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7mm Rem Mag Load Development Part 2: Audette’s Ladder Test

19 Oct

The first step in load development for my new custom 7mm Rem Mag started with gathering the real-world velocities I could expect out of three quality long range bullets I was considering. I then used those velocities to compare the ballistic performance between those three bullets, as well felt recoil during the test rounds. I eventually decided to pursure further load development for Berger’s 168gr VLD bullet over Hodgdon Retumbo powder because it struck the right balance for me between the competing characteristics (recoil, barrel wear, flat trajectory, low wind drift, ideal energy for medium sized game, etc). For more details on this first step, see my post about it.

After choosing the bullet, I used Hornady’s OAL gauge to determine the approximate bullet seating depth that would be just off the lands (.005″ – 0.010″ off the lands).  For my chamber, that was 2.770″ from the base of the loaded round to the ogive of the bullet.  Note: You should always measure cartridge overall length (COAL) to the ogive of the bullet, not the tip of the bullet.  Since I’m shooting a Berger VLD bullet (highly sensitive to seating depth), I will come back and fine-tune seating depth again after I settle into a powder weight … but for now, I just want it to be very close to the rifling.

My next step was to use Creighton Audette’s Ladder Test to identify a range of powder weight that show little sensitivity to minor changes in charge weight and/or pressure. Theoretically a range like that would allow a bullet to group even if a particular component varied slightly (e.g. primer was slightly hotter, neck tension wasn’t identical, there was .05 to 0.1 more or less powder in it, there was more/less bearing surfuce on the bullet, etc).

I’m purposely trying to stay in the mid-range of powder charges for the 168gr VLD for reduced recoil and barrel wear. You could definitely load the 7mm Rem Mag up or down more than what I’m showing here … but I felt like this range of powder charges was the right balance for me, because the ballistic advantage of loading it up a few more grains really isn’t that significant.

Here are the results of my ladder test. You should disregard horizontal dispertion. It was due to slight wind drift (targets were placed at 200 yards), because I used the same point of aim for each shot without holding for wind. With the ladder test, the things you want to focus on are vertical dispertion & muzzle velocities. After you settle in on a round, you can fine-tune grouping and figure out what adjustments you need to make for wind drift.

Audette Ladder Test Velocities for 7mm Rem Mag

Based on these results, its clear there might be a sweet spot somewhere between 69.6 to 70.0 gr of Retumbo.  I actually had a match come up before I could do further load development, and so I just loaded rounds at 69.8gr of powder based on these results.  Although I hadn’t ever shot that round past 200 yards and only had “theoretical dope” past that … I made it 80% of the way through that match before my first miss, and ended up placing 2nd.  The dope was good out to about 600, but beyond that I had a few misses. I also later fired a 0.188″ group with my first 3 shot group of that load.

Despite these stellar results, the ladder test isn’t the last step to load development.  Unfortunately it is very dependent on single shots, so results could be easily skewed by one outlier that had a hot primer, bad chronograph reading, or more/less neck tension or bullet runout.  Having said that, it’s still sometimes suprising how quickly the ladder test will help you zoom into an accurate load.

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4 responses to “7mm Rem Mag Load Development Part 2: Audette’s Ladder Test

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