6mm BR Norma Chronograph Results (discouraging)

[.300winmag]

So on new years eve, I took my new toy over to my folks house with the fiancé for some family bonding. The usual occurred, my fiancé, mother and sister gathered to talk about what is important to them, and my father and I started weighing cases, bullets and then measuring bullets with an ogive comparator for the next days range session to chronograph the loads out of my 6mm BR Norma. When all was said and done, brass had been sorted so that we had 10 identical weighing cases, 14 identical bullets in both weight and ogive measurements (there was a lot of time to kill before the ball dropped :mrgreen: ). We loaded them up with 30.0 grains of Varget, seated the 105 grain Berger Hybrid's 0.015" off the rifling. The results from my 8 year old Chrony chronograph were less than stellar. I've included a picture below. Shot 4 and 7 did not register. I excluded the first shot as an outlier and averaging the remaining shots yields an average velocity of 2,967 FPS. Based on my research and .257 Ackley's posts (which are more than useful), I take this number as hogs wash! I also would like to believe my extreme spread is not that large.

I had my ballistics calculator with me and decided to try a different approach to coming up with a decent ballistic solution for the competition coming up. I ranged a rock with the Leica at 613 yards, about the size of a softball, based on my educated guess through the Swarovski.Using Berger's advertised G7 BC along with the atmospherics from my Kestral and a MV of 2,925 fps. My father touched off the shot and the rock disappeared. I then found a rock at 584 yards, same size with the same results after plugging the yardage into the ballistic computer. After that, we focussed on a 14" gong my father and I had set at 1,078 yards according to the leica. We plugged in the info (including spin drift, excluding Coriolis) and fired. Dead nutz center. I was very pleased...While the chronograph may have not provided the proper information, I feel confident iny being able to ring the max point target at the next shoot (800 yards 16" diameter) with the first shot. Still no groups to show at extended ranges... Looking forward to stretching it out on paper however.

 

[wisconsinteacher]

I think you have it shooting well. As far as the number (speed) I feel that all published data runs on the high side. Your spread is a little more extreme than I would have guessed but the main point is that you are hitting what you are aiming at. Keep up the good work.

 

[DrMike]

Only one velocity is wildly out of line with the remainder. Unless you are absolutely certain that the load was accurate and that the reading is not an aberration, I'd discount it and continue enjoying shooting the 6BR Norma.

 

[257 Ackley]

Do you use a bushing die or regular die with a button? Did you check the thickness of the necks to ensure they were all the same along with the weights? The reason for asking is that those two factors will effect ES quite a bit even if you have an accurate load. Even neck tension, so the bullet is released the same every time plays a key role in the ES, especially if you are using a bushing die.

It can be the barrel needs a few more rounds through it as well to settle in. I had a barrel on a 6BR that didn't want to shoot with any consistency until it had about 90 rounds down the tube. That was a HV Bartlein barrel, so it should have shot well out of the gate.

 

[.300winmag]

Only one velocity is wildly out of line with the remainder. Unless you are absolutely certain that the load was accurate and that the reading is not an aberration, I'd discount it and continue enjoying shooting the 6BR Norma.

Dr. Mike,

I agree that only one shot was outrageous. I was however was very disappointed in the extreme spread and standard deviation in velocity. For the scrutiny we put into those choreographed loads, I expected much better. However, I believe .257 Ackley is correct in the fact that the barrel is not broken in fully, resulting in large velocity changes. The load used has performed consistently every time I've taken it out. I've taken it out 6 times now and only shot it at 100 yards, but each time the groups are averaging around 0.13-0.16" inches. That is with once fired and new brass. I'll chronograph them again at a later date and see what the numbers tell us.

 

[.300winmag]

Do you use a bushing die or regular die with a button? Did you check the thickness of the necks to ensure they were all the same along with the weights? The reason for asking is that those two factors will effect ES quite a bit even if you have an accurate load. Even neck tension, so the bullet is released the same every time plays a key role in the ES, especially if you are using a bushing die.

It can be the barrel needs a few more rounds through it as well to settle in. I had a barrel on a 6BR that didn't want to shoot with any consistency until it had about 90 rounds down the tube. That was a HV Bartlein barrel, so it should have shot well out of the gate.

.257 Ackley,

I'm using a bushing die. I have a .269, .268, and .267 bushings. I started usin the .268 bushings but have moved to the .267 bushing. I have a .271 neck on myrifle and the loaded cases are measuring .270". This seems a little tight with a no turn neck, but my gunsmith said it should work.

I've measures the neck thickness of about 30 cases all the way around and they are within a thousanth around the entire circumference. I understand neck turning cN be beneficial As long as the rifle, shooter are capable, and it is done properly. However, I've tried neck turnig in other competition rifles and never noticed a difference. I've never had a rifle that shoots his small of groups however.

I'm new to bench rest, I'm still trying to learn to drive this thing, but with free recoil, it seems to shoot. You seem to have much more experience with this cartridge Nd these types of rifles than I, so I would greatly appreciate any an all advice in loads, shootin techniques etc. Would be greatly appreciated.