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Old 12-04-2006, 12:41 PM
tim simbari tim simbari is offline
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Default 20 TAC freebore

My buddy just got a nice Cooper in 20 TAC this weekend and I let him borrow my seater for my Dakota Predator. We noticed that the Dakota seems to use a much longer throat in it's chamber, and I mean a lot longer, something like 25-30 thou. Anybody have any thoughts on this?
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Old 12-04-2006, 02:13 PM
Daryl Daryl is offline
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.025 to .030 wold be normal for a 'standard-type" chamber in .17 cal, perhaps slightly more in .20 cal.
: When we order reamers, we generally specifiy for "0" leade, so the throat (rifling origin) starts at the end of the case chamber. Note there is normally about .011" taken up by the 45 degree angle between the chamber's case end, and the start of the leade or throat. All this, including the actual length of brass, normally trimmed to .010" short of the chamber's end, adds up to what people call freebore. With cases .010" short, + the /.011", we're already putting in .021" of 'leade(freebore) added to the normal .030", makes a total of .041", + the angle of the throat, be it 2 degrees or 1-1/2 degrees, allows seating out much further than a tighter settup. Lets take the numbers for a "0" leade chamber with max length brass. Lacking the .010" shorter case and the .030" leade, we see only .011" bullet shank out of the case compared to the .041" of the 'other' mesurements. Both of these, if seated to the lands, would be longer, of course, depending on the angle of the rifling in the throat, however for the intent of this post and for comparrison's sake, the numbers are relative.
: Some of us have found that with normal length bullets, especially in .17 calibre with all bullets to 30gr., a "0" leade with 1-1/2 or 2 degree throat gives the best seating length(bullet in case) for that calibre, and works in .20 cal as well. I thank Dan C. for suggesting to me to have Dave Kiff make my .17 cal reamers with "0" leade. I used Bobd's suggestions of a while back, for the 2 degree throat rather than the 1-1/2 degree throat, in the interests of a longer lasting throat. The 2 degree throat with "0" leade chamber allows both 20 and 25gr. bullets to seat to the neck's base, or almost there for good grip and straight-line seating. The same senario applies closely to the .20's when shot mostly with the short bullets.
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Old 12-05-2006, 01:38 AM
tim simbari tim simbari is offline
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Daryl, Maybe I asked this wrong. What I'm getting at is the Dakota has that much more freebore than the Cooper, not 25 thou total. What's up.
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