I purchased a M17s at a gun show with a flash hider, but it turned out to be a post ban (the SOB lied). Here is what I have learn:
1) Post bans have .75" of barrel thread, Pre bans have 1.00" of thread, possible exceptions.
2) Post bans serial numbers generally start with a "P" and Pre bans serial numbers generally start with a "F". There are exceptions and Bushmaster will take a week to find the built date... thanks Bushmaster for the search.
3) The sleeve covers up about .125" or more of the thread so if you place a post ban upper on a pre ban lower, you could shorten the tube to gain the extra .125" to attach a flash suppressor. Or if the assault ban is lifted.
4) Bushmaster recommends using Loctite 271 on the lock nuts, flash suppressor and the barrel assembly retaining bolt.
5) If you have an older pre ban, bushmaster will upgrade it with the new lower and upper parts for $100 plus shipping.
There is a post on the M-17 in the general discussion forum if anyone wants to see some pictures and read more about these.
My post ban built in 1995 was P 035XX, the new one at the store is a 2001 with P 065XX. It does not seem they have sold that many based on the serial numbers on post bans.
And yes, it is safe to place a flash suppressor if the tube covering the barrel is shortened and loctite 271 is used (this is per BM tech department). Of course, this is only legal if you placed the post ban upper on a pre ban lower. IF the brady bill is not renewed by Bush, the ban is null and void.