Accurizing barrel shim

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  • Magnumite

    Ultimate Member
    Dec 17, 2007
    6,571
    Harford County, Maryland
    Tisas 9mm GI type pistol. Had it for awhile. Goal was keeping as many parts factory while getting ot to where I like this format 1911. The accuracy wasn’t as consistent or tight as I desire. Accuracy doesn’t need to be fit match barrel tight groups but a group of bullet impacts a little more aseociated was desireable.

    So removing the link, doing some math, cutting a shim for the rear barrel upper locking groove. Cutting the shim just a squeak wider than the groove swages and keeps the shim where it belongs. This is a chrome coated barrel so silver solder would probably be ineffective.

    Net result
    - upper lug lockup. .042”;
    - link about .002” long but allow pin contact on lower lugs in full battery position;
    - zero vertical clearance when the slide is pulled back about .010”.
     

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    Magnumite

    Ultimate Member
    Dec 17, 2007
    6,571
    Harford County, Maryland
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    Results of shimming the barrel:

    Shot standing, two hands, WWB 115 grain ball ammunition. POA was bottom of bullseye (black).

    Two target pics shown one 10 and 17 yard composite, 3 shots at each distance. The other pic is 10 shots at 25 yards on top of the 10/17 yard group. A third pic shows one 9mm case and 10 22LR cases. The 9mm represents the aiming point and the 22 cases are the called shots. You can see I called the high flier low and the low flier called high. Looking close at the called shots you will see two groups formed. That’s called need new shootin’ spectacles…front sight is a bit fuzzy. Also means I am not a machine rest…

    During warm up and subsequent shooting the pistol was shooting tighter and more consistently. Chasing the rabbit I caught the rabbit more readily. Yeah…shim works.
     

    Melnic

    Ultimate Member
    MDS Supporter
    Dec 27, 2012
    15,333
    HoCo
    Would a slightly longer barrel link and/or a matched barrel bushing have done the same thing?
    Barrel Bushing is what I did for my Kimber. I measured it but just ordered 2 of them with one slighty tighter and ended up using the tighter one that I pulled some metal off of. What a difference it made. Polished up the barren and the bushing for reliability.
     

    Boats

    Broken Member
    Mar 13, 2012
    4,109
    Howeird County
    Barrel Bushing is what I did for my Kimber. I measured it but just ordered 2 of them with one slighty tighter and ended up using the tighter one that I pulled some metal off of. What a difference it made. Polished up the barren and the bushing for reliability.
    I have done that as well, usually an EGW one, have also used old Colt collet bushings. Hence why I asked the OP why he went with shimming the barrel lugs instead
     

    Magnumite

    Ultimate Member
    Dec 17, 2007
    6,571
    Harford County, Maryland
    Would a slightly longer barrel link and/or a matched barrel bushing have done the same thing?
    Bushing typically helps noticeably. I had fitted a bushing to this barrel previously. I did long link it also when I fitted the bushing. Both helped. For most this would have been adequate.

    Problem with a long link is the barrel can tilt left or right during and at lockup. Also during the recoil event before the barrel begins to link down. Putting the shim in the groove prevents the barrel tilt.

    When you do long link, the link shouldn’t jam the barrel into the slide lugs during TDC of barrel travel (as the barrel ‘pole vaults’ on the link while going into battery).

    This barrel actually has a considerably ‘taller’ lower lug bridge (.1015”) -distance from edge of barrel pin hole to lower lug flats. Very close to a #4 link bridge dimension (.103”). So I did install a #5 link. There was still some vertical clearance. I made a few measurements, one with the link, the remainder without the link. I did some grade school math to determine needed shim thickness AND if there would be adequate lockup depth in the upper lugs.

    I cut a 3/4” inch long shim from my sacrificial feeler gauge, filed it a couple thousandths wider than the groove. Cut it to 1/2”. I pre-arced it. Set it in place with a light tap. I assembled the pistol without the recoil parts and cycled it vigorously several times. Removed the barrel, checked shim stability, did the barrel timing checks, and reassembled it.

    FWIW, most stock barrels are well under what this barrel has…the spec #3 link .098” (about .095” is common place). On top of that the big end of the link is nominally .003” - .004” larger diameter than the slide stop pin. Considering all that, its no wonder consistency can go to blazes in a stock pistol with all that room to rattle around. Some drop-in accurizing barrels are not much better, though the upper locking grooves are filled a little more.

    After installing the shim I could feel the difference in how the pistol went into battery.
     

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