oh man here we go again......
The understanding is that by adding a factory pistol grip to a factory grizzly you are not shortening it to less than 26" my telescoping, folding, or "by other means" so you are then still non-rest/non-prohib. UInless there has been new developments, or a court case to this scenario this is the arguable concencus around here.
there is also the "30 day rule" where you can modify a firearm, and then have 30 days to report and change it's classification therfore you're ok to have a pistol grip on for 29 days, then take it off etc...
Be prepared however to try to explain these to a LEO or CO if you are questioned about your short gun... you likely will not get a gun savy, or convincable officer
I am not a lawer, nor do I play one on TV.... I have stayed at a holiday inn express once though.... so take my comments for what they're worth,
do a search for dlask pistol grip, or grizzly pistol... lots of reading
do you see rest, non rest, or prohib??