I decided to measure the exact COL for the Sako tonight not difficult and pretty straightforward. Well it wasn't difficult but equally it wasn't straightforward. The idea is to use an empty fire formed case and a bullet head (i.e. no charge) by loading it carefully and then removing and measuring the exact COL can be calculated.
Here's the first 10 measurements all in millimetres:
67.00
65.70
66.61
66.87
66.21
67.04
66.19
66.92
66.87
66.37
Absolute garbage, so why was this happening ? I decided that the case wasn't holding the bullet head tight enough so I tweaked it a little and things got better:
66.41
66.59
66.76
66.50
66.89
66.58
66.60
66.54
66.43
66.44
Much better but still nothing to write home about, there was still something I had overloooked. Then I realised, the lands were gripping the bullet head as the bolt extracted, pulling the bullet a very small amount back out of the case. Once I found this I pushed the bullet close to the expected COL and carefully slide it back into the rifle, now the bullet was only being moved a miniscule amount by the bolt reducing the pressure on the lands and preventing the bullet head "sticking". The values improved dramatically:
66.42
66.42
66.43
66.42
66.41
66.41
66.42
66.43
66.42
66.42
As you can see the average is 66.42mm which if you are interested gives an overall COL minus the comparator of 77.80mm. Measuring SP bullets without the comparator is not very accurate as there is some variation due to the exposed lead tip.
I also noticed the Hornady Interlock, at 66.42 was seated 3.9mm off the cannelure (indented ring around the bullet) however this equated to 7.5mm of bullet in the case. Apparently a rough guide is that the bullet should sit inside the case a little over the diameter of the bullet, so 6.5mm calibre seated to 7.5mm ~ OK.
Determining the COL is all very well but no good if the bullet will not fit in the magazine, I checked it and thankfully there was no problem.
So I can now start the next batch of test rounds.
I hope you aren't all asleep and that someone found that useful.
Here's the first 10 measurements all in millimetres:
67.00
65.70
66.61
66.87
66.21
67.04
66.19
66.92
66.87
66.37
Absolute garbage, so why was this happening ? I decided that the case wasn't holding the bullet head tight enough so I tweaked it a little and things got better:
66.41
66.59
66.76
66.50
66.89
66.58
66.60
66.54
66.43
66.44
Much better but still nothing to write home about, there was still something I had overloooked. Then I realised, the lands were gripping the bullet head as the bolt extracted, pulling the bullet a very small amount back out of the case. Once I found this I pushed the bullet close to the expected COL and carefully slide it back into the rifle, now the bullet was only being moved a miniscule amount by the bolt reducing the pressure on the lands and preventing the bullet head "sticking". The values improved dramatically:
66.42
66.42
66.43
66.42
66.41
66.41
66.42
66.43
66.42
66.42
As you can see the average is 66.42mm which if you are interested gives an overall COL minus the comparator of 77.80mm. Measuring SP bullets without the comparator is not very accurate as there is some variation due to the exposed lead tip.
I also noticed the Hornady Interlock, at 66.42 was seated 3.9mm off the cannelure (indented ring around the bullet) however this equated to 7.5mm of bullet in the case. Apparently a rough guide is that the bullet should sit inside the case a little over the diameter of the bullet, so 6.5mm calibre seated to 7.5mm ~ OK.
Determining the COL is all very well but no good if the bullet will not fit in the magazine, I checked it and thankfully there was no problem.
So I can now start the next batch of test rounds.
I hope you aren't all asleep and that someone found that useful.
