors like brass friction, chamber roughness, brass hardness and stretch but this is the basic. Area of circle= pi (3.1416) x radius squared

  8. #8
    Boolit Man
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    As you can see,thrust is not great,about 2 1/2 tons.A small gr 5 bolt can easily support a similar weight with a margin of strength.I doubt that you can buy factory ammo in 308 NM,so you can simply reload to the pressure level you think best.I have rechambered a great many guns to 308NM on a production basis.Norma used to sell a gunsmiths kit to do the P14/M17 without removing the barrel,for the sum of $29.95.IMHO,308 NM and 7x61 S&H are two of the very best magnums.I would think a big increase on 308 performance at a much lower pressure level.With equal backthrust.

  9. #9
    Boolit Master
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    308 Norma Magnum - Head Diameter .512 - Pressure 60,000 psi Do the Math - bolt thrust = 12,353 quite a bit more than a 30-30

  10. #10
    Boolit Man
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    Quote Originally Posted by MostlyLeverGuns View Post
    308 Norma Magnum - Head Diameter .512 - Pressure 60,000 psi Do the Math - bolt thrust = 12,353 quite a bit more than a 30-30
    What’s the math on 308 win, which would be the donor gun?


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  11. #11
    I thought someone made and then rather needlessly deleted a query about my last sentence. It may have been for the mention of a .208-sized case head, which I have now corrected to .308-sized. For the rest, rechambering to an Improved case shape can in some rifles produce problems with how the rounds stack and feed in the magazine, but that doesn't apply when you don't have a magazine.

    I can tell you the bolt thrust on my .40-82 Winchester 1886, with assertive but moderate smokeless loads. It exerts no thrust whatever, since the primers are extruded to the limit of its considerable but not excessive headspace, and the case doesn't move back to push them in again or mushroom them. The case body is held to the chamber by gas pressure, and the case head is held to the body by the strength of the brass. The modern brass case had pretty much the size and work-hardening of a quarter-inch brass rod. Similarly Ackley found that a .30-30 Improved could be fired with his Winchester 94 locking lug removed.

    The reason we can't dispense with bolts and breechblocks, when they are such a nuisance, is that chambers can be wet or oily, and people do reload cases until they get head separations, and the strength of brass becomes none whatever. Rifle design normally allows for the worst, but I do believe H&R, in not going for the magnum case head, knew what they were doing.
    Last edited by Ballistics in Scotland; 11-13-2017 at 03:06 PM.

  12. #12
    Boolit Master Texas by God's Avatar
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    Quote Originally Posted by Ballistics in Scotland View Post
    I thought someone made and then rather needlessly deleted a query about my last sentence. It may have been for the mention of a .208-sized case head, which I have now corrected to .308-sized. For the rest, rechambering to an Improved case shape can in some rifles produce problems with how the rounds stack and feed in the magazine, but that doesn't apply when you don't have a magazine.

    I can tell you the bolt thrust on my .40-82 Winchester 1886 Winchester, with assertive but moderate smokeless loads. It exerts no thrust whatever, since the primers are extruded to the limit of its considerable rbut not excessive headspace, and the case doesn't move back to push them in again or mushroom them. The case body is held to the amber by gas pressure, and the case head is held to the body by the strength of the brass. The modern brass case had pretty much the size and work-hardening of a quarter-inch brass rod. Similarly Ackley found that a .3xe 0-30 Improved could be fired with his Winchester 94 locking lug removed.

    The reason we can't dispense with bolts and breechblocks, when they are such a nuisance, is that chambers can be wet or oily, and people do reload cases until they get head separations, and the strength of brass becomes none whatever. Rifle design normally allows for the worst, but I do believe H&R, in not going for the magnum case head, knew what they were doing.
    I didn't reply directly, but yes you had me baffled for a bit with talk of .208 heads and magazines haha. I agree about not rechambering to the belted magnum as well. NEF would have if they could have because they would have sold a bunch.

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  13. #13
    Boolit Master

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    It does seem strange that a 30-30 with 42,000 psi will back out the primer with head space but a 7MM with around 49,000 will stretch the brass so the primer is flush. Some of the test P O Ackley did proved that the brass on some cartridges would stick to the chamber wall and have no bolt thrust. I would not depend on that as I have seen some head separations that would be dangerous. The bolt thrust on a 308 is around 8,000 pounds. On a 12 gauge it is around 5,000.
    John Taylor, Taylor Machine, gunsmith

  14. #14
    Boolit Man
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    Quote Originally Posted by Texas by God View Post
    I didn't reply directly, but yes you had me baffled for a bit with talk of .208 heads and magazines haha. I agree about not rechambering to the belted magnum as well. NEF would have if they could have because they would have sold a bunch.

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    NEF could have sold alot more of several different calibers if they just offered it. But...they did offer a .444 and .500 s&w...aren’t those heavy recoilers?


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  15. #15
    Boolit Master
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    Recoil is based on bullet weight, powder charge weight, bullet velocity and weight of the firearm, pressure is not directly involved. A lightweight 45-70 with a 300 grain bullet at 2000 recoils far more than a 11 pound 243 with an 85 grain bullet at 3100, though the 243 runs at much higher pressure than the 45-70.

  16. #16
    Boolit Man
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    Quote Originally Posted by MostlyLeverGuns View Post
    Recoil is based on bullet weight, powder charge weight, bullet velocity and weight of the firearm, pressure is not directly involved. A lightweight 45-70 with a 300 grain bullet at 2000 recoils far more than a 11 pound 243 with an 85 grain bullet at 3100, though the 243 runs at much higher pressure than the 45-70.
    Ah, good point!


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  17. #17
    Boolit Master
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    308 Norma Mag is 64,000 PSI under CIP. The maximum SAAMI pressure cartridge that the H&R Handi Rifle was chambered in was 65,000 psi (270 Win). SAAMI max pressure for 308 Win is 62,000 psi. No magnum rifle cartridges were ever chambered in the Handi Rifle.

    Reaming the 308 Norma Mag chamber is technically feasible but would likely require having a new extractor cut, which may be tricky.

    FWIW, in my opinion, it represents a greater risk than it does reward.

  18. #18
    Boolit Man
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    Thanks for all the replies. I did learn a few things. I guess the 308NM will have to be an ‘opportunity knocks’ rifle.

    On second note...I find tons of .308 H&R Handi rifles out there...what would be some ideal rechamberings for these?


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  19. #19
    Boolit Master Texas by God's Avatar
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    The only factory round I can think of that would clean up the chamber is the 30-06 (if it will). Whoops-there is the 7.5x55 Swiss but good luck finding a reamer....may I ask what's wrong with the #1 30 caliber cartridge in the world?

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  20. #20
    Boolit Master
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    Quote Originally Posted by sukivel View Post
    Thanks for all the replies. I did learn a few things. I guess the 308NM will have to be an ‘opportunity knocks’ rifle.

    On second note...I find tons of .308 H&R Handi rifles out there...what would be some ideal rechamberings for these?


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    30-06 Ackley Improved or 30 Gibbs come to mind.
    Hell, I was there!

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BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
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HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
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LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
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