diameter vs. pressure

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diameter vs. pressure

Post by mtngun » Thu Feb 14, 2008 8:06 pm

from the old forums:

RSI asked me to shoot some 180 jacketed bullets so that the barrel exit time could be compared to the exit time for my 180 cast. They have confidence in their calculations for jacketed bullets but aren't so sure about cast, so they were interested in a direct comparison.

Also, I compared 0.310" jacketed to 0.308" jacketed, just to see what would happen. Both jacketed bullets were Remingtons, but the 0.310" had a completely different ogive that had to be seated deeper and was a tighter fit in the throat, so it wasn't 100% apples to apples.

All loads used 55.7 gr. WW760 and a CCI 250 primer. The cast loverins had to be seated deeper than either of the jacketed bullets, so as usual, it wasn't 100% apples to apples, but it's as good as it's going to get.

Here's the jacketed traces. The 0.310" bullet had slightly higher pressure but nothing radical. The higher pressure may be due to the diameter or it may be due to the deeper seating or it may be due to the tighter fit in the throat -- or all of the above. Anyway, the oversize bullets did not blow up the gun.

Here's the 0.314" cast. Spikes were smaller than last time. The first group was a nice 1.8" but the next group was a wild 5.3". :evil:

And here's the 0.310" cast. Groups averaged a lousy 3.3". The spikes appear to be bigger, on average, compared to the 0.314".

Accuracy was too poor to draw any conclusions. Let\'s just say that diameter doesn't seem to make a huge difference with this particular load. The load seems to have some other problem that is not related to diameter.

The 0.310" spikes averaged 43.7 ksi while the 0.314" spikes averaged only 36.4 ksi. I didn't collect enough traces to determine if the difference is statistically significant. For now, the 0.314" seems to be the safer bet. I am not sure that 0.314" is better, but I am sure it is not worse.

The ET's (the "+" mark on the trace) were about the same for all bullets and show that the spikes occured while the bullet was inside the barrel, not after the bullet exited.

Considering that this bullet gave consistent 2" groups with WW760 powder at the last session, why did it misbehave today? What changed?

1) Warmer temperature -- maybe the lube works better in cold weather?

2) I lowered the powder charge 0.3 gr. ???????

3) I sized the noses in a straight 0.3085" die whereas previously I sized the noses in a tapered die. It only made a small difference in the final diameters.

Since the primary pressure is still too high, I will try dropping the charge a little more, and go back to using the tapered nose die. I can't think of any other good ideas at the moment