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Need educated. Pressure, FPS, and seating depth

 
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Old 10-13-2013, 10:36 PM
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Need educated. Pressure, FPS, and seating depth

Question what is the relationship between Pressure, FPS, and seating depth? Does more seating depth create more pressure or vice versa? Does seating depth have a direct impact on FPS?

Here is my issue:

The rifle I am shooting 6.5x284. 140 vld. 24" barrel. 56.5 grains retumbo. .100 off lands. Lapau brass. Fed 210M primer.

There is another thread regarding this chambering that had got me to thinking, I know I know, I should not think..... but hey too late. Thinking about whether I could load to the lands and have enough bullet in the case neck for proper tension and alignment. I wanted to test and see if I could do this and gain accuracy.

I tested some loads where the bullet was seated to the lands, in fact, .003 in the lands. I tested a 3 shot group and they all flew in the 2970 range. I am seeing faint extractor marks. Accuracy was OK, .75 at 100, the wind was horrible, gusting from every direction up to 16 mph. It blew my chrono over, the target over about 5 times, it was a pain to say the least.

I shot my normal load that is .100 off the lands, 2900, which I expected. I am still seeing some extractor marks, little stronger, with 56.5.

Hmmm, so I load up another group and pulled them back a little, 002 off the lands, and they flew 2950. Still pressure signs, but faint.

OK, so I am baffled. I load up another 3, .002 from the lands and change from 210m to 215m and I am getting 2980 to 3000. .75 MOA in the crappy wind.

Load up another 2, the last I had to test in the field, and pulled back to 56 grains, .003 into the lands, and a solid 3000 shows up on the chrono and .30 group. I wish I had a 3rd to at least get one more on paper, but I saw what I needed to at least do another field test, no pressure signs.
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Old 10-13-2013, 10:43 PM
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Re: Need educated. Pressure, FPS, and seating depth

I'm not an internal ballistics expert but I've seen pressures rise when seating deeper and also when jammed into lands. Anything in between is a toss up depending on powder charge.
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Old 10-13-2013, 10:49 PM
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Re: Need educated. Pressure, FPS, and seating depth

Every load for my rifles that I have done with Berger bullets likes them between .005" - .030" off the lands. But my sweet spot seating depths for those rifles have been between .010" and .015". Now that is not proven science but just what I have found in my rifles.

I never load my hunting reloads into the lands because on of these days you will go to eject a loaded round and get a chamber full of power and a stuck bullet.
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Old 10-13-2013, 11:13 PM
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Re: Need educated. Pressure, FPS, and seating depth

I agree and have always been concerned too. However this was a test and I don't understand the result.
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Old 10-14-2013, 06:06 AM
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Re: Need educated. Pressure, FPS, and seating depth

Quote:
Originally Posted by BrentM View Post
Question what is the relationship between Pressure, FPS, and seating depth? Does more seating depth create more pressure or vice versa? Does seating depth have a direct impact on FPS?
Hornady 9th edition reloading manual has an excellent explanation with illustrations on pages 25 and 26. I tried to scan it but not very good.

ADDED:

Decided to attach it anyways just in case you can decipher it at your end.


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Old 10-14-2013, 06:44 AM
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Re: Need educated. Pressure, FPS, and seating depth

I agree with Derek M
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Old 10-14-2013, 07:48 AM
BMF BMF is offline
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Re: Need educated. Pressure, FPS, and seating depth

In my 6.5x284, I lost about 15-25 fps every .015 I loaded away from the lands. It looks like your velocity is acting like it should. And certainly when you go with the hotter primers your velocity is going to increase. But regardless of your extractor marks on your brass, if you're using the same powder and primers and your velocity goes up or down due to seating depth, so does your pressure. They will be related.

Pressure signs are a tricky thing, especially with Slow burning powders like retumbo. I've found that with Retumbo I was getting the "occasional flyer". When I say flyer, I mean on my chrono and paper. Even though I wasn't near the max load, sometimes I would get an unusually high or LOW ( which is odd) chrono reading.

If your seeing ejector marks, especially on Lapua brass, that's the first sign ( not a good gauge though) of pressure. It's not necessarily a red flag point, but......you know. From there just monitor your velocity, which is a good gauge of pressure. Do at least 4 shot strings. Your ES will will start to spread when pressure is elevated.
Brent
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