Rebarreling and Home-Shop Machining

Thursday, April 21, 2005

Long Range Results 200 grain Accubond

            I shot a few shots at American Shooting Center in Houston Texas on 4/28/2005. The conditions were a windy 10 mph average, gusts to 25. I took the shots in pairs prone off a backpack. The wind direction was from 6:30 where 12:00 is toward the target. I used the .300 WSM long leade long action rifle with a 70.5 grain load of H4831 SC and a 200 grain Nosler Accubond Bullets. Photo 1 shows the 200 and 416 yard target from the shoot. Groups were tight and well placed at these two distances. At 620 yards I had some windage errors that I believe were caused by wind spilling around the edge of the 400 yard berm. The vertical spread on four shots at 620 yards was about 2 inches. Here are the corrected elevations at some even yard numbers and the corresponding windages for a 10 mph wind. These are averaged over 3 range sessions. The windage is estimated by adusting the ballistic coefficient in the free web based JBM ballistics program to fit my shooting data. The data is given in Minutes of Angle or MOA. Wind speeds are in miles per hour (mph)

100 yard 0.00 MOA up, windage 0.60 MOA @ 10 mph

200 yard 2.00 MOA up, windage 1.25 MOA @ 10 mph

400 yard 6.25 MOA up, windage 2.70 MOA @ 10 mph

600 yard 12.00 MOA up, windage 4.30 MOA @ 10 mph

Note that this load was bit hot at the long range in Cases fired three times, although it worked fine in new cases. I believe the new cases cushion the maximum pressure a bit as they expand. I have 50 more new neck turned cases that I will use for a while.

180 grain Nosler Accubond Results

I shot a couple of groups at Baylies House of Guns in Houston, Texas on April 23, 2005, one with 180 grain bullets and one with 200 grain bullets. Winds were quartering and about 10 mph on average. This time I pulled out all the stops as far as case preparation is concerned.

1. I used once fired and then neck turned cases. Runout on the bullet was less than .0005 inch (5 tenthousandths). I describe a neck turning technique using the lathe in Precision Rebarreling.

2. I remeasured the maximum overall length after 150 rounds of use. I set the overall length for .006 inch bullet jump which these Accubonds had preferred in a couple of previous rifles. I describe the homemade gage in Precision Rebarreling.

3. I full length sized the cases, so the cases would just fit the chamber as described in my Journal, but the bolt with the firing pin and cocking piece removed, would close easily on the case.

4. I used vibration (described in my Journal) to reduce the powder volume in the case before seating the bullet.

            I was rewarded with a .255 inch 3 shot group, shown in photo 2, which included the cold barrel shot from a clean cold bore. The incremental load target in my Journal and on the CD had predicted good conditions between 73.0 -73.5 grains. I chose 73.2 grains, but I used a slightly shorter overall length so the cold barrel shot was a tad hot. I will reduce the load a bit. The 180 grain load averaged 3072 feet/second. The rifle seems to be more accurate with this 180 grain bullet.

200 grain Accubond (Typical Results)

            Photo 3 shows a typical 3 shot group from the same rifle at 100 yards, but with the 200 grain Nosler Accubond. The rifle is zeroed for this load. It is usually a nice triangle like this. Notice that the radial miss error from the center is only about .3 inches for each shot even though the group is .725 inch. The bullets in this group had about .001 inch runout. Previously with new cases my bullet runout was about .002 inch in spite of necks with less than .001 inch runout. I have not fired the cases since I turned the necks, so the runout and possibly the group size will improve. These cases tend to develop a bulge on the inside of the neck near the base of the neck (a do-nut) when unturned cases are full length sized. The donut was causing the extra runout on the bullet. The K&M neck turning tool’s inside neck reamer does a nice job of removing the excess brass from the do-nut. This improves the bullet runout even though the cases are not fireformed again yet.

            The 70.5 grain load of H 4831 SC (with vibratory compaction) averaged about 30 feet per second faster at 2988 feet per second in once fired cases than in new cases with no vibratory compaction. That is excellent ballistics from this case. This load turned out to be a little hot over 85 F. I reduced it and report the results in this topic below.

Accurate Arms MagPro Powder

180 grain Nosler Accubond

            Accurate Arms MagPro Powder is a relatively recently developed ball grain powder that was designed to be used in the short Magnum cases. I loaded up a pressure series using once fired cases and Accurate Arms MagPro powder which is suitable for my .300 WSM long leade long action rifle. Some tabular data from Lee had led me to believe this powder was slightly more dense that Hodgedons H4831 SC. I found that exactly 76.00 grains of AA’s MagPro powder filled my once fired .300 WSM case to the top of the neck without any compaction. This is the same as H4831 SC which also takes 76.0 grains to fill the case to the top of the neck. So the effective Density is about the same for the two powders. They also compact a similar amount with vibration.

            The load containing 75.5 grains of AA’s Mag Pro under a 180 grain Nosler Accubond went 3081 and resulted in a sticky case extraction. The maximum load in my rifle is thus about 75 grains. I did not test for accuracy. I get about 3130 feet per second and good accuracy from H 4831 SC so I returned the Mag Pro to the shelf.

Loads for Once Fired Cases

200 grain Nosler Accubond

            I developed a maximum load for the 200 grain Nosler Accubond in once fired cases. I max out at 68.7 grains of H 4831 SC. Five shots averaged 2944feet per second with an extreme spread of 32 feet/second and a standard deviation of 6.5 feet per second. A five shot group in windy conditions went .75 inch center to center. This load is .8 grains under and 15 feet per second under what I can use in a new case. The case expansion that occurs in a new cases apparently cushions the maximum pressure a bit.

180 grain Nosler Accubond

            I developed a maximum load for the 180 grain Nosler Accubond again with once fired cases. I maxed out at 3112 feet per second average for five shots with an extreme spread of 64 feet per second and a relative standard deviation of 14 feet per second. A five shot group in windy conditions went .75 inch center to center with four shots in a tight cluster. I will probably start using this load at the long range at it is more accurate on average.

Powder Compaction using a Drop Tube vs Vibratory Compaction

            A fellow shooter and I did a powder compaction test with a drop tube. I filled my .300 WSM case to the brim with H 4831 SC which is a short cut extruded powder (76.0 grains). I made a drop tube by nesting three different sizes of brass tubes taped together to a 30 inch length. The largest tube is on the bottom. Its diameter is slightly larger than the case neck. I took a cut with a tubing cutter on the end of the large tube which made a gently tapered section just small enough to fit over the neck. I poured the powder in through an RCBS funnel into the tube. The powder was compacted the full length of the neck. I get the powder to compact down about 2/3 of the neck with vibration using a dog shear. See the previous topic for a photo.

            My fellow shooter did a similar test with IMR 4831 which, I believe, is a long cut extruded powder using the .300 Winchester Belted Magnum case. The powder compacted about 1/4 of the neck with a 3 foot drop tube. I did not receive a report on the material in the drop tube. If it is plastic a static charge may inhibit the velocity that the particles can reach.

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