I need to submit a rifle with a pistol brace attached to the ATF for tech evaluation, to certify the design of the pistol brace. The firearm it is designed to interface with is currently sold only in rifle configuration. The rifle will remain legally configured after the pistol brace is attached, with barrel length and OAL requirements met. Are there any problems with this? I don't think this presents any issues but just want to be sure. I'm sure the ATF would rather I submit it attached to a pistol but that's not possible. I tried asking the Tech Branch about this and they gave a vague answer of "submit it attached to a firearm" which didn't really address the issue of a pistol brace attached to a rifle.

Why not just submit it attached to a pistol?

What do you hope to accomplish?

A pistol brace attached to a rifle is still a rifle, it would not allow you to use a <16" barrel

If you are trying to get a determination that it isn't a stock, you'd likely have to get a bare receiver or make one

This explains it all...

"submit it attached to a firearm"

   pistol - barrel <16", overall length <26", cannot use a vertical foregrip or a stock legally

non-NFA firearm - barrel <16", overall length at 26" or greater, cannot use a stock legally,

short barreled rifle (SBR) - barrel length <16", must be approved by the ATF for legal use of a stock

So what thing are you looking to get banned OP?

Your competitor running a similar product?

Just asking for historical accuracy.

As said in the OP, "The firearm it is designed to interface with is currently sold only in rifle configuration."

The gun is not currently sold or manufactured as a pistol. I cannot attach it to a receiver that was not originally manufactured as a rifle, because it is only sold as a rifle. The goal is to design a pistol brace for this platform and then produce and sell the firearm as a pistol.

This is exactly what I'm trying to do, just get the brace legally classified as a brace and not a stock. I know attaching a brace to a rifle is pointless, I just want to make sure its not creating any legal or classification problems. I would think they could classify the brace based on its design intent regardless of whether it is attached to a rifle or a pistol, but they gave a vague answer.

Not going to get anything banned.
There are no competitor products.
There are no similar products or any arm braces at all for this firearm.

Please explain how submitting a pistol brace on a rifle can get the rifle banned. Just want to get the historical record clear.

Mind naming what firearm this is?

This is why the whole pistol brace concept started with SIG.

The only way to have a chance is to partner up with the rifle manufacturer.

If the manufacturer isn't going to cooperate, there won't be a market for your product anyway.

Exactly.  
Unless the OP is planning to manufacture a complete copy of this receiver?

At the trade shows, I've asked Savage and Ruger to do something similar, selling stripped actions, or even complete pistols -with a brace, but neither claims they're interested.  Will see.