As a custom ammunition mfg, I do a lot of custom load developments for rifles. Sometimes I get calls from hunters/shooters that have tried multiple factory ammunition brands/types to get their rifles dialed in. Sometimes, no matter what they feed it, it's not good enough. They want something better. A few weeks ago I got a similar call for 2 rifles. One a 7 STW and one a 257 Weatherby. The factory ammunition was very inconsistent and did not shoot well in either of these custom rifles. The customer asked me to pull the factory bullets, and use the high end brass to load for my ammo. No problem! Whenever I pull bullets on factory ammunition, I weigh their powder charges to see how (in)consistent it is. I am using a high end Sartorius digital scale accurate to .02 grains. Here is what I found!7 STW- 140 grain Accubond factory ammo
. I pulled 28 rounds and weighed the powder charges.
The average powder charge on 28 rounds was 80.08 grains. The high was 80.52 grains and the low was 79.74 grains. That's a .78 grain extreme spread.257 Wby 110 grain AB factory ammo
. I pulled 25 rounds and weighed each powder charge.
The average was 66.56 grains. The high was 67.24 and the low was 66.08. That's a 1.16 grain extreme spread on 25 rounds!
The 7 STW had a .78 grain powder extreme spread and the 257 Wby had a large 1.16 grain powder extreme spread. So, what does this high extreme spread mean?
When loading precision ammunition to tune the ammunition to the rifle, you often load your test loads up in about .3 grain increments. For example, Load A may be 44.0 grains, load B may be 44.3 grains, Load C may be 44.6, Load D 44.9, and Load E 45.2. These 5 test loads have a 1.2 grain spread from start to finish. Within these A-E test loads, you can find a sweet spot where the rifle likes to shoot. Generally, a .3 grain increase in powder charge will increase the bullet velocity about 25-30 fps. So a full grain of powder will generally increase your bullet velocity about 90-100 fps. This .3 grain increment is small enough to see where the rifle wants to shoot well to find the sweet spot. If the factory 7 STW ammunition on 25+ rounds already has a .78 and the factory 257 Wby ammo has a 1.16 grain spread, that will not allow the rifle to stay within a given accuracy node. This will open up the groups and cause inconsistent shots. Plus, the .78 grain spread on the STW ammo will cause about 65-80 fps spread. The 1.16 grain powder difference on the 257 Wby ammo will cause about 96-120 fps spread. That's a lot!!!
Also, what I noticed on the 257 Wby ammo is a lot of charges were on the high side and on the low side. The average charge was 66.56 grains, but most of the powder charges were closer to the max and the min charges, not the average. There were not many close to the average charge. Also, when I have done these in the past, the smaller the opening (diameter of the round), the worse (or higher) the spread is on the powder charges. The 257 Wby held true to this also.
If you look at these 2 targets below, I am going up in .3 grain increments in powder charge with these test loads. In the first target, you can see a noticeable decrease in group size with Load C. This was a 1.032" group shot at 300 yards. The second target is a 243 Win. You can see the top right group about 1" at 300 yards, also. I like using these test targets, because it shows a perfect load work up with the normal signs of what the rifle likes when shooting. You can see some openness, then the groups start to string a little, then you find your sweet spot and the group tightens up. This sweet spot is what you load to. When your scales are accurate to .02 grains, that's accurate to 1 kernel of powder. When the powder charges have a difference of .78 and 1.16 grains, this alone will open up your groups and make for inconsistent ammo. When you pull the bullets and weigh the powder charges, you can see what's really on the inside!!!!