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lube, vibes, and powders vs. pressure

Posted: Sun Feb 10, 2008 6:20 pm
by mtngun
from the old forum:

3006 pressure trace: lube, barrel vibes, powders

Here's an assortment of experiments attempting to shed light on the secondary pressure spike.

Gage #4 (Omega with JB weld), the usual validation load, except the barrel was swabbed generously with Break-Free oil before each shot. Based on the slippery bullet theory, I expected the oil to increase the secondary spike, but it didn't happen. Velocity was 55 fps lower than normal.

Gage #4, the previously tested load of 53.0 gr Ramshot Big Game, 180 gr. loverin, known to have mega-spike. Just a sanity check to confirm that the mega spike shows up with this particular gage.

Same as above except with a card applying upward pressure to the muzzle at the foreend. The spikes are still there, but look a little different.

Same load as above except with a 15 pound (approximate) weight hung near the muzzle. The idea was, if the spikes are caused by barrel vibrations, the 15 pound weight should seriously alter the barrel vibrations.

Gage #5, located 2" from the muzzle, with the 53.0 gr. Big Game / 180 gr. loverin load. It does seem to confirm the existance of a pressure spike. This gage flatlines at 69,007 psi, so we can only say that the spike pegged the gage.

Gage #4, 180 gr. loverin, CCI #250 primer. Traces 1 - 3 = 47 - 49 gr. WW748.

47.0 gr. 748 at 2583 fps, QL predicted 46.8 ksi at 2497 fps.
48.0 gr. 748 at 2640 fps, QL predicted 47.5 ksi at 2549 fps.
49.0 gr. 748 at 2710 fps, QL predicted 50.5 ksi at 2585 fps.

Quickload seriously underestimated the pressures. I stopped at 49.0 gr. because of the high primary pressure. WW748 did reduce the spike compared to other powders.

Gage #4, 180 gr. loverin, Varget powder. Obviously, Varget did not eliminate the spikes.

47.0 gr. at 2569 fps, QL predicted 46.8 ksi at 2500 fps.
48.0 gr. at 2599 fps, QL predicted 49.6 ksi at 2545 fps.
49.0 gr. at 2609 fps, QL predicted 52.6 ksi at 2591 fps.
50.0 gr. at 2645 fps, QL predicted 55.7 ksi at 2636 fps.
51.0 gr. at 2721 fps, QL predicted 59.1 ksi at 2682 fps.

Quickload was pretty close on velocity, but failed to predict the big increase in pressure when the charge was increased to 51.0 gr.

Conclusions???? If the 15 pound muzzle weight had made the spike go away, then we could be sure that the spike was caused by barrel vibrations. If the 15 pound weight had no effect on the spike, then we could be sure that the spike has nothing to do with barrel vibrations. What are we supposed to conclude when the 15 pound muzzle weight drops the spike from 106+ ksi to 81 ksi????? :|

If I take the spike seriously, then 748 seems to be the most likely powder, and the next logical step is to try a heavier cast bullet with 748 powder.

I am interested in shortening the barrel one inch at a time, to see if it makes the spike go away. The question is, if chopping a few inches off the barrel does make the spike go away, what am I supposed to conclude, since shortening the barrel changes both the barrel vibrations and the gas dynamics?

I am also tempted to install a muzzle brake, but again would be faced with changes in both the barrel vibes and the gas dynamics.