Rebarreling and Home-Shop Machining

Incremental Load Method

The purpose of developing a load is to find conditions in the Loading Process (for example-Powder Charge Weight, and Overall Length) that minimize the effects of barrel vibrations on the size of your groups.

I used the Incremental Load Method to try and tune up a long range accuracy load for the 27 inch Douglas Barreled 30/06 which is described in the topics below.

I made some changes to my loading method based on some information on the Speer Web site. They suggested vibrating the powder down when using a compressed load of 4831 SC in the 30/06, using either a vibratory cleaner on which you rest the cases, mechanical vibration or a long stem funnel. The benefits were not given, but I decided to try it anyway. The target that I used for the pressure series that is graphed in the topic below did not show much accuracy potential, so I reduced the bullet jump to .020 (bullet ogive to lands). From .060. I vibrated the powder down in the case by tapping the side of the case with my metal funnel while holding the top closed with my finger. This lowered the powder enough so that with the bullet moved farther forward there was not much compression of the load by the bullet itself. Lower velocities (all about 2800 feet/second) and pressures resulted, but some accuracy potential is apparent in the target. All of these fired cases rechambered with no cocking force with the cocking piece removed (as described in the maximum pressure topic below). The velocity graph for this second set rounds is shown as the red diamonds and line on the graph in photo 2. The black line and squares are the same as in the topic below. The steep increase in velocity shown on the black line which is probably due to the load being compressed is not apparent. The vibration of the powder and the longer overall length has reduced the tendency fo the bullet to compress the load.

I loaded up 10 rounds consisting of Lapua cases (these charges are probably too high for Federal cases) 180 grain Nosler Ballistic Tips and Hodgedon H4831 SC starting with 60.3 grains and increasing each round .1 grain up to 61.2 grain; this is the incremental load. I shot the 10 rounds at 80 F with plenty of cooling time between sets of 3 or four rounds. I started shooting in the middle of the sequence to the highest loads and then shot the lower five loads. The target that resulted is shown in photo 1. I labeled the shots on the target from lowest powder charge (1) to highest (10) rather than in the order it was shot.

The incremental load method uses the principle that if 2 to 4 shots in sequence group well, then that is a good set of conditions for an accuracy load. It is also helpful if the shots are at the extremes of the set of shots, say at the lower left or upper right. This means the bullet is exiting the bore when the barrel is at the extreme of one of its vibrations and where the barrel is moving slower. Shots tend to group better under this condition.

If you study the photo 1 with a 180 grain Nosler Ballistic tip, shots labeled 1 - 4 which correspond to 60.3, 60.4, 60.5, and 60.6 grain of H4831 SC were clustered fairly tight. Shots 5 and 6 which corresponds to 60.7 and 60.8 are high and left; this might also be a good place. I’ll shoot some 3 shot groups at 60.4 and 60.8 grains in the next session.

Photo 3 shows a similar Incremental load method with the same rifle using a Nosler 165 grain Ballistic tip, Lapua Brass (this is harder than Federal brass, these loads are a bit too high for Federal Brass). Hodgedon H 4350 from 59.1 to 60 .3 grains increasing the powder by .1 grain for each of the 13 rounds. I vibrated the powder down to avoid the bullet compressing the load. Shot 3, 4, 5, and 6 which corresponded to 59.3-59.6 grains of H 4350 were in a nice cluster. I’ll try a couple of 3 shot groups at load number 4 which is 59.4 grains of H 4350 with .020 bullet jump. Photo 4 shows a graph of the velocity vs charge weight for this series of rounds. The selected load went 2960 feet/second. Rechambering the fired cases, with the cocking piece removed as described below, indicated my pressure limit was at about 60.0 grains at 3000 feet/second

Results Photo 5

I shot two 3 shot groups with 59.4 grains of H4831 SC and two with 59.7 grains. I am using new neck turned cases which are not real straight yet. I selected the best cases with case neck runout of about .001 -.002.inch to shoot the upper groups on both targets. I used my worst cases which included mostly 002 & .003 inch runout and one .004 inch runout (in lower left group) to shoot the lower groups in both targets. The loads averaged .711 CC for the load at 60.4 grain and .781 for the 60.7 grain load. I expect the accuracy to improve .1-.2 inch when I start using once fired cases which will be very straight from this match chamber. This is a virtual tie considering the somewhat random nature of group size. I will use the 60.7 grain at 2784 feet/second at the long range, when I get a chance, to get some trajectory numbers.

Results with Once Fired Cases (new)

I now have a good supply of once fired cases. When I neck size + shoulder bump these cases they are very straight. Less than or equal to .001 inch runout on the neck. The cases are also a bit bigger in volume than new cases so the load needs to be redeveloped. I shot a series of rounds starting with 60.7 grains and increased each round by .1 grain to 61.4 grains of H 4831 SC with a Nosler 180 grain Balllistic tip at an overall length of 3.362 to give about .020 inch bullet jump, CCI Magnum Primers. These loads are too hot for Federal Cases; the Lapua 30/06 cases are a bit harder throughout. The resulting targets are shown in photo 6. All the shots were in a 1 inch square. The maximum pressure was in the range of 60.9 - 61.1 grains. 61.4 grains showed a slight indication of the plunger ejector on the case head. Shot 5 at 61.1 grains was toward the right bottom of the shots and close to shot 6 and 7. I’ll try a couple of groups here at 61.1grain H4831SC at the next session and try to get velocities. I was unsuccessful getting velocities for this series. I vibrated the powder down in the case with my vibratory case cleaner before seating the bullet. This lowered the powder volume about ½ the neck.

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