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Old May 13, 2013, 09:00 PM   #1
browninghunter86
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Reamer Advice

I have a 308 win match rifle and am wanting to shoot the Berger 185 Match, 168 Hybrids, and the Hornady 178 BTHP bullets.

Anyone have any info on the freebore spec I need for the reamer design so that the shank of bullet is above the neck/shoulder point to avoid the donut from forming and to allow more powder in the load with the listed bullets?

Thanks for any help
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Old May 14, 2013, 07:22 AM   #2
Bart B.
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I've never had a donut problem wearing out several .308 Win. match barrels with 180's, 168's and 155's seated such that their heel is in front of the shoulder-neck junction. Even after firing and reloading the same Federal case 47 times shooting Sierra 165 SBT bullets with max loads whose heel was past that point full length sizing that case every time; no donut problem nor its effect.

You can calculate where the bullet ogive will contact the leade once you have determined the ogive diameter for the leade angle of your choice, then measure a case from head to that point seating a bullet in a dummy round for reference. Here's a link to SAAMI specs for the .308 Win. that might help you.

http://www.saami.org/PubResources/CC...Winchester.pdf

Personally, I don't think it's worth the expense of a custom reamer. That contact point will move forward about .010" every 300 to 400 rounds anyway.

I don't think it's good to cram as much powder under a bullet as possible. That raises peak pressure, shortens barrel life, case life and accuracy usually suffers.
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Old May 14, 2013, 07:53 AM   #3
Dixie Gunsmithing
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Barts correct. Years ago, when the first wildcatters were experimenting with heavy loads, not only did they find a much lower barrel life, but at a certain point, the higher powder charge can change the ballistics enough that the bullet starts to do some squirrelly things, and accuracy falls over it.
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Old May 14, 2013, 06:16 PM   #4
browninghunter86
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so what freebore did that barrel have Bart?
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Old May 16, 2013, 10:42 AM   #5
Bart B.
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Browninghunter, what barrel?

All my .308 Win. match rifle barrels had standard minimum SAAMI spec chamber dimensions. No freebore at all past SAAMI specs, a couple with less for 155-gr bullets. A Sierra 168 HPMK seated out to a 2.84" hinch length would be a few thousandths short of rifling contact.

Dixie 'smith mentions heavy/high-pressure loads changing the ballistics enough that the bullet starts to do some squirrelly things, and accuracy falls over it. I totally agree. There's two reasons in my opinion.

All bullets are not perfectly balanced. When they spin at 140,000 to well over 200,000 rpm leaving the barrel, it doesn't take much out of balance to have the centrifugal forces upon muzzle exit make the bullet's flight path divert from the bore axis at the muzzle. This is the reason why all bullets fired at the same muzzle velocity have a very tiny spread in BC. Those most unbalanced have the most drag and that slows them down a bit more that the "perfectly balanced" ones. Most of the time, BC spreads are typically less than 1% in the best match grade bullets, but that's enough to be one of the reasons group size in MOA gets larger as range increases.

The other reason; supermaximum loads do not have their pressure curves as repeatable as normal max loads do. Nor do slower powders producing maximum velocity possible. That causes a greater spread in barrel time for the bullets; case mouth to muzzle exit. While the barrel whips at the same frequency for every shot, a spread in barrel time causes bullets to leave at a greater angular spread of the muzzle axis and the amount also gets bigger. While a little bit of spread in the right place helps accuracy, too much spread degrades it.
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Last edited by Bart B.; May 17, 2013 at 05:45 AM. Reason: Clarifying "freebore"
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Old May 16, 2013, 07:58 PM   #6
Harry Bonar
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?

Sirs;
"Freebore" is a meaningless term! I have seen English 22 rifles that are rifle-less almost to the muzzle!
The "paralel throat" espoused by now defunct A-Square" (a shame) is the answer and several reamer companies will furnish it on order!
Real freebore is the distance the bullet travels (if any) till engraved by the rifling! "Funnel" throats ( in use today) and the paralel throat are just terms for this event!
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