“E. C.” stands for the “Explosives Company.”
The following is from doublegunshop.com/forums: "Originally established as “The Explosives Company”, the name was changed to E C Powder Company. Exactly when the firm became a limited liability company is not known, and its relationship to Explosives Co of Stowmarket, Suffolk is unknown. The company was established in 1884 at 11 Queen Victoria Street. In 1887 the firm moved to 20 Bucklersbury. In the early 1890s a Mr Norton was the chairman or managing director of the company. In about 1895 the company had manufacturing facilities in Green Street, Greenstone, Dartford, Kent. In 1897 they moved to 40 New Broad Street. In 1900 the firm’s name appears in the American E C & Schultze Gunpowder Co Ltd, but it seems the E C Powder Company continued as a separate company. There has been a suggestion that the company was bought by Curtis & Harvey, this has not been confirmed. In 1907 the firm were back at 20 Bucklersbury. In about 1913 the company moved its facility in Greenstone, Dartford, Kent, to Bean, Dartford, Kent. In 1918 the firm became part of Explosives Trades Ltd, but it appears to have continued trading under its own name. Reportedly, the company was bought by ICI. In 1923 the Ardeer gunpowder factory commenced production of E C Powder.
In 1868 Captain E Schultze established Schultze’s Granulated Wood Gunpowder Co Ltd at 9 Northumberland Street, Charing Cross. In 1869 the name changed to Schultze Gunpowder Co Ltd and the company moved to 2 Bond Court, Walbrook, but the whereabouts of the firm’s factory at this time is not known. In 1871 the company moved to 25 New Broad Street, and in 1873 to 62 Bishopsgate Street Within. In about 1886 (and up to about 1910) the company’s factory was at Eyeworth Lodge, Godshill, Salisbury where the chief chemist was a Mr Griffiths. Other factories were in Germany at Ludwigshaven, and in Belgium. In 1877 they moved to 8 Bucklersbury, and in 1879 to 3 Bucklersbury. Between 1883 and 1885 the company moved to 32 Gresham Street, moving to 28 Gresham Street in 1899. From about 1890 to about 1897 the company had additional manufacturing works at Lyndhurst in Hampshire.
In 1890 The Smokeless Powder Co Ltd was established at 9 New Broad Street, why and who established it, and whether or not it had any connection with the Schultze company is unknown. In 1899 this company changed its name to The Smokeless Powder & Ammunition Co Ltd and moved to 28 Gresham Street where it traded until 1904; presumably it was bought by The Schultze Gunpowder Co Ltd. In 1893, a Mr Borer represented the company at the inaugural meeting of the Inanimate Bird Shooting Association.
In 1898/9 the E C & Schultze Powder Company was formed, Richard William Smith-Griffith was managing director in about 1900, but in 1900 the name of the company appears to have changed to American E C & Schultze Gunpowder Co Ltd at 28 Gresham Street. Who the American investors were is not known. Presumably this was a new company but it could have been an amalgamation of E C Powder Company Ltd, The Schultze Gunpowder Co Ltd and The Smokeless Powder and Ammunition Co Ltd. At about this time the firm had a factory at Barwick, near Ware in Hertfordshire (the manager at the time was a Mr F W Jones) and at Barking Creek in Essex. For a few years the company’s production facilities were at Eyeworth, Redbridge and Totton, and in Southampton.
In 1910 American E C & Schultze Gunpowder Co Ltd was taken over by E I Dupont, and The Schultze Gunpowder Co Ltd was taken over by Eley Bros Ltd; prior to it being bought by Eley Brothers the name “Schultze Gunpowder Co Ltd” was changed to Schultze & Co Ltd. In 1918 the company amalgamated with others to become part of Explosives Trades Ltd, and in 1919 it became a subsidiary of Nobel Industries Ltd. In about 1918/1919 the company’s production facilities were at Eyeworth and Fritham, Lyndhurst and Redbridge, other facilities appear to have been closed but from this time a manufacturing facility operated in Glasgow. In 1926 the company became a subsidiary of ICI Ltd and in 1928 of Eley Kynoch. Footnote: In 1920 the Ardeer Gunpowder Company commenced production of Schultze powder (in 1923 it produced E C powder and in 1934 Smokeless Diamond powder)"
Not sure where I heard it, but I’m pretty sure that it’s very important to not confuse E. C. sporting powder with E. C. blank powder, which the U.S. military used a lot of in blank cartridges. As I understand, E.C. blank powder is extremely fast-burning and is suitable only for blank-type rounds with very little restriction (no bullets, just a paper wad). If you load E.C. blank powder in a normal ball round, it will probably explode and destroy the gun and maybe you.