I think the Impulse has a unique feed tube thread design. The PTP adapters are angel feed tube, bushmaster feedtube.
So, as far as I know, there are no specific warp adapter feed necks. But you can just buy a regular aftermarket low rise and stick a 90 degree adapter in it.
Btw, most low rises have at least 4 balls "waiting" in the feed tube. As long as you can stick to 3-4 shot bursts, you're fine for feeding. (Don't forget you have a ball in the breach already)
EDIT: If you really want a warp only adapter to make the lowest feed tube profile possible, you can find someone like Doc Nickel to make a custom one for you. He can also probably convert you Imp to a warp feed only body as well, but that's a little more extreme.
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