Heat treat effect on pressure

[sundog]

We've kicked around alot of stuff about heat treat or water quench, and I just don't remember any discussion on what it does to pressure. All things being equal, it seems a 20+ BHn should produce higher pressure that -20 BHn. Really the comparison might be better if we talk straight WW and heat treat alloy. Velocity and resonance changes. sundog

[Bass Ackward]

We've kicked around alot of stuff about heat treat or water quench, and I just don't remember any discussion on what it does to pressure. Velocity and resonance changes. sundog


Corky,

The theory is that softer lead will throw higher pressure.

I had done a lot of testing since Christmas to show how and why opinions vary on rifling height and bore condition using two control loads. In other words why we get into such arguments during barrel discussions. Unfortunately, all the test data was tossed and thus lost. So this is all you get and you have to trust my memory.

One low velocity load and one high velocity load was selected using both hard and soft bullets. Soft was 14 BHN bullets and the hard were 28-29 oven heat treated. I started with a virgin barrel. The first test using the 1850 fps load showed no pressure difference if velocity was the judging criteria. The high velocity load was my 56 grains of RL19 for 2600 fps. Here, there was a velocity difference of 200 fps on a new bore with the soft bullet throwing more pressure and leading.

Then I lapped the bore so that rifling height became even on all lands which also cleaned up the bore. Land height was .0033 if memory serves. Once the bore was fire lapped, even the high velocity loads were the same velocity to the point that you couldn't discern which was which as my dad handed them to me. As rifling height dropped below .003, the soft bullet, high velocity load went haywire and velocity slumped below the hard bullets. I assume gas cutting occurred when the bullets stripped.

So my point is that here were three examples from the same barrel where all three possibilities occurred based on the life span of the SAME barrel. 1. Soft bullets threw more pressure. 2. No difference was noticed between soft and hard. 3. Soft bullets threw less pressure. It just depends on how you shoot lead and the condition of your barrel.

IF you are interested in accuracy results this was even more stunning to understanding peoples positions. The 1850 fps load went from 1 1/4" in the new barrel and dropped to 7/8" at it's best point and then increased back to 1 1/4" as I stopped testing at .0014 rifling height. I stopped at this height because the hard bullets still gripped, but the soft ones stripped at this height too. So the slow load guy's positions would be: 1. Rifling height makes no difference for lead accuracy. 2. Fire lapping was a waste of time. 3. Cheap / old barrels shoot lead just as well as expensive / new ones.

For the high velocity load, accuracy sucked with soft bullets not on target at all and about 4" for the hard ones. Then the bore was smoothed up at the .0033 height. Then both loads grouped about 1 3/4". By the time I went below .003, the soft bullet load went crazy again while the hard bullets maintained 2". The HV load was totally lost for both hardness bullets at the .0024 height. So a guy shooting this style would have had positions that: 1. Rifling height and smooth bore condition are everything to cast especially if you wanted to go soft for hunting purposes. 2. Fire lapping is the greatest thing since sliced bread. Without fire lapping he would have probably lost patience and sold his gun or abandoned high velocity testing as impractical for cast bullet use. 3. You need the best quality barrel possible for shooting cast.

[sundog]

Bass, good report. I'll stew over it fer a spell.... sundog

[fecmech]

Sundog--I'm guessing you guys are talking about rifles but I rememberd an article from the 1973 Gun Digest by Wm. M. Caldwell on pressures in revolvers. He worked in the Speer lab and they fitted a pressure transducer to a revolver cylinder and did some testing. One portion of the test involved cast bullets of different alloys and the scan I'm posting lists the pressures he got during the test. In the limited testing he did it does appear that softer alloys in solid barrels give higher pressure but just the opposite in revolvers. Nick

[David R]

Good stuff guys.....Thanks

HI Nick!
David