Gunsmithing Boring bar question
- Thread starter Turner421
- Start date
I've thought about doing what you're talking about, but I always use piloted reamers. It's more important to me that the pilot starts in the bore at the beginning of the cut. If I were to use a boring bar, I would only run it in far enough to accommodate the bottom of the shoulder of the reamer. Otherwise, the pilot is out of contact with the bore when you first start and I think that would cause more problems than it would solve.
I prebore to 20 thou under the shoulder diameter far enough that about half the pilot will still engage the bore. Then I prebore about 1/8" to a diameter where the reamer will just start. This gives 2 spots spaced reasonable far apart where the reamer starts concentric with the bore.
The prebore winds up being about 1 1/2' deep. As mentioned above, do a rough prebore and then clean it up with a boring bar. I do the roughing with an end mill rather than a drill to give a cleaner end of the prebore. You'll need a boring bar that'll do at least 1.5" depth of cut. Do not let the boring bar bottom or you'll cut a groove the reamer may not remove.
The method is fast and consistently gives great concentricity and minimum chamber diameters. I'm using a flush system, so reaming goes very quickly. If you're not using a flush system, putting more effort into a more detailed prebore might pay off.
The prebore winds up being about 1 1/2' deep. As mentioned above, do a rough prebore and then clean it up with a boring bar. I do the roughing with an end mill rather than a drill to give a cleaner end of the prebore. You'll need a boring bar that'll do at least 1.5" depth of cut. Do not let the boring bar bottom or you'll cut a groove the reamer may not remove.
The method is fast and consistently gives great concentricity and minimum chamber diameters. I'm using a flush system, so reaming goes very quickly. If you're not using a flush system, putting more effort into a more detailed prebore might pay off.
So for a 308 which has about a .475 body diameter at the head and .450 at the shoulder a .375 boring bar should be good. You'll have to make sure you don't make a huge cut or have any chatter. I like the carbide solid boring bars from accupro like this one from MSC.
www.mscdirect.com
It has a .290 minimum bore so it will do the 308 fine. Get a cutting length that will be sufficient for the depth you need. Don't recall what 308 is....don't do any of those.
Frank
0.29" to 1.25" Bore Diam, 5/16" Shank Diam, 05253208 - MSC
Discover Accupro Boring Bars at MSC Industrial Supply. Over 1 million products that ship and deliver fast
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Frank
