I recently converted an MP5K pistol into an SBR.  Every once in a while, however, I thought it might be fun to shoot it with no stock but keep the forward grip on - essentially making it an AOW.
Any issues in doing this?

No, SBR registration covers it. Legally, at that point it would be a SBR without a buttstock.

Got it.

THX for the clarification.

If so, that would create a bit of a problem for those that contend an SBR that was "first a pistol" can be returned to Title I pistol status merely by reconfiguring it as such. If reconfiguring a "pistol first" SBR into an AOW makes it a "weapon made from a rifle," it would seem the same would hold true when reconfiguring the same weapon into a pistol. Conversely, if those who argue a "pistol first" SBR can return to being a pistol are correct, I can't see how configuring the same weapon into an AOW wouldn't make it exactly that.

I think both views have strong arguments in their favor, and I'm not sure what the correct answer is. On the one hand, the argument that filing a Form 1 nullifies all previous history of a firearm does make sense; on the other, it seems a bit ridiculous to imagine if one SBR'ed something like a Glock 19, that it couldn't return to being a pistol by reconfiguring it as such. I don't see how both can be true. If it's the latter, that would argue that indeed an AOW would be created in such a situation (rather than just an SBR minus a stock).