Best freebore diameter, for 6PPC?

Here's my take on things. BR bullets are subject less trauma than larger caliber heavier bullets. Think about jacket thickness and bearing surface length. Launch pressures are about the same regardless of cartridge capacity or bullet weight. BR bullets have thin skin and a short bearing surface that should be good but everything is a compromise. BR FB bullets all have a pressure ring and that's the part of the bullet that has to be addressed. Large caliber heavy BT bullets have thicker jackets. I think giving that pressure ring a little room can't hurt a thing over the life of the barrel. It takes a great deal more force to push heavy thick jacketed bullets into the rifling. One time I was having problems blowing up some custom 6.5 bullets. I cut a few dummy chambers and threaded them to fit a loading press. Made a punch and starting squirting bullets down barrels. I did it with 6 and 6.5's. Not sure what I learned other than it takes a hell of a lot pressure to push a bullet into the rifling. It is widely believed that a tight freebore is the best for accuracy. I'm not so sure. I have changed the reamer angle at the end of the neck from 45 to 30 degrees on some reamers per customers request. It doesn't hurt. I've tapered freebores for years and hundreds of barrels mostly larger calibers and I believe it has definitely helped accuracy and has contributed to consistent accuracy over the life of the barrel.

George

I guess it would be defined either freebore or leade angle by the diameter. By that I mean if you spec'd .2443" down to .2435" it would still be larger than groove diameter and that would be freebore. If you went .2443" to .2428" that would be a leade angle. I have always been suspect of the freebore just abruptly ending at a diameter something larger than the groove diameter. That transition can't be good on FB bullets with pressure rings. I know with a dull reamer it becomes a choke point.

Now the 64 trillion dollar question. How do you grind it in a very short freebore and then how do you confirm that's what you ordered? I've have it done on longer freebores without issue. Look up the SAAMI specs on a 300 Win Mag for a clue.

http://www.saami.org/PubResources/CC_Drawings/Rifle/300 Winchester Magnum.pdf

Dave
 
Dave, I think ???? the whole situation might be helped ir the leade were about 3-4 times as long as the present 1.5 degree leade.
 
Freebore diameter experiment

Interesting discussion. I'm not sure how the freebore diameter relates to accuracy, but I suspect that a little larger is better than a little smaller.

For example, when testing my 6.5mm wildcat, I found that I got smaller velocity spread with a 0.2648" than with a 0.2643" freebore diameter. I discovered this phenomenon totally by accident. The testing was with a Krieger and a Lilja barrel with the 140g Sierra HPBTM (discontinued) and Berger 140g VLD. These velocity spreads don't seem to matter much at short range, but they certainly do at long range.

I also believe that accuracy was better with the larger freebore diameter throughout multiple shot strings…but I never really shot enough rounds during this testing to be conclusive on the accuracy results. I've speculated that this area of the freebore becomes excessively fouled, and if this cylindrical shaped section of the freebore is cut a little larger, the accumulation of the fouling won't affect the bullet as much.

Note the 308 Winchester in both SAAMI and CIP specification has a freebore diameter 0.002" above nominal bullet diameter. Maybe this is one reason why so many different 308 rifles shoot so well.

Greg Walley
Kelbly's Inc.
 
Dave, I see where they started at end of 45* into .315 dia. then into 1* 26' 37" so I guess starting around .007 oversize would be a tapered throat. If you trig. out taper to length you can move table to degree you want to make tapered freebore to. an optical grind attatchment would be a big help but not imperative to have. Other than that a high power comparator. In years past I had two cutter grinder shops this type of job wouldn't bother me. If you really need to check or inspect angle you wanted without comparator I would ream a barrel stub just to where 45 starts on neck and calculate how far a gage ball would drop to certain depth figures dia. at a certain angle...
 
JerrySharrett said:
Dave, I think ???? the whole situation might be helped ir the leade were about 3-4 times as long as the present 1.5 degree leade.
Click to expand...

Jerry

I think you might be right. I know one well known high power shooter who uses 3/4 degree on a side. Without a lot more testing who can say which approach gives 1. guilt edge accuracy or 2. very good accuracy over the long haul.
 
geo.ulrich said:
Dave, I see where they started at end of 45* into .315 dia. then into 1* 26' 37" so I guess starting around .007 oversize would be a tapered throat. If you trig. out taper to length you can move table to degree you want to make tapered freebore to. an optical grind attatchment would be a big help but not imperative to have. Other than that a high power comparator. In years past I had two cutter grinder shops this type of job wouldn't bother me. If you really need to check or inspect angle you wanted without comparator I would ream a barrel stub just to where 45 starts on neck and calculate how far a gage ball would drop to certain depth figures dia. at a certain angle...
Click to expand...

