7mm Mag range work and Maneto Speed V3

[pescado]

Hi Guys sorry this is a long post. I was out at the range over the weekend to start the load work for my Mongrel Rem 700. Its and old action with a new stock and barrel. I have settled on Retumbo and Sierra Match King 168gr as the components. All projectiles have been coated with the David Tubb Boron Nitride coating system I have been using for years so nothing new there.

I have had this rifle for over 20yrs and done a heap of load work and shooting with it so much so that the barrel had to be replaced. This was done here in NZ by Hardy Rifle Engineering with one of their barrels made here in NZ. Several strange results baffled me while at the range and I though some of you might have some answers.

I loaded up 5 shot strings with 6 increasing charges starting at 69.6gr and finishing at the Sierra max of 71.1grs Two of each string were for velocity purposes then the other 3 were for accuracy indications. I was using my new Magneto Speed V3 crony. I started shooting the 2 round strings for velocity and from the lowest load to the highest there was no difference in velocity over the whole lot all around 2700fps. This has never happened before using my old crony there is always an increase in velocity as the powder measure increases. As an aside there was no increase in apparent pressure signs on the cases. I measured them in front of the belt and from the min to max no increase in size, also there is absolutely no difference in the primer appearance.

So possible causes:

My new RCBS Chargemaster doesn't know what its doing.
So I ran some of the charge weights again changing the charge input unit the same way I did when I loaded the ammo. I then weighed them on my other scales 3 different charges all checked ok

The Magneto Speed V3 doesn't know what its doing.
Not sure about this it recorded all the shots they all had a similar but uniquely different velocity. Will contact the manufacturer to see if they have any suggestions. I can also test another of my rifles to see what happens with different loads

My powder which is 10 years old has gone funky.
Well again not sure it all went bang had a nice solventy smell and showed no signs of degradation. I have a new batch so I can try this next time at the range.

Possibly the David Tub coating is soo slippery with the new barrel that sufficient pressure cannot be achieved to increase velocity. A possibility but I have used this coating system in 4 rifles while doing load work and this has not happened before.

It sure has me turned around, I am open to any suggestions from you guys.

 

[dsculley]

Did you shoot the 3 shot strings for accuracy? What did the groups look like on target? Was there a change if POI from group to group? Were there some groups that tightened up and others that opened up? FWIW, I always trust what I see on my target more than my chronograph. I do not own a Magneto Speed, but all the reports I have read about them have been good.

I would like to see the target with the 3 shot strings to assess the nodes.

Can you get another chronograph to shoot through? Then you can set it up, put your Magneto Speed on your rifle and compare the two. If they agree, then you can change one component at a time to find out what is causing this phenomenon.

 

[pescado]

Thanks Dennis yes I did shoot the three shot groups. There was very little difference and no vertical shift in them as the charge increased. So I don't think there was any increase in velocity. I will load the same again with uncoated bullets and a new batch of powder and shoot them this weekend. I have given my optical chrony away to my hrother so cannot use that. I will also shoot another rifle with the V3 and see how that goes as well
All the best Jeremy

 

[AZShooter]

Your velocities were very similar because your powder charge weights were also very similar! You split up the difference between 69.6 to 71.1 grains with four different powder charge weights inbetween?

I have used Retumbo in NON coated 168 berger classic hunter in several 7 Rem mags. My 7 mag and two of my friend's 7 mags worked best with more than your 71.1 grains. Powder charge weights ranged from 72 to 74 grains.

I think you should try 1 grain increments to find your load. Based on my experience with moly coated bullets, I would suggest you can go past Sierra's suggested max load by several grains due to your bullet coating.

 

[pescado]

Thanks I will try bigger increments between loads and shoot them again, I will also take some factory loads to run over the chrony to see what it says about them. I will keep you guys updated with the results.

 

[bigngreen]

It's common to see this with slower powder and real low pressure loads, until you start getting some pressure it just does not burn well so you end up with inconsistent results. Once you get it up where it needs to be you'll see velocity start marching up the ladder.

 

[pescado]

Been up to the range again to have another go at getting more velocity. I decided to persevere with the Boron coated bullets as they have worked well in other rifles. I took advise from several other shooters and continued to increase the powder loads. All other variables apart form using a new batch of powder remained the same. I went up in 0.5gr increments.

71.1gr 2713
71.5gr 2890
72.0gr 2898
72.5gr 2957
73.0gr 2976
73.5gr 3004
74.0gr 3024

The 73gr load produced the best accuracy of .0368" so that's where I will load to there is no point in going over board with pressures. Thanks for the encouragement guys.