Check the NMLRA website. Also, become a member, and order a copy of their Rule book. It lists all the matches, and the rules that apply. Because the Association has tried to open its Match program up to accommodate a wide range of firearms( including in-lines) You will find different rules for different kinds of matches. The only place I would not expect you to be able to use any substitute powder would be at the Primitive range, up on the hill. However, down on the line, where everything shoots, including black powder cartridge rifles, I suspect the rules vary from match to match. The association is attempting to be Inclusive, rather than exclusive. Itprovides a National venue for all BP target shooting sports, to the extent the grounds can accommodate a given sport.
The ONLY sport involving BP firearms that the association does not include at present would be Cowboy Action Shooting( CAS). I suspect that the only reason for that is that the BP shooters in that sport shoot the same targets, and program as those who shoot other powders and guns, but in a separate class. There is no reason for those shooters to turn to the NMLRA to find a venue at a national level for competition.
The NMLRA recently( now a few years back) arranged to have the use of a 1,000 yard range at a Military base, about 40 miles away from Friendship, to host the long range shooters, using both slug guns, and BP cartridge guns. Altho I have yet to get over to see the matches, I understand that this has been working out well for those shooters who want to fire matches at 600 to 1,000 yards with BP guns.
Members can buy powder from the powder magazine at very reasonable prices, regardless of quantity.
