Rebarreling and Home-Shop Machining

Determining Maximum Usable Pressure

There are several clues that help you determine the maximum amount of a certain powder that you can use for a particular barrel, bullet, and case combination. I was interested in experimenting with H4831SC in my Douglas Barreled 30/06 with Lapua Cases. Previously I had achieved a 2895 feet per second with a published maximum load of 57.5 grains Hodgedons H 4350 with a 180 grain Nosler Ballistic tip bullet in the 27 inch long barrel. An article on the 3 kings which included the 30/06 in the September October Issue of Petersons “Rifle Shooter” magazine had a fellow that liked using H4831 and H 4831 SC in the 06. I thought that my 27 inch barrel could probably profit in the speed department from using this slightly slower burning powder which is also suitable for the Winchester Short Magnum. By the way, the other two kings were the .270 and the 7 mm Mag.

First I found some published data. Speer listed a maximum load for a 180 grain Trophy Bonded Bear Claw as 59.0 Grains of a compressed load of H 4831 SC for a speed of 2689 fps

Hodgdon listed 60.0 grains of a compressed load of H4831 SC at 2710 ft/sec (24 inch barrel) at 44,000 CUP (copper units of pressure) Some of the loads went up to 50,000 CUP for a 180 SPBT (Soft Point Boat Tail)

I loaded up seven rounds starting at 58.5 grains increased by .5 grains up to 61.5 grains which is 1.5 over the maximum published loads. I intended to carefully examine the bolt lift, case head condition, and degree of primer flattening to stop shooting when the maximum load was reached. I ended up shooting all the rounds as the case head condition of the 61.0 grain load was not alarming. Note that I am using Lapua Cases which seem to be harder than Federal and Winchester Cases.

Photo 1 and 2 show the case heads of the rounds fired at 90 F with cooling after round 3. The top left round is 58.5 grains of H4831 SC. The top right 60.0. Bottom left is 60.5. The last 30/06 Lapua case on the bottom right is 61.5 grains. There is also a .308 case at the bottom right to show another over pressure condition discussed below.

Primer Flattening

In photo 1 you can see changes in the primer as I increased the powder. If you look at the very edge of the primer the top row shows some curvature. The bottom row of 30/06 primers are more flat; less curvature is visible. This is a subtle change and requires close inspection. These are CCI Magnum Primers.

Drag Marks from the Bolt

Photo 2 shows the same cases with slightly different lighting to show the shiny parts of the case head. The case on the top right (between the inscribed L and the .3) and middle 30/06 case on the bottom row (between the inscribed A and the 06) are starting to show some shiny areas as the bolt face drags on the case head during primary extraction. I think that this is OK but indicates that you are near the limit.

Extrusion into the bolt Face

The .308 case on the lower right shows a raised area of brass (between the inscribed 8 and the W) near the bottom of the Case Head. What has happened here is the pressure has extruded (pushed) the brass into the ejector plunger hole. This is too much pressure, back the load off a couple of tenths of a grain. The Lapua 30\06 cases that I have used don’t do this because they seem to be harder than Federal 30/06 brass. You have to look for other indications that happen first.

Rechambering the Fired Case Requires A Small Bolt Handle Force

This was probably the most sensitive test that I used. During Firing, I got easy bolt lift on all the fired 30/06 cases. At home, I removed the Cocking Piece/ firing pin assembly from the bolt with the Sinclairintl.com tool and tried the fired cases back in the chamber. Case one though six rechambered with no bolt tension closing the bolt. I could tell that case 6 (61.0 grains) had no room to spare. Case 7 with 61.5 grains had tension on about the last 5 to 10 degrees of bolt closing. This was light enough not to be felt if the firing pin spring was in place. This slight tightness of the case indicated I was at the limit. The steel of the chamber was over expanded and the brass was not able to spring back from the chamber slightly as is normally the case.

David Tubbs’ Method For Determining Maximum Pressure in the .223 for Service Rifle Competition

I read this article in a recent issue of a magazine. David fires a series of rounds increasing the powder by .5 grains per round. Overpressure is noted when the fired case has an expanded primer pocket. This changes the feel of the hand priming operation. The primer will seat too easily with hand pressure. Back of about .5 grains from this point. He also stresses the need to have rounds that will function in the heat so that a trip to a distant range does not throw off all your work.

Velocities

Photo 3 shows a graph of the Velocity of the 180 grain Ballistic Tip with respect to the charge weight of H4831SC that I used. From 58.5 to 59.0 there is only a modest increase in velocity; the line is not very steep on the left side of the graph. As we start getting in to the interesting pressures the velocity increases rapidly with the same .5 grain increase in powder charge; the line is much steeper on the right side. 61.0 grains produces a usable 3060 feet per second. 3223 ft/sec was the highest velocity attained. I will tune for accuracy in the future. Photo 4 shows a table of the same data that is shown in the graph for the 7 rounds.

About the SC in H 4831 SC

H 4831 SC is the same powder ballistically as H 4831 which has been around since 1950. The SC means Short Cut. The granules are about the same length as Varget and H4350 so I assume that the H 4831 has longer grains which make the H 4831 slightly more difficult to meter.

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