Cast in 7-30 carbine

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mtngun
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Cast in 7-30 carbine

Post by mtngun » Mon Feb 04, 2008 6:02 pm

From the old forum:

Today was Christmas, Fourth of July, and my birthday all rolled into one, because my pressure trace arrived !!!! :lol:

I mounted one of the strain gages on a 7-30 TC carbine, just because the TC is amenable to mounting a gage. This gage was mounted with superglue so that it could be removed later with acetone. I had a hard time getting the superglue to stay stuck, so I wasn't sure that the gage would work correctly, but it worked fine.

In theory, if the gage is mounted correctly, on a straight cylindrical section of the barrel, in the middle of the chamber area, you should be able to get good absolute pressure readings. But the manufacturer recommends using a known load to verify the pressure, and I felt like that was a good idea, since this is new to me and I'm not real confident in the setup yet.

For a verification load, I used a 130 gr Speer spitzer, 39.0 gr. IMR 4350, 2.80" COL. Quickload predicted 44587 psi and 2562 fps. Actual velocity was 2421, and the pressure trace was seeing 46 - 48 ksi, so I elected to subtract 3000 psi from the pressure trace readings to bring the PSI down to about 44 ksi. That seemed reasonable, considering the velocity was a little low, and Quickload usually errs on the conservative side.

Now for a cast bullet load. Today it was a 150 gr. cast driven by 36.0 gr. IMR 4350, with Felix lube and heat treated WW. Quickload predicted 41576 psi at 2351 fps. Actual velocity was 2199 fps, and pressure trace was seeing 39 ksi. So lower velocity but lower pressure, too, which seems reasonable. Thanks to pressure trace, we can see that this load generated some nasty secondary pressure spikes.

Image ** missing pressure chart, darn it **

The secondary spikes are real, as Charlie Sisk proved by blowing the muzzle off a gun or two. They tend to be pop up with slow powders and light bullets, and especially with "slick" bullets, like moly coated. Now we know that they happen with cast bullets, too. The solution is to switch to a faster powder or a heavier bullet. You can also make it go away by shortening the barrel, but I'll pass on that option. IMR 4350 is slow for this cartridge and I only used it because I have a bunch of it and because traditional wisdom says to use slow powders in high velocity cast bullet loads.

Accuracy was 9 shots in 3.3" at 100 yards, nothing to brag about. This gun shoots well with a 170 gr. bullet, so I may try a 160 grainer next..... and a faster powder.

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Re: Cast in 7-30 carbine

Post by mtngun » Mon Feb 04, 2008 6:27 pm

from the old forum:

This post is going to talk about my experiences with the pressure trace system in boring detail, but some of you who are also learning to use the pressure trace or who are thinking about buying a pressure trace may find the details useful.

Since last time, I repositioned the gage's connector to face downrange. That makes it easy to secure the cable to the barrel with some masking tape.

Here's two shots with the "validation" load, just to make sure the trace is working properly. I believe the true pressure for this load is 43,000 psi, or maybe less, since Quickload tends to be conservative. The traces are not very consistent but the pressure is in the ballpark so let's proceed. My normal benchrest technique for this gun is to hold the scope with my left hand. That happens to make my hand brush against the pressure trace connectors. More on that later.
Image ** missing pressure chart, darn it **

Here's a string with a 160 gr. bore rider. 33.4 gr. 748, CCI 250, HTWW sized 0.278" x 0.285", lube #3, 2.715" COL. Quickload estimated 43 KSI at 2327 fps. Today\'s actual velocity averaged 2274 fps in the 30 degree temperatures. Including the two strings fired at the previous range session, the average 5-shot group at 100 yards was 3.6".

The blip on trace #7 was probably my hand brushing against the cable as the gun recoiled.

Image ** missing pressure chart, darn it **

Image
Here's a string with a 160 gr. loverin. 31.0 gr. 748, CCI 250, HTWW, sized 0.2855", lube #3, 2.410" COL. There were some complications. The rounds were difficult to chamber due to lack of clearance at the case neck. The cases are 375 Wins leftover from another experiment, with necks turned for a benchrest fit with 0.284" jacketed bullets. Next time I will use 30-30 brass, which should result in proper clearance with the oversize cast bullets.

Quickload told me to use less powder owing to the bullet seating deep into the powder space, predicting 2300 fps and 45 KSI with 41 gr. 748. I was skeptical because there is excess powder space, only about 90% loading density.

Indeed, the loverins averaged only 2170 fps. The five-shot groups averaged 3.0". One group teased me with a nice 1.4" cluster, but I couldn't repeat it.

By now I had become aware that my left hand was rubbing against the trace cable, and had moved my hand to the bottom of the forend to avoid bumping the cable. Notice how much more consistent the traces are now. Notice that there is absolutely no secondary spike such as we saw a few weeks ago with 4350 powder.
Image ** missing pressure chart, darn it **

What's next for this gun? I will try bumping the bore riding noses up an extra 0.001". The loverin acted like it wanted to shoot so I will dink with powder charge and seating depth. I've never been able to shoot worth a hoot in cold weather, so I am optimistic that groups will shrink come summer.

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Re: Cast in 7-30 carbine

Post by mtngun » Sun Feb 10, 2008 5:33 pm

'More 7-30 Pressure Trace data: 4895, 3031', 'First off, my validation load traced at 46584 psi compared to 46732 psi at the previous range session, telling me that the strain gage was working normally.

Today I was using the usual 160 gr. loverin in the 7-30 contender carbine, except with different powders.

Here's 29.7 gr. H4895, 2.540" COL, Fed #210, EL #3, heat treated WW. Quickload predicted 2178 fps at 43 ksi, it actually averaged 2159 fps at 42 ksi. One group didn't stay on the target, the second group was 12.7", and I gave up after that. Note the mild secondary spikes. Trace #6 seems to be delayed -- I dunno if that was due to delayed ignition or due to an electrical problem. Either is possible.
Image

Here's the same bullet except with 29.9 gr. IMR 3031. Quickload predicted 2299 fps at 43 ksi, it actually averaged 2222 fps at 46 ksi. Accuracy, if you can call it that, averaged 16.2". Once again, we have mild secondary spikes. The powder charges were not weighed, and since 3031 does not meter well, that may explain the shot-to-shot variation.
Image

By comparison, with WW748 powder this same bullet averages 3.6", 2274 fps, and 43 ksi, and no secondary spikes, as noted on a previous thread.

So what have we learned? Regarding secondary spikes, they weren't supposed to happen with fast stick powders like 4895 and 3031, but they did. Accuracy really sucked with the faster powders, just like Col. Harrison said it would. So far WW748 is this gun's best powder for full throttle cast bullet loads. Next time, I may try a 170 gr. bullet and maybe Ramshot Big Game or WW760.

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Re: Cast in 7-30 carbine

Post by mtngun » Sun Feb 10, 2008 6:22 pm

730 pressure trace: Varget, muzzle weight

Here's a test to see if Varget would make the spike go away. Not!

160 gr. cast bore rider.

30 gr. Varget at 2049 fps, QL predicted 34.6 ksi at 2106 fps.
32 gr. Varget at 2166 fps, QL predicted 41.4 ksi at 2237 fps.
33 gr. Varget at 2224 fps, QL predicted 45.2 ksi at 2302 fps.

Quickload underestimated pressures for these loads.
Image

Here's the validation load. The weight did seem to reduce the spike, though a one shot comparison doesn't really prove anything.
Image

I don't like WW748, but it looks like I am stuck with it, if I decide to take the secondary spikes seriously.

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