Written by Sierra Bullets Ballistic Technician Rich Machholz
Nearly every day and for sure every week each of us technicians get asked how we work up our own personal loads. Now I don’t live in a fantasy world so I think it is safe to say we all use a somewhat different method to achieve the same perceived result. So my method is outlined below. But I have to warn you, I have been known to throw a load together only to find that it is good enough as is without further testing.
Case in point – I got a new-to-me IBS Heavy Gun and since it was new-to-me, I borrowed some off the shelf RCBS dies from my brother in-law. The gun came with some Norma brass luckily because it is a caliber I would not have picked on my own. Since it is a 300 Magnum I wanted to shoot our 210 grain HPBT MatchKing bullet #9240. I have a good supply of Retumbo on hand so I dreamt up a load and threw it in the first 20 cases I could grab that had been primed with W-W WLRM primers.
That chore completed, I mounted a Burris XTR 8-40X to the integral 20 MOA rail and headed to the farm where I have a 250 yard range. After I got all the benchrest paraphernalia set up I positioned myself so I could look through the bore of the rifle and proceeded to bore sight it. The first shot was on paper low and slightly left so I clicked to the hole and fired five more shots.
I had another such stroke of good fortune with my primary hunting rifle a Ruger M77R 7×57. For years I had shot IMR4350 and the Sierra 140 gr SPT ProHunter #1910 but a Montana Mule deer hunt brought with it the prospect of longer than normal shots so a search began for a flatter load. I chose the streamlined Sierra 140 gr SBT GameKing #1730 for a bullet and VihtaVuori N165 for the powder. I had some new W-W 7×57 Mauser brass and plenty of WLRs so I checked the manuals, picked a load and assembled three shots. I used the same COAL as my original ProHunter load. Over the years that load has produced many 1/2 inch 3 shot groups, just like it did originally. They don’t all work like that but it is nice to be lucky.
So, when I’m not being lucky how do I really find my loads?
I do have a method besides being lucky and it is as follows:
First things first, select a bullet and a powder. Find the powder charge that works for that bullet. This is easily done with a “Ladder Test”. You may need to shoot several tests to find the load that suits you.
Next, swap whatever available and appropriate primers you have through the above powder charge to establish a balanced ignition.
Shoot this test in one trip to the range as you want everything to be as similar as possible. This is best done on the same day and same target so you can get a quick visual comparison of group size and POI.
Then, using that powder charge and primer as established previously, start working the bullet seating depth from magazine length back at least .030″ to .045″ less than max mag length in increments no greater than .010″ to establish the best COAL for this combination.
After firing your tests in .010″ increments there will be a true standout dimension. To narrow that down even closer, load groups at your previously determined dimension AND .005″ greater AND .005″ less than the previous best dimension to determine the true performance corridor.
The result of all this testing is a stable, well-balanced load that will be consistent day in and day out tailored specifically to your firearm.