Wednesday, July 17, 2013

Ever heard of Ballistics by the Inch?

Skidmark reminded me of them in the comments to the carbine/pistol post.  They take a bunch of different cartridges and loads and start with a 19" barrel, then cut it down an inch at a time and check velocity along the way.

They also take standard off-the-shelf firearms and check velocity through them.  Well worth looking at.

Borrowing one table, .357 Mag, something I'd read before:
barrel length Cor Bon
110 gr.
JHP 		Cor Bon
125 gr.
JHP 		Cor Bon
140 gr.
JHP 		Cor Bon
125 gr.
DPX 		Federal
125 gr.
JHP 		Federal
158 gr.
JHP 		Federal
130 gr.
Hydra-Shok
JHP
low recoil 		Federal
158 gr.
Hydra-Shok
JHP
18" 1718 2113 1941 1936 2072 1719 2017 1721
17" 1686 2067 1947 1907 2026 1689 2009 1712
16" 1790 2119 2004 1946 2051 1739 2044 1741
15" 1728 2069 1942 1915 2032 1708 2012 1718
14" 1691 2054 1933 1897 2017 1732 1993 1715
13" 1682 2024 1916 1885 1979 1674 1984 1687
12" 1725 1994 1851 1858 1945 1654 1934 1679
11" 1667 1978 1814 1831 1973 1643 1922 1671
10" 1619 1943 1802 1804 1943 1630 1922 1638
9" 1615 1901 1745 1779 1881 1563 1812 1586
8" 1600 1851 1717 1728 1817 1570 1810 1593
7" 1578 1784 1671 1682 1784 1516 1765 1536
6" 1461 1715 1580 1648 1702 1465 1662 1485
5" 1377 1614 1493 1552 1571 1373 1586 1402
4" 1286 1496 1394 1471 1511 1293 1453 1332
3" 1109 1257 1172 1271 1255 1102 1194 1122
2" 928 904 911 1050 949 858 919 914

Notice something?  All loads are faster from a 16" barrel than a 18".  I'd read that most pistol cartridges give maximum velocity from a barrel 16-18" long; beyond that you actually lose velocity due to the pressure no longer being able to overcome the friction of bullet to barrel.  In this case all but one load loses some velocity in that extra 2".
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3 comments:

  1. Keith2:46 PM

    Thanks for the link, very useful, I'll be using that.

    I'm trying to work up a piece on the 1950s Kimball .30 carbine and .22 hornet auto pistol

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  2. Sailorcurt8:32 PM

    Something weird this way comes.

    Notice that every loading slowed down from 16" to 17", but then sped back up again a bit from 17" to 18".

    That doesn't make sense. apparently, a 16" barrel is the right length to reach maximum pressure just as the bullet leaves the barrel. At 17" maximum pressure is reached before the bullet leaves the barrel and so begins to drop off due to friction (as you stated) and the bullet begins to slow before leaving the barrel.

    Why, then, would adding an additional inch of barrel speed it back up again? If maximum pressure is reached at 16" then the additional inch between 17" and 18" should increase friction and decrease speed, not the other way around.

    Can anyone explain this to me? Or did they just get the numbers reversed?

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  3. Keith11:32 PM

    I had a look through a few of the calibre tables, and the same thing seems to be happening at shorter barrel lengths too.

    What I haven't checked yet is their sample size at each barrel length and load, or how they control for things like ambient and gun temperatures.

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