How do you determine optimal freebore dimensions for the reamer
- Thread starter Cassidy
- Start date
Dusty Stevens
Shiner
Neck clearance has never been important in the accuracy game. Some thought close was good but the winners always had plenty of clearance. It is of utmost importance if theres not enuf so dont cut it close. Freebore is determined by seating a bullet right where you want it and sending that dummy round to your favorite reamer mfr or by asking someone who has already used that particular combo. The reamer grinders already know what you want unless youre using a lesser known bullet.
I know of one of our top shooters that Throated his out more because the bullet was down in the case more than what he thought it should be, and then set the bullet out into the lands where he had it before. The rifle went from shooting 4 and 5 inch groups to 7". He then went back to the original seating depth, and it started shooting small again. Harmonics!
Joe Salt
Joe Salt
I think the far more interesting question is, "what is the optimal freebore diameter for best accuracy?"
Joe, my cal is 6.5 x47, but as Richard (KingX) indicated above, I would like to get your opinions on how, for any given cal, to precisely determine how to determine freebore dimensions (diameter and length) for precision shooting for any given round. On the "Neck Clearance Trend" thread, recently Alex said he orders his reamers with a freebore that is 0.0002 greater than the diameter of the bullet at the shank regardless of the diameter at the pressure band. This is the sort of guidance I am looking for. It would be helpful to get broader feedback from a number of people familiar with this matter. I don't think that just asking the reamer maker to select these freebore dimensions is always the best approach; another guy on this site has a 6.5x47 and ordered his reamer from the same mfr as I did, and we got way different freebore length dimensions. It would be helpful if some of the experienced shooters on this site can help us determine how we ourselves can better precisely determine/select/calculate the freebore dimensions on our reamer.
Don't know if this helps but my 6.5-47 has 140 freebore for 130 VLD's
Dusty Stevens
Shiner
Cassidy said:
So for a 6.5mm he orders a .2642 freebore? We must be talking about 2 different things cause half that is about normal.
Roeder
Silver $$ Contributor
What you seem to be asking is not easily answered because The most accurate freebore diameter and length varies somewhat with different barrels and bullets. The easiest way to measure (and the method the reamer makers use) is to view the sample cartridge on a high magnification optical comparater and take the measurement from the image. Different comparators allow different methods of doing this. Some even have software that can calculate measurements from the image (shadow) of the object being viewed. One has to determine how the freebore will engage the bullet and measure to another reference point such as the mouth or shoulder of the case. It is harder to describe than it actually is to do.
It is also possible to use a CAD program to draw the relationship between the bullet and the chamber throat and determine a pretty close estimate. This all assumes that you know exactly where you want to seat the bullet in the neck of the case and how much jump (if any) you want.
Reamers are ground to a tolerance of about plus .0004. If you have one reamer with a nominal freebore diameter of .2642 and another reamer ground to the same spec. but with an (in tolerance) freebore diameter of .2646, the 2 resulting chambers will be noticeably different in functional throat length. This may be exaggerated by machining conditions such as a loose pilot bushing or misalignment of the lathe tailstock or just the gunsmith's machining technique.
For these and other reasons, I tend to give the reamer grinder the benefit of the doubt if a reamer does not hit the exact throat length intended. Many smiths use short throat chamber reamers and lengthen the throat carefully with a separate throating reamer to achieve the exact results desired.
A gunsmith with plenty of experience building 600 and 1000 yard BR guns would have valuable insight into this subject.
It is also possible to use a CAD program to draw the relationship between the bullet and the chamber throat and determine a pretty close estimate. This all assumes that you know exactly where you want to seat the bullet in the neck of the case and how much jump (if any) you want.
Reamers are ground to a tolerance of about plus .0004. If you have one reamer with a nominal freebore diameter of .2642 and another reamer ground to the same spec. but with an (in tolerance) freebore diameter of .2646, the 2 resulting chambers will be noticeably different in functional throat length. This may be exaggerated by machining conditions such as a loose pilot bushing or misalignment of the lathe tailstock or just the gunsmith's machining technique.
