Greg, I believe you have brought THE issue into the open -  in the Varmint Classes (for both organizations), "tuners" have always been "legal" - there are no restrictions on barrel "attachments"  . . at least, not in the rule books I ever had in my possession!  Further, until the NBRSA expressly prohibited the use of Muzzle Brakes on Varmint Class rifles (immediately following the "legalization" of them for NBRSA HUnter Class), MUZZLE BRAKES would have been legal during Varmint class events: there were no restrictions on either barkes or, barrel"attachments"!

I suspect that, for Varmint Class( HB;HV;LV;Sporter) events, this is true under the current IBS rules; I do not believe either a brake or, a tuner could be disqualified - at least not under any clearly stated rule - there are no restrictions on either barrel attachments or, muzzle brakes! ;)

The rule(s) prohibiting barrel attachments are Hunter Rifle doctrine. I must confess to not having read an IBS rule book lately, but, I believe that  the only reason brakes are not allowed is that they are "attachments". However, there is NO rule [in the IBS book] expressly prohibiting the use of a muzzle brake which is integral to the barrel (as in "milled" into the barrel)! Therefore, it appears that, as happened in NBRSA country about ten years ago, when Mike and "Bud" Schniedeman (son/father team, then, from St. Louis, MO) showed up at a NBRSA registered HUnter tournament, at Mason City, IA, with muzzle brakes "milled" into their barrels, the referees had no choice but to allow their use! Three years later, the attendees at the NBRSA Hunter Nationals (Raton, NM, 1997) voted to allow thread-on brakes . . . and this year, they voted reapproval, then, ten mimnutes later, voted to completely disallow MBs! Go figure . . .

It appears the the IBS is as wide open as Ddoge City - just awaiting some rowdy gunslinger to raise a little HELL! ;)