I have been working up a load for the Nosler Custom 48 in 300 WSM. I got back into reloading last year to allow me to practice more at distance. Factory ammo was producing sub MOA groups, but I was burning through at least $300 in ammo each year.
During the fall of 2012, I experimented with Ramshot Hunter and IMR 4831. Ramshot Hunter accuracy was decent but I was only getting about 2930 FPS out of the max load. I learned that the spherical Ramshot and the IMR 4831 were not temperature insensitive.
In Montana, temperatures during hunting season can typically range from 65 degrees to minus 15 degrees. After reading an article on powder sensitivity to outside temperature, I decided I needed to load a powder designed to produce minimum deviation in velocities at wide temperature ranges. The Hogdon family of Extreme extruded powders was designed to produce low dispersion in FPS. Below is a graph that shows the extreme spread in FPS between 0 and 125 degrees F for several popular powders. H4831 SC, one of Hogdons Extreme powders, only varies 10 FPS.
Cartridge: 300 Win. Mag. Case: W-W Primer: WLRM 180 gr. Sierra Boat Tail SP
You can read about the Hogdon Extreme line and view other test results at the following link: http://www.hodgdon.com/smokeless/extreme/index.php
I started at 64 grains of H 4350 behind a Nosler 180 Accubond. According to Hogdon this would push a 180 Swift Sirroco at 2950 FPS. I cleaned the bore on the rifle and set up the Chrono. I found the 64.0 grains averaging right at 3000 FPS in the Nosler with cases showing plunger marks on the case head and slight difficulty in ejecting the shell. I shot a five shot group which measured around 2.75″. Horizontal dispersion was less than an inch, but verticle stringing was just under 3″. Some of this may be the result of starting with a clean bore. (more on that below) The top two shots were the last and both exceeded 3050 FPS. One five shot group is not necessarily indicative of the load’s accuracy, however the pressure signs and high velocity told me I needed to back this one off a bit. Here’s the group:
I loaded six rounds at 63.5 grains H 4350 and cleaned the bore again. The difference in the group size was amazing. The first three shots grouped right 3/8 ” or around .4″. I shot a second three shot string which grouped a little larger, at .6″. The chrono showed the load had slowed to about 2980 FPS. The first shot from a clean barrel was at 2969, three shots between 2980-2984, one and one shot at 2999. One shot didn’t pass through the chrono correctly. Below are the two groups.
At this point I am pretty happy with these results and it looks like I have a good load for the Nosler that will average around 2980-2990 FPS. I am a little concerned with the dispersion in velocity (2969-2999) and will need to run more of the 63.5 grain loads through the chronograph to arrive at a final average velocity. I suspect the lower velocity of the range was a result of the clean bore. I found the reference below about shooting through a clean bore on the web.. and the Nosler exhibits the same characteristic increase in FPS as the bore fouls.
One thing I’ve noticed in chronographing loads is the first shot from a clean barrel is typically at a lower velocity than the second one. And the third will be higher than the second one. At about the fourth or fifth shot, velocity has stablized and shots tend to stay at the same elevation. This has been seen in tests and normal shooting conditions at all ranges through 1000 yards. And it doesn’t make any difference whether the barrel is hot or cold when starting the series of shots. Once in a while, a barrel won’t
exhibit this situation, but more often than not, it’s very normal. The reason seems to be that as fouling builds up in the bore, that increased resistance causes higher pressure; hence higher velocity for each succeding shot. After the fouling/resistance has `normalized,’ velocity stablizes for
the most part. Velocity in my .308 Win. rifles for the first shot is often 30 to 50 fps lower than normal for the first shot from a clean barrel.
Next time I chrono the 300 WSM I will shoot 12 shots with 5 minute spacing. I then plan on letting the bore cool completely then shooting a round through the cold, fouled bore.
Results to come.