George

I knew you would get around to gage balls for measuring the taper. I think the Win Mag solution would work for the BR game. Make the angle shallow and trig out the starting diameter at the end of the neck. That would be a simple grind for a reamer maker. I think the body and pressure ring could be protected until fully inside the rifling.
 
Greg Walley said:
Interesting discussion. I'm not sure how the freebore diameter relates to accuracy, but I suspect that a little larger is better than a little smaller.

For example, when testing my 6.5mm wildcat, I found that I got smaller velocity spread with a 0.2648" than with a 0.2643" freebore diameter. I discovered this phenomenon totally by accident. The testing was with a Krieger and a Lilja barrel with the 140g Sierra HPBTM (discontinued) and Berger 140g VLD. These velocity spreads don't seem to matter much at short range, but they certainly do at long range.

I also believe that accuracy was better with the larger freebore diameter throughout multiple shot strings…but I never really shot enough rounds during this testing to be conclusive on the accuracy results. I've speculated that this area of the freebore becomes excessively fouled, and if this cylindrical shaped section of the freebore is cut a little larger, the accumulation of the fouling won't affect the bullet as much.

Note the 308 Winchester in both SAAMI and CIP specification has a freebore diameter 0.002" above nominal bullet diameter. Maybe this is one reason why so many different 308 rifles shoot so well.

Greg Walley
Kelbly's Inc.
Click to expand...

Greg

What I learned in LR BR game I carried over to my current work and it has proven out. I started opening up throats probably 15 years ago with the larger cartridges. Fewer flyers. Then went to tapered throats. Everything has shoot better, more consistent since then. So I agree that loose is better than too tight. Just as we found out fitted necks worked most of the time. A little more clearance on the neck gave more consistent results.
 
As is the usual for this sort of experiment, it will fall to someone who has the skills, tools, and budget to pursue it. If that person comes along, I think that it would be interesting to use throating reamers to explore this, all with the same barrel and chamber, starting with the tightest configuration, and working toward the largest. Perhaps a tapered freebore that started a couple of tenths over the largest pressure ring to be used would be interesting for the flat base short range bullets. I think that due to their relatively short bearing surfaces, that short range bullets may present more of a challenge in maintaining alignment as they enter the bore than their longer brethren. It may also be that the popularity of BT bullets with some shooters for short range, may be related to how they interact with the common throat configurations as they begin their journeys, rather than being about external ballistics.
This thread took a while to get going, but it has certainly drawn in some interesting and illuminating responses. Thank you all for sharing.
 
Boyd Allen said:
As is the usual for this sort of experiment, it will fall to someone who has the skills, tools, and budget to pursue it. If that person comes along, I think that it would be interesting to use throating reamers to explore this, all with the same barrel and chamber, starting with the tightest configuration, and working toward the largest. Perhaps a tapered freebore that started a couple of tenths over the largest pressure ring to be used would be interesting for the flat base short range bullets. I think that due to their relatively short bearing surfaces, that short range bullets may present more of a challenge in maintaining alignment as they enter the bore than their longer brethren. It may also be that the popularity of BT bullets with some shooters for short range, may be related to how they interact with the common throat configurations as they begin their journeys, rather than being about external ballistics.
This thread took a while to get going, but it has certainly drawn in some interesting and illuminating responses. Thank you all for sharing.
Click to expand...

Boyd

I would think alignment issues would be the same. How the two react to pressure and current throat configurations is what I see as the issue.
It would very difficult to do that test with one barrel/chamber. I don't think it would be possible to control and document the changes made with the throating reamers. Done properly I wouldn't have doubts about the results if the barrel was set back a turn for each new throat. As far as that goes if you could get a reamer maker to do this make a reamer with the only cutting area the throat. The non cutting shoulder or neck would act as a stop. All dimensions would be laid out the same as the original reamer.

Dave
 
Dave Tooley said:
Boyd
It would very difficult to do that test with one barrel/chamber. I don't think it would be possible to control and document the changes made with the throating reamers.
Click to expand...

