Warning

Discussion in 'Gunsmithing' started by J E Custom, Oct 22, 2018.

  1. J E Custom

    J E Custom Well-Known Member

    Messages:
    8,258
    Joined:
    Jul 29, 2004
    Occasionally I run across something poorly done and feel like it needs to be shared with the membership.

    I just started working on a good friends rifle (A re-barrel) and this is what I found.

    It was a 340 Weatherby with a very small barrel contour. (.608 muzzle diameter) and had a homemade brake installed that had 1/2'' x 28 threads that were not cut very well and had a minor
    thread diameter of .460. This left the barrel wall .061 in thickness.

    In addition to the dangerously thin barrel wall thickness the thread fit was poor to say the least and offered little if any support for the bore. (The brake was the same outside Diameter as the barrel and was also very thin walled (Just over .054 thousandths).

    The brake was also under bored for a .338 bullet and had a .347 bore. add all of this to a misalignment of .003 thousandths off center the bullet was just barely missing the bore of the brake.

    After checking the barrel bore at the muzzle, it ended up .340 Groove to groove diameter for .562 in length, indicating the barrel had been expanded due to the thin barrel wall and lack of proper support. I am very surprised that the homemade muzzle brake was not shot of off during firing.

    This subject has been discussed many times and opinions vary, But I can't stress the importance of
    doing this type of work correctly.

    J E CUSTOM
     
    Last edited: Oct 23, 2018
  2. ShtrRdy

    ShtrRdy Well-Known Member

    Messages:
    1,264
    Joined:
    Jan 14, 2012
    Yikes!
     
  3. Heavyiron

    Heavyiron Well-Known Member LRH Team Member

    Messages:
    234
    Joined:
    Apr 11, 2016
    Scary to say the least
     
  4. WildBillG

    WildBillG Well-Known Member

    Messages:
    55
    Joined:
    Sep 5, 2018
    JE is there a certain diameter the break should be. I am asking as I have a Sako with a break on it and I have a question about the break. I had not noticed it before but the last 2 ports on the break have a sharp pointed piece of metal pointing down the bore. They do not appear to be into the bore of the break but I am not entirely positive of this. To me the gun is not as accurate as I feel it should be. Could this condition be part of the problem. I would take the break off but it is on there very tight. Any help you could offer would be great.
     
  5. J E Custom

    J E Custom Well-Known Member

    Messages:
    8,258
    Joined:
    Jul 29, 2004
    There are three critical dimensions on brakes in my opinion.

    1= Bore size in the brake should be .020 to .025 larger than the bullet diameter.

    2= Selected Minor thread diameter should leave at least .140 to .150 thousandths barrel wall thickness.

    Depending on the design of the brake, the thickness of the brake wall should be a minimum of .100 thousandths to handle the pressure requirements and with the help of a good thread fit, support the reduced barrel wall thickness.

    As to the pointed objects in the brake, they are turning vanes that aid in diverting the gas out the port. this is now common with ported brakes with angled ports and are part of the machining process on/in some brakes because the mill used to cut the ports are not milled past center. (If it were milled past center the turning vane would not be present).

    With my first design these were present and found to slightly improve performance if they were not removed. And no, they do not interfere
    with the bullet clearance.

    http://www.jecustom.com/index.php?pg=Home&cmd=Cell&cell=FAQ

    In my FAQ section I give the recommendation of barrel size and bore diameter using the minor diameter of the thread size and pitch you should use. Take the barrel diameter at the muzzle, subtract the minor thread diameter from that and as long as the total barrel wall is greater than .300 thousandths plus the bore diameter you will be safe.
    Example: .300 + 308 + .608 and if the minor diameter is .038 the barrel would have to be .646 plus a minimum of .030 of shoulder to seat the brake on the barrel for a grand total of .676 muzzle diameter.

    But Keep in mind that some barrels were not designed for muzzle brakes and are two small to safely thread for there bore size. so a clamp on brake should be used if a brake is needed.

    J E CUSTOM
     
    7stwnut, CaptnC, Doublezranch and 2 others like this.
  6. WildBillG

    WildBillG Well-Known Member

    Messages:
    55
    Joined:
    Sep 5, 2018
    Thank you JE this has helped me alot.
     
    7stwnut likes this.
  7. tobnpr

    tobnpr Member

    Messages:
    5
    Joined:
    May 30, 2013
    I share your concern with barrel wall thickness. There is no agreement among experts, near as I can tell as to minimum thickness. General consensus that I have read is .100-.150 minimum.

    Recently had a customer send me brake from a well known, popular manufacturer of brakes and custom rifles- along with his Tikka T3 Lite in .300 WM. Brake was also 1/2"-28 tpi, I called him and told him no-go.

    He said he'd spoken to them before purchase, and he was told it wasn't optimal but could be used. So, I called and spoke with the company owner about it. He agreed it was borderline- but insisted that it would work as long things were done correctly on my end. Customer gave the go-ahead despite my misgivings, and while I was tempted to turn it down I did the work for him.

    Why? A quick check of common assault and battle rifles that have been used and abused for many decades have less than what we consider optimal barrel wall thickness- and the fact that they have withstood the test of time without their muzzles cracking off led me to believe there's more to "this". We also know that being mass-produced in wartime, often in third-world countries- that its a safe bet class 3 threads were probably not to be counted upon.

    How many tens of millions of AK's, with M14 muzzle threads and a barrel wall thickness of .073?

    Same for the FAL- 9/16"-24 with a minor diameter of .513 leaves a barrel wall of just over .100 ( and I have threaded dozens of light contour Mosin-Nagant barrels like this).

    This is a subject that comes up often, and as with everything firearms, huge safety margins are rightly recommended. I'm not disagreeing with you- just saying that the above examples are proof positive that less than optimal recommended thicknesses don't result in failure.
     
  8. newmexkid

    newmexkid Well-Known Member

    Messages:
    764
    Joined:
    Apr 11, 2011
    What are your feelings on clamp on brakes?
     
  9. J E Custom

    J E Custom Well-Known Member

    Messages:
    8,258
    Joined:
    Jul 29, 2004
    They are better than threading an already thin barrel.

    I personally don,t like the looks of them but for the small contours they are better than nothing.

    J E CUSTOM
     
  10. Good

    Good Well-Known Member

    Messages:
    413
    Joined:
    Jul 15, 2009
    It is unfortunate when a smith tells you he can do something he cannot. I had a brake put on my wife's 7RM and actually shot the brake on the first couple shots. He opened it up a little bit, but I am not satisfied with it. Giving him another chance I had him put my Painkiller on a 30-06 and he messed up the contour of the barrel/brake junction, and still charged full price. No more.
     
  11. coop22250

    coop22250 Member

    Messages:
    14
    Joined:
    Nov 6, 2018
    Wow, good thing you caught that. I’m guessing an avg joe like me would not know the diff, good way to get into trouble.