effect of coal on pressures and velocity?

Discussion in 'Reloading' started by fumder, Jan 27, 2010.

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  1. fumder

    fumder New Member

    Nov 20, 2009
    i haven't ever played with COAL until i started using berger VLD's. up until now if the loading manual stated a 3.290 coal for a 7RM i would seat to 3.290 and work my load until i got it under 1 inch at 100 yards.
    i am looking to extend my range, so i picked up a new rifle, sendero in 7rm, and the previous owner did use bergers and found the optimal length of 3.475 worked in this rifle. i did some measuring, and testing, and 3.475 did indeed end up being .080 off the lands. worked up a load, and got a pretty impressive *to me* group sitting at .53 inch at 100 yards.

    i was shooting the breeze with another shooter who happened to mention that when he started using bergers in his 300 win mag, he got great consistancy, but was lacking in velocity once he seated teh bullets far out. he supposed it was due to the added head space in the case leading to lower pressures. now, he has the benefit of having a chrony to verify his findings, and adjusted his load to match normal velocities and noticed no signs of over pressure.

    so if his semi scientific tests are accurate... could this be the case? i haven't ever read anything about it online, nor in any of the loading manuals i have kicking around *lee , berger, nosler, all latest prints* or is it a case of me over thinking it, and should just get a chrony and answer my own questions...
  2. eric2381

    eric2381 Well-Known Member

    Feb 27, 2009
    This is something I wonder a bit about as well.

    With the bullet seated out close or into the lands slightly, the pressures will raise. But, when the bullet is seated out farther, it increases the internal capacity of your brass.

    What is the sweet spot for distance from lands to get the highest case capacity, and lowest pressure??

    I'm sure it's different for each rifle though.

    Does Quickload have a function for this??
  3. fumder

    fumder New Member

    Nov 20, 2009
    well, since no one seems to have an answer...

    lee reloading makes no mention of COAL making a lick of difference on case pressures nor does the barnes, and nosler manuals. i don't have the lyman one, but i can't see them having it.

    got a chrony, found the following.

    64.5 grains of 7828ssc, rem 9.5 magnum primers. 168gr berger hunting vld

    32 deg F, 1500 m above sea level, 89% humidity.

    average of 4 shot

    coal 3.333 = 2780 fps
    coal 3.323 = 2800 fps
    coal 3.313 = 2830 fps

    i have no means of measuring case pressure, but the chrony did show a steady increase of velocity when seated deeper into the case. all loads were non compact loads. it is note worthy to mention that with the coal at 3.333 (.080 off lands) it was the most accurate, and again at 3.313, it seemed to start closing up again.

    in this particular rifle, browning a bolt 7rm, having a shorter case length will be beneficial due to hinged floorplate mounted internal box magazine internal dimensions.

    the nest rounds of tests will be done with my sendero 7rm, using retumbo, and as high as 72gr powder. thus far my results were as followed with the same conditions

    69 =2790
    69.5 =2810
    70 =2830
    70.5 =2845
    71 =2920

    69-70 were most accurate, and for some reason sounded like a black powder gun going off. at 70 it started to crackle a bit, by 71 it sounded like a rifle. i don't know why it would be a steady increase of 20+-, then jump 70fps in .5 gr.

    i'm not too sure where some guys are saying that they are getting over 3000fps with the same set up, and 72gr of retumbo, i'm missing something. unless outside temp matters that much.

    more to come.