Freebore diameter question

[memilanuk]

So... by and large, I've stuck with either factory tubes with fairly generous freebore diameter, or just went with whatever the gunsmith put in the barrel. By and large I was more concerned with the *length* of the freebore than the diameter. A few conversations I've had with some folks over the last year or so has got me wondering, though...

One person, a highly respected competitive shooter in the Sacramento area, had suggested a fairly tight (to my mind) freebore diameter of .3085 for a .308 Winchester chamber in a Palma/F-TR rifle. One could hardly argue with his results, but in the back of my mind I was thinking that just the tiniest bit of fouling in a bore so close to bullet diameter could be very bad news. I never did get a chance to revisit the subject with him.

More recently, I went to a tournament and one of the top F/TR shooters from last year pulled out because his gun (.223 Rem) was (still) not grouping competitively. Talking with him before hand he said he'd gotten a new barrel with a different chamber, and had been fighting over-pressure and accuracy problems ever since. As an interesting corollary, another person there (normally a pretty solid TR competitor) had barrel done by the same 'smith with the same chamber, and was having similar problems. Given the relative difference in target sizes between TR and F-Class, it was more of an annoyance for the TR guy than a show-stopper - but he was seeing the exact same issues. They said looking at the reamer print after the fact it looked as though the freebore diameter was spec'd at .2240" - which all of us thought was kind of odd. From the sounds of things, with a freebore that tight some bullets were acting 'jammed' way before actually reaching the lands, causing all sorts of pressure excursions - and that was before fouling started piling up.

So... what would you consider the minimum *safe* freebore diameter relative to the bullet size? .0005" over? 0.0010" over? More?

Would this affect things like cleaning frequency? There is an old joke that Palma shooters clean their gun once a year, whether it needs it or not I may not stretch out the cleaning on my F/TR .308 gun *quite* that long, but I'm pretty comfortable letting it go upwards of 500rds before it *needs* cleaning... usually I'll clean it after a long weekend tournament (200-300rds, depending on the match program). Then again I think my gun has a pretty generous freebore diameter (factory 12 F/TR tube). I've been looking at putting together another F/TR gun with a custom barrel, etc. and I'm kind of wondering how much I need to keep an eye on the freebore diameter of the chamber I choose...

TIA,

Monte

 

[mikecr]

Mike Bellm has done a bunch of testing about this. Keep lookin for his 'coaxial throats' article.
http://www.bellmtcs.com/FAQ/ChamberThroats101.htm

I set up a 6.5wssm with .0005 over bullet dia, coaxial throat, for 139Laps. Never had any cleaning issue, and my ES was below 10fps.
But I'd also be interested in other experiences here.

 

[queen_stick]

Mike Bellm has done a bunch of testing about this. Keep lookin for his 'coaxial throats' article.
http://www.bellmtcs.com/FAQ/ChamberThroats101.htm

I set up a 6.5wssm with .0005 over bullet dia, coaxial throat, for 139Laps. Never had any cleaning issue, and my ES was below 10fps.
But I'd also be interested in other experiences here.

This subject is interesting... anyone reading this should read the article posted by Mikecr. I just finished reading through it... it added yet another variable to the accuracy equation

 

[alf]

Another perspective.......

http://benchrest.com/showthread.php?75751-Chamber-and-Reamer

 

[mikecr]

dmoran, I just pulled my reamer print and find that I set it at +.0001 over cal. My bad.
I knew it was very tight
By coaxial, I mean it is .2641" full length of the freebore, with no taper that is common.
But this may be a misnomer.

 

[memilanuk]

Thanks for the input guys!

The articles by Mike Bellm and the posts by Gordy Gritters were very interesting. The bit about the reamer flexing slightly makes sense... but I'm not sure I can wrap my mind around how that makes the resulting freebore diameter *smaller* than the actual reamer diameter as mentioned in one of those posts. Seems like if the reamer was flexing, it would hog out a hole somewhat larger than the actual reamer dimensions...?

At any rate, I'm still wondering what is too small, and what is too big?

I think the whole co-ax throat thing is a Good Thing, but its not the end-all be-all, though. Why do I say that? Having shot a few factory tubes that have chambers that are very definitely *not* co-axially aligned w/ the bore... they may not win the Super Shoot, but they still put the bullets in the X-ring when I do my part Definitely something to be desired if you're paying for a custom chambered barrel, no doubt. And I have sent a couple back because they were just too far off to be usable. :-\

Not sure I grok how the freebore diameter being too big has to do with the barrel being almost 'shot out', either... does the freebore diameter become measurable larger throughout the barrel life? If so, I'd love to see some pics of sectionalized barrels at end-of-life showing the wear in this area?

FWIW... I pinged a reamer manufacturer (JGS) for some prints... it showed their .308 Win (CIP) chamber as having a freebore diameter of .3098", and their other .308 'match' reamers (Palma, Obermeyer, & Match/Tactical) as having diameters of .3083-.3085". Interestingly, both of the .223 Reamers they sent me (5.56 Target & .223 Wylde) show a freebore diameter of .2240" - which seems just stupid tight, seeing as how the Berger 82gn BT bullets sitting on my bench measure .2240"...?

Thanks,

Monte

 

[mikecr]

All I know is that a 26cal with a .2641" freebore seemed unaffected performance wise, over other barrels that I doubt are so tight(I have no way to actually measure it).
It shoots well(~1/4-3/8moa), no sooting of necks, and very low ES. But a Cooper I have shoots as well, and I can only wonder about it's dimension here, or assume it doesn't matter much.
There really would have to be an apples to apples test done anyway.

My thinking(for benefit) was low effective runout. Afterall, .0001 over with a very long bearing provided by a 139gr Lapua(which my reamer was based on), pretty much ensures a straight shot at the rifling. But my loaded ammo for this barrel holds runout very nearly this low anyway. They're turned -fitted necks(.0005 neck clearance), and I don't FL size.
So I've done all I know to do, but don't know the affects of any single attribute.
It might be that I don't 'see' benefits of this, because I'm not a good enough shooter, or a good enough load developer..
I also clean my barrels to the metal every time, dry prefoul, & store dry. So there has never been any carbon buildup in the throat area. This I suspect could be an issue for some with such a tight freebore. Their velocities could potentially climb, and cause timing issues that wreck any tune.
If their runout was high, and the freebore long & tight, there could be a lot of bullet tension created when chambered that could cause flyers.

I would not go tight anywhere without a focus on loaded ammo dimensions.