There is no set grading that I know of... The vender sets his own grading system & what HE thinks is is. And even that will change from what he gets out of the kiln at dif time. If all the curl is getting less, the #5 grades are now #6 & the #6 is now a #7, etc., because that is the best they have. I have bought some #5 that was every bit as good as a #7 two years later.
And one vender a #3 may be a #5 when compared to wood at another vender. To me you cannot compare grades when they are graded on a dif system. The only way I know to do that is comparing the TOP Grade to another top grade, or a plain one to a plain one. And even then we get into a Hardness factor......
Two different times I have bought a Grade #7 and when I got down into the blank where the stock finished out, it turned out to be a grade 3 or 4, & just lost all the curl. But you have to consider out of well over 100 blanks I have bought, this has only happened 2 times. But wood is wood, ya can't see Inside it, and it is what it is. Sometimes it just happens.
I like to hand pick all my stocks or blanks & pretty much do that. Once in a while I will buy one from photos if I have some good photos & feel comfortable with the vender. The reason I do that are a couple guys cutting & selling stocks have no idea what they are doing & don't care, as long as it sells. Most all the stocks have grain runout at the wrist. I want ALL of my blanks curved & good grain thru the wrist.
I just looked at some stocks a new guy is cutting the other day & it has some really nice curl in it & was thinking of buying 5 maples & 2 curly ash. But best I can tell, all will have grain runout at the wrist. Curl means absolutely Nothing if the stock is broken in two......
Now if I build rifles from those blanks, who do you think is going to get the blame if one breaks at the wrist ? The guy that bumped that stock too hard, or the guy that built the rifle on a weak blank ?
Can't remember the last time I bought a precarved stock sight-unseen but it has been many moons. I do remember the last 2 I bought that way I returned, as they were not the quality I wanted for the $ spent.
In closing.... Buy the best wood ya can afford. Grain structure & hardness of the wood comes first, curl comes last........
Keith Lisle