May I suggest another possible option. You can bore your throat or entire chamber for that matter on your TL-1. As long as you can remount your barrel the same I see no reason you couldn't rethroat your chamber accurately with great flexibility in design. I've come up with a system that works well and I would be glad to pass along to you the things I've learned if you were interested. I use a tiny Iscar insert cutter with the shank ground to just clear the chamber on the X-minus side at its deepest point to maintain the most rigidity as I can. For setting up to recut the throat on your test barrel I would adjust in the tools X offset by test boring a scrap chunk of barrel and gaging the bore until you are certain of the tools exact offset at your throats cut diameter. After indicating the barrel back in, it should be a simple Z offset, new program and cut a new throat.

My unsubstantiated thoughts on throat design is the center of the bullet at the point of contact with the leade angle should be right on center as good as the throat was cut to the groove diameter. We can't precisely control the back of the bullet kicking to the side somewhat due to the necessary neck clearances. In my opinion we are somewhat at the mercy of the throat to straighten it out upon entry. I would imagine the smoother the transition the less deformation of the bullet. I started thinking about this when I was setting up to cut a 30BR Robinette chamber with no freebore. Off the end of the case was a 45 degree chamfer to the freebore diameter, then the leade angle. I just didn't like that sharp corner and so I started designing mine with a radius transition to the leade angle.
 
Joe

From experience it would be very difficult to set a barrel up exactly the same and then be able to go in with a boring bar and redo the throat. A reamer following an existing hole yes. The time and trouble verifying what you did right and what you did wrong would pay for a couple three of reamers as I described. A 30 cal. throating blank could be ground for a 6mm throat very easily.

I have to question the argument about bullet alignment in tight neck chambers. Old wives tale? It hasn't made any difference with SAAMI bodied chambers and factory ammo. As always your mileage may vary.


Dave
 
Dave

I certainly yield to your practical smithing experience and by no means want to come across as argumentative. From a CNC machining perspective, I look at the realignment of the barrel and the tool setup as being rather simple. Maybe our setups differ though. I use a fitted bushing on the muzzle end that centers in my spindle bore. I do not adjust the muzzle end to align the chamber coaxially with the "banana". I do use a spider type contraption on the chamber end. I have had to rechuck barrels before to rebore the chamber and I find it easier than setting up on a new barrel, mainly since I can indicate in on a true cylinder of the previously cut chamber. Once the barrel runs true I calculate my Z offset and hit the go button. Probably the biggest difference is I use a CAD/CAM system to program the toolpaths and it just doesn't make a mistake. We have come to trust the numbers the CAD produces and don't even read the programs anymore. Not having that confidence can be a bit nerve racking I'm sure.

I'm not sure we are on the same page regarding tight neck bullet alignment and I certainly have no argument. I just aproached my throat design by cutting a radius between the 45 degree angle off the neck and the leade angle rather than leaving a sharp edge there. Whether that's a fix to a problem that doesn't exist I don't really know. I don't see how it could hurt though.
 
Joe,

No problem here. An exchange of ideas is always welcome. We each have own perspective. Sometimes I come across a little blunt. I'm still learning and continue to glean/swipe bits of information where I can. I set barrels up essentially the same way you do. I think there can be a small problem setting one back axially. Radially no problem. No matter what we do a spider can have some influence on the center line of the bore. The trick is to know it can and how to minimize it. From my perspective I know a reamer will always be self centering unless outside forces affect it. I'm not a CNC guy. I will never fully utilize the capabilities of my TL-1. I understand the capabilities of a CAD-CNC and I'm always amazed at what a little creativity can do.

I get it now you're putting a radius at the end of the freebore as it transitions to the lead angle. Makes sense. As you say it can't hurt. I'd think it would happen naturally after few hundred rounds.

Dave
 
Dave Tooley; said:
Now the 64 trillion dollar question. How do you grind it in a very short freebore and then how do you confirm that's what you ordered? I've have it done on longer freebores without issue. Look up the SAAMI specs on a 300 Win Mag for a clue.

Dave
Click to expand...

Dave,
I'm sure it would be difficult as I have not even been able to have a short free bore reamer resharpened without the free bore diameter becoming undersize.

This discussion has piqued my interest. Most of my focus has been on short range PPCs with short free bore. I am starting to work with the 6mm BR and 8 twist barrels for longer range. The idea of tapered free bore over about .200 or a little more makes sense. Approximately how much taper would suggest?

Scott
 
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