3N37 is an excellent powder for .40 S&W and flash is pretty low. I was at a gunshop in a small town here in Texas about 60 miles east of Abilene yesterday and found a pound of 3N37 that had evidently been sitting there for a long time. The pricetag said $24.95 which was good for 3N37 20 years ago, so naturally I left with it.
In later years, the best powder that I've found as a replacement for 3N37 at a much better price is Silhouette (formerly WAP as mentioned). I have used a lot of it in 9mm, .40 S&W and high velocity JHP loads in .45 ACP and performance and accuracy are excellent, plus Silhouette is chemically treated to give low flash. Luckily I have about 6# left on the shelf and I'll put the 3N37 to good use as well. They are very similar powders, IMO, and Silhouette is just above 3N37 on most burn rate charts.