7mm Chamber Neck Size

Discussion in 'Gunsmithing' started by merbeau, Oct 22, 2018.

  1. merbeau

    merbeau Well-Known Member

    Messages:
    114
    Joined:
    Nov 1, 2011
    Hi
    I am currently building a long range (1000 yard rifle) in 7mm caliber, using the 7mm SAUM cartridge as the driving platform. I plan on using Norma brass for the rifle. Norma brass seems very consistent for neck size. I need advice on the chamber neck size.

    I just purchased a set of Whidden dies including the click die with bushing. According to the directions, the expander can be removed and the neck sized using a bushing. Bushings seem capable of “squeezing” the neck down two to three thousandths of an inch. So if the chamber is cut to where it exceeds the bushing's capability, i.e., the neck needs to be turned?

    Should I measure 20 necks and then have the chamber cut 0.002 above the neck size and then neck turn 0.002 and use a bushing 0.002 to 0.003 smaller to obtain a consistent neck tension. Or with good brass not a major issue.

    Thanks

    Robert
     
  2. Rhovee

    Rhovee Well-Known Member

    Messages:
    360
    Joined:
    Jun 29, 2017
    Neck tension and chamber clearance are two different things. You can use whatever busing you want and the loaded round will still be the same when bullet is seated. For a hunting round just get a .318-.319 neck clearance. .004-.005 clearance to loaded round is what I did. More importantly is going to be freebore length.
     
  3. J E Custom

    J E Custom Well-Known Member

    Messages:
    8,258
    Joined:
    Jul 29, 2004
    What you want to do is push a bullet in one of your cases and measure
    the outside diameter of the loaded neck. This will also give you a good neck wall thickness for your brass. if you are not going to turn the necks,
    add a minimum of .003 thousandths to this dimension for the reamer neck diameter. I would recommend .004 thousandths (.002 on each side)to cover differences in neck wall thicknesses and spring back.

    Most chambers have .004 to .006 loaded neck clearance and some dangerous game rifles have .008. If you make the chamber neck clearance .003, you should turn all necks to be sure that it will allow the bullet enough clearance or you may experience excessive pressures.

    J E CUSTOM
     
    MNbogboy likes this.
  4. Rhovee

    Rhovee Well-Known Member

    Messages:
    360
    Joined:
    Jun 29, 2017
    I have a 7 saum and use Norma brass and loaded round is .3145
     
  5. Deviant

    Deviant Well-Known Member

    Messages:
    96
    Joined:
    Jul 12, 2018
    I like to turn all of my necks so I no that will never be an issue with consistency. I am having a 6.5 Saum built now and am using a .296 reamer. I necked down Bertram 7 Saum brass and turned the necks to .014 and with a loaded round they measure .292 leaving me with .004 clearance for expansion. Turning the necks gives you lower SD and better accuracy in my opinion. I also use a Whidden click adjustable FL sizing die with the expander removed and then use a sinclair mandrel die to open it up for .003 neck tension. I use the neck turning mandrel first before I turn the necks as it is slightly smaller.
     
  6. merbeau

    merbeau Well-Known Member

    Messages:
    114
    Joined:
    Nov 1, 2011
    I have a 7 saum and use Norma brass and loaded round is .3145

    Many thanks for all the excellent responses. I should have mentioned this rifle is more for competition (not national but local) than for hunting. I have ordered Lapua 180 grain Scenar L bullet and Norma brass. I have had good luck using the 136 grain Scenar L in 6.5 mm round at 600 yards and their 105 Scenar L in 6mm at 300 yards. I can certainly see how close it comes to 0.3145 and then I can go for the 0.004 clearance.

    Robert
     
  7. Rhovee

    Rhovee Well-Known Member

    Messages:
    360
    Joined:
    Jun 29, 2017
    I’m on the prowl for Norma 7 saum brass.
     
  8. Deviant

    Deviant Well-Known Member

    Messages:
    96
    Joined:
    Jul 12, 2018
    The Bertram 7 SAUM is very good and was in stock a few days ago.