Determining freebore from sammi drawing (dummy needs help)
- Thread starter Shawnba67
- Start date
Cloudrepair
Silver $$ Contributor
It looks like
2.0788 is referance of were the datum of bullet ogive hits lands. so free bore is 2.0788-1.9393=.1395 1.9393 looks like end of chamfer.
2.0788 is end of 2* lead in to bore or where the ogive hits the lead in end of free bore.
Edited
2.0788 is referance of were the datum of bullet ogive hits lands. so free bore is 2.0788-1.9393=.1395 1.9393 looks like end of chamfer.
2.0788 is end of 2* lead in to bore or where the ogive hits the lead in end of free bore.
Edited
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I've not had coffee yet but what cloud repair said with one change . I would say it's the end of the case which is 1.924 so using his math just substitute case length # .
A lot don't take the chamber neck chamfer into the number but I do . It actually adds a small amount to the freebore .
Coffee time , ,,,,'
Ok forgot to add not all use this saami chamber . If the leade angle changes to a 1.30 deg it'll change the freebore a little .
A lot don't take the chamber neck chamfer into the number but I do . It actually adds a small amount to the freebore .
Coffee time , ,,,,'
Ok forgot to add not all use this saami chamber . If the leade angle changes to a 1.30 deg it'll change the freebore a little .
Cloudrepair
Silver $$ Contributor
Yes, the freebore is from 1.9393" to 2.000". The key is that the freebore is defined as the area, forward of the case mouth that has no grooves. Typically it has no taper and it is very close to the groove diameter. In this drawing you can tell where the freebore is by noticing that the diameters at the 1.9393" and 2.000" lengths are specified to be the same. That means that there is cylindrical space (a free bore) where there are no lands or grooves just forward of the case mouth.
A reference dimension is NOT a Datum point, it is just what they say...a reference. Reference dimensions are not true or exact dimensions, they are either mathematically calculated values based on the drawings other toleranced dimensions or they are just a dimensional value that is used to help visualize a part's general size.
If you look at the reference dimensions in the SAAMI drawing you will notice that there are toleranced dimensions on either side of the reference dimension. That means that the reference dimension will vary depending on the tolerance of the dimensions to either side of it. There is no tolerance connected to the reference dimension so it can be any value between the toleranced dimensions on either side of it. But if the diameters at the toleranced points and the angles are correct then the referenced dimension will be fairly accurate. Basically it's a way for you to estimate that all the angles and dimensions, from the bolt face forward, should be right if the reference dimension comes out to the value specified. It makes for an easy reference to check to see of the case is generally dimensioned correctly without having to check very small angles and diameters that would require complicated setups.
Both the 1.9393" and the 2.0778" dimensions are reference dimensions that are included to help you visualize the size and shape of the case, they are not true dimensions that define the features and dimensions of the case, you do not use them as dimensions to be used to build a gauge or other precision tool. The reason that they give you a reference dimension is that the angle of the second shoulder (the small angled shoulder directly in front of the case mouth) can vary to some degree because there are no angle tolerances associated withe this drawing's angles so where the freebore starts can vary due to changes in that angle. The other end of the freebore (2.000") is a toleranced dimension and can change by + .015" and the lead (the area where the barrel is cut as a taper from the groove dimension to the land dimension) can vary by quite a lot so the length of the freebore can change a lot.
The 2.0788" is another reference dimension and it is where the angle of the lead reaches the diameter of the lands. As mentioned earlier, the angle of the lead can vary and that will change where the full diameter of the lands are.
A reference dimension is NOT a Datum point, it is just what they say...a reference. Reference dimensions are not true or exact dimensions, they are either mathematically calculated values based on the drawings other toleranced dimensions or they are just a dimensional value that is used to help visualize a part's general size.
If you look at the reference dimensions in the SAAMI drawing you will notice that there are toleranced dimensions on either side of the reference dimension. That means that the reference dimension will vary depending on the tolerance of the dimensions to either side of it. There is no tolerance connected to the reference dimension so it can be any value between the toleranced dimensions on either side of it. But if the diameters at the toleranced points and the angles are correct then the referenced dimension will be fairly accurate. Basically it's a way for you to estimate that all the angles and dimensions, from the bolt face forward, should be right if the reference dimension comes out to the value specified. It makes for an easy reference to check to see of the case is generally dimensioned correctly without having to check very small angles and diameters that would require complicated setups.
Both the 1.9393" and the 2.0778" dimensions are reference dimensions that are included to help you visualize the size and shape of the case, they are not true dimensions that define the features and dimensions of the case, you do not use them as dimensions to be used to build a gauge or other precision tool. The reason that they give you a reference dimension is that the angle of the second shoulder (the small angled shoulder directly in front of the case mouth) can vary to some degree because there are no angle tolerances associated withe this drawing's angles so where the freebore starts can vary due to changes in that angle. The other end of the freebore (2.000") is a toleranced dimension and can change by + .015" and the lead (the area where the barrel is cut as a taper from the groove dimension to the land dimension) can vary by quite a lot so the length of the freebore can change a lot.
The 2.0788" is another reference dimension and it is where the angle of the lead reaches the diameter of the lands. As mentioned earlier, the angle of the lead can vary and that will change where the full diameter of the lands are.
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Cloudrepair
Silver $$ Contributor
Maybe after my last edit of first reply will make what I was trying to say clearer.
Dusty Stevens
Shiner
Saami freebore is very long to keep from building pressure. Theres not a bullet out there that will reach the lands in a 22-250 saami chamber. Your dtac example shows a more believable freebore that we would look for in our world. An example is you cant reach the lands in a saami 243 even with dtacs
.
Correct answer. The 22-250 reamer print in Dave Kiff's book of reamer prints clearly shows .061 freebore from the same reference lines. This amount of freebore leaves a fair amount of jump with factory 40 gr loads, but heavier bullets such as 69 grains and up can almost certainly be seated to touch the lands.
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