For these and other reasons, I tend to give the reamer grinder the benefit of the doubt if a reamer does not hit the exact throat length intended. Many smiths use short throat chamber reamers and lengthen the throat carefully with a separate throating reamer to achieve the exact results desired.
A gunsmith with plenty of experience building 600 and 1000 yard BR guns would have valuable insight into this subject.
Alex Wheeler
Site $$ Sponsor
Free bore diameter, not length is the question. Right or wrong I have had good success with .0002 over bullet shank diameter. Speaking with some reamer makers that is not an uncommon number to shoot for. Now whether your grinder can hit that number is another matter. I know for fact that the pressure ring of the bullet being larger than the freebore diameter does not hurt accuracy. I spec my dasher reamers .2435 -.0000+.0002. If I get a lot of bullets .2434 or .2435 at the shank I would either send them back or sell them. Some world record holders are having good success with ever larger freebore diameters, maybe they will chime in. I think like most things in this sport you are better to error on the side of too much clearance rather than not enough.
OP's question:how do you determine the optimal freebore dimensions for the reamer.
I agree with all of what ZFAST said, but you may not get the .0002" tolerance you ask for, when the manufacturers tolerance on the print says +.0004". With that being said, bullet tolerances are are pretty much the same way, they fluctuate (tolerance wise). Knowing that, there is not an optimal freebore dimension for the reamer, but more like a close to optimal freebore dimension. Even the best optimal reamer it may not make the barrel shoot, if the barrel is not optimal. Barrels come in different diameters ( back to tolerances) that will make that reamer not cut the chamber the exact way you thought. So if you get a reamer ordered with the proper diameter throat and length, so you can, EXAMPLE: (shoot heavy bullets ) the free bore should be long enough so when the round is loaded the bullet pressure ring is above the donut zone of the case ( thats the reason for the dummy round). But Not so far above the donut zone to limit the type and weight of the bullets you want to shoot. And that optimal freebore length you were shooing for will get longer, in in a matter of 700rounds down the barrel as burns farther and farther . I think freebore length of .145" to .155" is optimal for 6.5x 47 shooting 130 VLDs. If you are lucky enough to get that optimal reamer, optimal barrel and optimal bullets you are half way there to an optimal group.
If you know for sure what bullet you were going to shoot, then it would be easier to spec a reamer to that
I agree with all of what ZFAST said, but you may not get the .0002" tolerance you ask for, when the manufacturers tolerance on the print says +.0004". With that being said, bullet tolerances are are pretty much the same way, they fluctuate (tolerance wise). Knowing that, there is not an optimal freebore dimension for the reamer, but more like a close to optimal freebore dimension. Even the best optimal reamer it may not make the barrel shoot, if the barrel is not optimal. Barrels come in different diameters ( back to tolerances) that will make that reamer not cut the chamber the exact way you thought. So if you get a reamer ordered with the proper diameter throat and length, so you can, EXAMPLE: (shoot heavy bullets ) the free bore should be long enough so when the round is loaded the bullet pressure ring is above the donut zone of the case ( thats the reason for the dummy round). But Not so far above the donut zone to limit the type and weight of the bullets you want to shoot. And that optimal freebore length you were shooing for will get longer, in in a matter of 700rounds down the barrel as burns farther and farther . I think freebore length of .145" to .155" is optimal for 6.5x 47 shooting 130 VLDs. If you are lucky enough to get that optimal reamer, optimal barrel and optimal bullets you are half way there to an optimal group.
If you know for sure what bullet you were going to shoot, then it would be easier to spec a reamer to that
Alex Wheeler
Site $$ Sponsor
Tom, I sent my dasher reamer back to Hugh to shorten the freebore .020". That should put it at 2.8 to the ogive with our tools. My his numbers that is .140". I would say he's the best. So if you have to guess I would say .140-.150. But like you I had a .124 that put me at 2.795 and a .135 that put me at 2.775. My last 2 JGS reamers came out very close. Maybe I got lucky?
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