Prebore and floating reamer holder

Discussion in 'Gun Project Questions & Gunsmithing' started by Shawnba67, Oct 3, 2019.

  1. Shawnba67

    Shawnba67 Silver $$ Contributor

    Joined:
    Aug 22, 2016
    Messages:
    1,071
    So I am feeling froggy and want to attempt to prebore a chamber. I have read most folks are using a pusher set up. I do not have a pusher. I am worried about potential issues using the floating holder.
    I know the reamer will try center itself, but worried by allowing the tail to move it may not do it as well as Id like. Meaning it may start cockeyed then stay that way?
    Am I overthinking this?
     
    waverly likes this.
  2. Stan Taylor

    Stan Taylor Gold $$ Contributor

    Joined:
    Sep 2, 2017
    Messages:
    1,514
    I've never pre-bored a barrel in my life and never wore a reamer out yet.
     
    spitfire_er, Matt P, DickE and 5 others like this.
  3. Roeder

    Roeder Silver $$ Contributor

    Joined:
    Mar 22, 2014
    Messages:
    165
    Pre boring is not about saving wear on the reamer.
     
    Nightraider and Stan Taylor like this.
  4. Roeder

    Roeder Silver $$ Contributor

    Joined:
    Mar 22, 2014
    Messages:
    165
    Yes.
     
  5. Stan Taylor

    Stan Taylor Gold $$ Contributor

    Joined:
    Sep 2, 2017
    Messages:
    1,514
    Enlighten this dumb ole country boy what's the purpose of pre-boring.
     
  6. Shawnba67

    Shawnba67 Silver $$ Contributor

    Joined:
    Aug 22, 2016
    Messages:
    1,071
    My hopes are for better alignment.
    A gross overestimation of theory is why would I indicate so carefully to put a loose fitting bushing into a crooked hole as a guide.
    Loose fitting being the needed clearances for proper operation.
    Crooked bore being the 0.0015 or less a bore wanders.
     
  7. Danny1788

    Danny1788

    Joined:
    Feb 24, 2011
    Messages:
    415
    I predrill and use a JGS floating reamer holder. My chambers come out concentric and not oversized at the bases. But I’m a nobody so take that with a grain of salt. A lot better Smith’s on this forum then myself. I use a bushing that fits the bore past the throat.

    Are you just trying something new or are you having accuracy issues?
     
    bsheats2 and ridgeway like this.
  8. Shawnba67

    Shawnba67 Silver $$ Contributor

    Joined:
    Aug 22, 2016
    Messages:
    1,071
    Trying something new.
    I have never prebored a chamber. Lots of guys way smarter than me do it. Thought Id give it a try and see what I thought of it.
    I think that while maybe being way overkill the theory and mechanics are sound.
     
    Cloudrepair likes this.
  9. Nightraider

    Nightraider

    Joined:
    Sep 19, 2018
    Messages:
    188
    Expedition of the work, ensuring a straight chamber, dissipation of heat.

    Last Creedmoor chamber I cut took 3 cuts with reamer, less than 15min from start to finish, with very little heat transference and no measurable runout on a .0001" interapid. No sense in digging with a shovel when you have a backhoe.
     
  10. Dud

    Dud

    Joined:
    Oct 7, 2017
    Messages:
    553
    A reamer will self center but it can only follow the hole it's cutting in to.

    Once you have your barrel dialed in, a boring bar will cut a nice concentric hole, perfect to start a reamer in.

    Of course it works without boring but use what works for you.
     
  11. Dusty Stevens

    Dusty Stevens COVFEFE- Thread Derail Crew Gold $$ Contributor

    Joined:
    May 8, 2014
    Messages:
    14,909
    That shows your lathe is running true. If i cut a part in my lathe without removing it i cant get it to runout.
     
    B Nettesheim likes this.
  12. Stan Taylor

    Stan Taylor Gold $$ Contributor

    Joined:
    Sep 2, 2017
    Messages:
    1,514
    Okay now that your Chambers straight after you Fire case a couple times What's the total indicated run-out of that case.
     
  13. Nightraider

    Nightraider

    Joined:
    Sep 19, 2018
    Messages:
    188
    Doesn't matter on fired cases, but certainly does on loaded ammo. Straight ammo in a bad chamber is just as bad a crooked ammo in a straight chamber. Again, you asked.
     
  14. Nightraider

    Nightraider

    Joined:
    Sep 19, 2018
    Messages:
    188
    I would like to think so, but it isn't. The tail stock that I push with is about .003" lower than the centerline of the headstock. That's the main reason I started boring, to offset that number, the other reasons came along with it and were a pleasant surprise. If everything is left to its own devise and given a straight path it'll follow that like a little puppy. The path of least resistance is the one that typically yields the best results in machining, I've found.
     
  15. Stan Taylor

    Stan Taylor Gold $$ Contributor

    Joined:
    Sep 2, 2017
    Messages:
    1,514
    Well shucks I guess I have got several thousand crooked chambers floating around hope nobody complains like I said in earlier post never have pre-bored before always have used a reamer all the way.
     
    Jesswil61, Hopkins and DickE like this.
  16. Bob3700

    Bob3700

    Joined:
    Jul 16, 2006
    Messages:
    890
    Shawn,

    There are as many ways to fit a bbl and cut a chamber as there are individuals with a lathe. One thing that I have found is that the one and only correct way to cut a chamber is being given to you by the person you are talking to. There are a lot of ways to skin a cat, and in the end the cat is still skinned!

    Give the pre-bore and floating holder a try and see how it turns out. I have been doing it that way for years and never have had an accuracy issue.

    You want to open up another can of worms? Ask what the best method is to crown a bbl. Ha, just get the popcorn out and sit back and watch the fireworks. :)

    Good luck and have fun with your experiment.

    Bob
     
    swd, Shawnba67 and Cloudrepair like this.
  17. Cloudrepair

    Cloudrepair

    Joined:
    Aug 28, 2016
    Messages:
    646
    My experiance it is hard for most to measure. 0001 accurately
     
  18. boltfluter

    boltfluter

    Joined:
    Jan 3, 2012
    Messages:
    2,922
    Another little trick you can try is prebore the chamber straight, and then come back and cut the back of the prebore at the same angle as the reamer. A .25 inch deep or so cut will leave a nice starting spot for the reamer. If done correctly, the reamer has no choice but to start straight. Good luck! :D:D

    Paul
     
    Geno C, tac284, butchlambert and 2 others like this.
  19. swd

    swd Gold $$ Contributor

    Joined:
    Nov 25, 2008
    Messages:
    2,607
    This^^^ There are lots of excellent gunsmiths that do it both ways. Try pre boring and see how you like it. Chances are you’ll find you like one method better than the other and thats the right way for you. It’s about that that simple.
     
    boltfluter likes this.
  20. Bamban

    Bamban

    Joined:
    Jan 22, 2010
    Messages:
    364
    I often wonder about 0.0001 precision and accuracy measurements in the garage or shop environment.

    I retired from the semiconductor industry, I still remember how our calibration/metrology labs demand strict requirement for vibration, temp and humidity requirement to guarantee measurements at that level. Maybe they just like to waste money.

    How good of a spindle TIR and environmental conditions in the shop is required to get to that level of measurement?

    I am not a gunsmith nor a machinist, just a hobbyist trying to learn.
     
    Ares, Jesswil61, zeke06 and 3 others like this.

Share This Page