I did a search for a GM 21" IBS and couldn't find anything. So I don't know the stats on it, I cain't tell ya about that barrel.
but', I did try a Cabelas .54 "carbine" just a few years ago. that rifle had a shallow groove barrel .004-.005 1-28 twist and was 21"s long. I tryed everything I had through that thing,prb, maxiball,minneball and several sabot/style projectiles with a varied amount of lubes and load volumes. In short I gave the barrel a work out. Now I'm a tech kinnda shooter, I enjoy accuracy,,I also hunt and I do some what well with my co-horts at vous'.
That little 21" fast twist barrel seemed very forgiving. At 20yrds several combinations gave me 1" groups,at 25yrds they opened up a bit to 1.5"s...At 50 yrds,,everything I tried,,no matter what,, did no better than 3.5",,the prb was 10"'s! not suprising for that barrel. Anyways, the short barrel "carbine" lives up to it's word, good for short brush work, but beyound 30 steps,,?,,,,, imho,luck too ya.