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".