The six loads tested separated by 0.1gn.

The ladder test for rifle load development


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In the last issue, I briefly explained Creighton Audette’s Ladder Test or Incremental Load Development Method (ILDM). In order to personalise the concept, I illustrated the initial stages by picking a powder and projectile for my .30-06 Sauer 202, abbreviating the test parameters somewhat by taking an educated guess at where a set of components may start to work and giving it a go.

The five increments which I chose were 0.4gn between 60.7gn and 62.3gn of Reloader 22 behind the Sierra 180gn Spitzer Boat Tail bullet, seated at around 3.280″ (thirty thou’ off the lands) give or take 5 thou’. That first test revealed what could be interpreted as a “sweet spot” at 61.5 and 61.9gns. The groups of both charges were fine, around 1 MoA for three shots and the initial group of one shot of each charge indicated that the charges of 60.7, 61.5 and 61.9 gns all fell within .75″. Surprisingly, the 61.1gn charge fell well outside the group, as did the 62.3gn charge (understandable at 0.3gn over Nick’s maximum and this gave some case head expansion but no sticky bolt lift or ejector marks).

At this stage I’ll explain that my aim coincided with Audette’s rationale of finding a workable range of charges, whereby picking a middle charge from several well-performing, sequential charges would give me a “tolerant” load, one where you could fairly confidently change parameters like brand of primer or maybe even similar weight of differently branded case, for example, and be back in the reasonable load ballpark that performed well.

I decided to do a 0.1gn increment between 61.4 and 62 grains, without the 61.9gn step, which I considered to be a known quantity ie good. The results again gave some minor surprises but achieved the aim.

Firstly, the repeated 61.5gn charge was ho-hum, with a 1.5MoA span an inch higher than the generally occurring zero. 61.6gn was a wild card that gave a 2.2MoA group with vertical stringing. The addition of 0.1 gn to 61.7gn reduced group size to .65MoA with .3MoA elevation spread – pretty good. Adding another 0.1gn to 61.8gn closed it up to a ragged hole of 0.4MoA with 0.35MoA elevation. Woo-hoo!! 

What has this process revealed to me so far?  First up, the “sweet” range of charges has narrowed to 61.7 to 61.9 grains. I am happy to standardise on the median 61.8gn as it is below the particular manual’s recommended maximum. What I am slightly disappointed by is that the accurate, consistent range, in such a large capacity cartridge, was so narrow. There were two surprises where individual charges, within larger ranges that were overall pretty similar, shot out of the normal zero and into a comparatively large group. I believe, from my reading that, if I had used a standard very mild starting load and worked up in twenty 0.2gn increments, that another sweet range, possibly over a wider range of charges than my experiment proved, would become evident lower down the range. My lack of patience and unwillingness to spend money so freely forbade me from doing so. 

The rifle is obviously seriously accurate, but I have had to exercise some patience (something I am not used to doing) to allow the barrel to a) shoot in, b) let the barrel cool between groups and c) work up a highly accurate load it likes. Speaking of accuracy, it is unrealistic and probably not productive to conduct this exercise at 100 metres (Audette recommends 200-300m) unless the firearm is accurate enough to identify emerging trends reliably; if its best grouping potential is, say 2 – 3MoA, then you will chase your tail. I would also observe that the results I have obtained are not unimpeachable but merely a pretty solid indicator that I have arrived at the ideal load. Were I to repeat the tests several times, I may come up with somewhat different results but I am happy nonetheless. If I was to shoot in the heat of summer, I would reduce this load by a grain and take any consequent accuracy fall-off in preference to making unwelcome pressure excursions. Throughout, I have not been anally retentive by trimming all brass to minimum spec’, uniforming primer pockets or noting which brass has been fired how many times, but I have used only Remington brass, uniformly factory crimped and, in the first test, weighed all charges electronically with the second test using balance-beam charge weights. The reasons were to build a bit of real world small error margin into the process so that results are more robust for use by your average beginning reloader.

Note. Having chosen the 180gn Sierra and, particularly Reloader 22 propellent, I would offer the following observation. Close to maximum loads of very slow burning powders with heavy projectiles from a short-barrelled rifle really test the shooter’s ability to concentrate through muzzle blast that would curl my hair, if I had any. I used a Sierra manual recommended accuracy load (quite mild) of AR 2213SC behind a 155gn Sierra HPBT Match King for occasions when I may “plink” with this gun and shoot extended strings with it. It was still pretty severe. My best man, Mark Adamson (an experienced centrefire rifle shooter) took a few shots with that load off the bench recently and handed it back saying, “That’s enough, mate” prior to completing his allocated ammunition. Also, people have commented on the blast from other parts of a range where 7.62 NATO loads are the norm. In the hunting scenarios I will experience, this will not be an issue due to me walking lots and shooting little. 
Additional note. There are other factors that could affect the validity of results eg increasing fouling with progressive shooting, barrel heating and cooling, environment conditions eg wind changing etc but, in field conditions I defy anyone to hold so they can determine the difference between 0.5 and 1.5 MoA in larger calibres. Nevertheless, I believe the process is useful for those chasing better accuracy overall.
Incidentally, including the one over-max load, all loads fired exhibited normal pressure signs through measurement of non-existent case head expansion. Were I to repeat such a test as a purely academic exercise in future, I might pick a 125gn bullet and AR 2206H or AR 2208 powder so my extended bench time is a bit more pleasant.

Where to from here?  I will load up several lots of five 61.8gn loads with different seating depths and see what gives the most consistent results. Who knows! When that happens, the range of accurate loads may expand from a 0.3gn range up to something a bit more forgiving of changing variables. Hopefully, I won’t have worn out the barrel by then.

In the next instalment, I’ll be using another load development method, that of Dan Newberry’s, known as the Optimal Charge Weight (OCW) method, which is supposed to be a significant improvement on the Ladder Test.  Finally, keep shooting and never lose your curiosity.

 

This article was first published in Sporting Shooter, October 2009.


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