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seating depth

 
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  #1  
Old 03-12-2006, 02:52 PM
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seating depth

How much can seating depth affect preasure? I just finished my first project with a throating reamer. A 300 RUM and I throated for the 210 vld but went a little far. I only have about .100 parallel in the case neck. So the base is about level with the bottom of neck parallel. I am not showing usual preasure signs and am getting lower than posted vellocities. 28" lilja. 99.5 grn retumbo 190 vld 3150, 96.5 grns retumbo 210 vld 3000. Primers still not flat small cratering around pin, but I think that is from after market pin and extrusion of .060 ???? Do I keep going higher or look for something else??

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Old 03-13-2006, 07:24 AM
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Re: seating depth

I just read an article in the May 2005 issue of "Rifle" magazine that covered your exact question. In the article by Chub Eastman, he was able to test two 300 WSM barrels at the Nosler ballistic labs. His goal was to be able to load the .300 WSM to an OAL of 3.000 inches. One barrel had a SAAMI chamber, the other was chambered with the same reamer except for a straight .250 inch leade (similar to what you did to your barrel?).

Quote from the article:
"Starting OAL was 2.820 inches and was increased .060 inch at a time until an OAL of 3.000 inches was reached. Three shot groups were fired at each AOL. In theory, as the bullet was seated farther out, the subsequent drop in pressure would require an increase in powder to maintain loads at factory pressure. The first surprise that proved this theory was questionable came when the second series of shots were fired when AOL was set to 2.880 inches. No increase or decrease in powder was needed to obtain near the same pressure or velocity using the barrel with the modofied chamber. Surprise again, when we found the same results at 2.940 inches OAL and at a full 3.000 inches AOL."

Although there was virtually no change in pressure or velocity with the modified chamber and seating depth changes, the SAAMI chamber acted more like expected with higher pressure and velocity the nearer to the lands the bullets were loaded, so anyway, this may be similar to your situation. As for your velocity and pressure signs, again, your barrel modification probably is why you see lower velocities and pressure than expected, (although they are in the ballpark).

I personally don't like to play around max pressures if I don't have to, hoping to find the best accuracy and barrel life a few percents below max.

I hope this helps. Just remember, there are so many variables in reloading and individual rifles it's hard to pinpoint an exact explaination for why you get certain results.
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Old 03-13-2006, 07:38 AM
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Re: seating depth

The reason Weatherby has a lot of distance to the lands (freebore) is to allow higher velocites at the same pressure. (He also built stronger actions to take more pressure). In theory, starting a bullet engaged in the lands will have higher pressure than allowing the bullet to move a little and increase the expansion volume and have some momentum built up to hit the lands.

Theory and reality are not always the same and your gas mileage may vary with your driving habits.
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Old 03-13-2006, 09:21 AM
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Re: seating depth

"How much can seating depth affect preasure?" This is where I wish I had a strain gage pressure measurement device to go with the chrono. Maybe it would prove my 'obversations as incorrect'? But my observations have been when the bullet is seated further out, pressure and velocity go down noticably. I then increase powder to usually achieve a higher velocity at the same pressures as w/the shorter seating.

I tend to load at close to maximum pressures and velocities and I think its a bit of a different world up there. The rate if increase of pressure line it trending pretty steeply as a small amount of powder increase will result in a very large increase in pressure if the powder is all getting ignited w/in the bbl. So 'careful' is the key word.

W/my 27" Lilja in 270Win the first indication of pressure is a titch of a sticky case upon extraction. The cam action of the bolt has to move the case very little then it just falls out. This makes for a very distinctive indicator of too much pressure. Using this indicator I increase the COL then increase powder till I get to that "sticky point" then increase COL, repeating the process until I get max out on the velocity or achieve the velocity I desire. With the 270 the goal was 3200 FPS w/130gr bullet. When I got there I stopped figuring that was sufficient for the cartridge. Could have gone more but 3200 was enough. This process achieves maximum velocities and most accurate loads and cases that last more than 20 reloads.

My experience is the same with a 338 Win. Less pressure with longer COL. I have had it rebarrelled to 338 RUM and would expect to find the same trends.

This seems to conflict with the above post (Rifle Mag article) which apparently was a bit more scientific/lab approach???

HTH.
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Old 03-13-2006, 05:22 PM
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Re: seating depth

"But my observations have been when the bullet is seated further out, pressure and velocity go down noticably. I then increase powder to usually achieve a higher velocity at the same pressures as w/the shorter seating."

This can be the case, depending on how much the base of the bullet extends into the case. The more powder room the bullet takes up, the higher the pressure spikes when the shot is fired.

As you seat the bullet farther and farther out, you relieve the pressure in the case by enlarging the space. However--when you get the bullet into the lands, the pressure begins to go up again. Since the bullet is already pressed against the lands when the powder ignites, it wants to hesitate a bit before entering the rifling (and this is one plausible explanation as to why seating close to the lands sometimes helps accuracy; it allows for a more uniform ignition of the powder). For bullets seated well away from the lands, they collide with the lands at some significant speed, and there would be little to no hesitation here.

Also, when you deep seat very long bullets, such as a VLD types, you can have--according to some ballisticians--what is called a "nail head effect." This is when there is so much of the bullet protruding into the case that it sort of "mushrooms" slightly at its base before leaving the case neck. Naturally, this creates a rather large pressure spike.

So basically, pressure goes high when you seat too deep, and then it reduces as you seat longer until you get against the lands, whereupon pressure once again increases.

I've been testing the 200 grain Sierra Matchking in my Savage LE2B .308 win. That's a long bullet, and it protrudes well into the .308 case. I found that pressure signs were MUCH worse with the bullet seated deeper into the case than they were with the bullet actually crammed into the lands about .010" or perhaps a bit more. With the bullet seated to an OAL of 2.840", I got severely stretched primer pockets and snug bolt lift with 43.8 grains of W748. But with the OAL set at 2.900", which put the bullet so far into the lands it had to be forced by the bolt to chamber, I found no pressure signs at an even higher charge of 748, which was 44.2 grains.

Dan
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