I got a KAC M5 for this A4 build and realized the Delta ring was going to be a tight squeeze, so I decided to hit the LGS for a handguard tool and maybe a little help.

The smith got the guard about 90 on but it didn't seat in the ring completely. After a struggle or two in the back, he pointed out to this piece mounted to the top end at the back towards the delta ring that sat over the gas tube. Apparently one or both of its pronged tips were interfering with the teeth of the barrel nut. They hammered the part's retaining pin out and removed it, which resulted in a successful mount, but my question is WTF is this and what is it for?




Here is the part seated in the bottom rail, far right

If I remember correctly, you loosen the part and it is suppose to latch on the other side of the teeth on the barrel nut. Once the rail is secure, you are supposed to tighten it down and lock everything in place

here are the instructions - Page 2 has the info you are looking for

KAC RAS Installation Instructions

Thanks.

As already posted, it's the part that slides underneath the inside of the barrel nut and clamps the rail down tightly.

You should probably find a new gunsmith if he couldn't figure this out, as he doesn't sound like he knows what he is doing.  You have to make sure the clamp is loosened all the way and that it's hanging pretty low to get enough clearance to clear the barrel nut.  It also helps if the rifle is completely perpendicular to the ground which allows gravity to hold the clamp out of the way while sliding it underneath the delta ring.

FWIW, installing this rail is seriously about the easiest thing you can do to an AR, next time just do it yourself.

Your "smith" isn't much of a Gunsmith if he can't install a part the way it was intended to be used.

That is part of the clamp that holds a KAC rail on solid.  Those "tips" go under the barrel nut and a screw is used to rigidly lock the KAC rail in place.

no point in beating a dead horse here. I'll just say that I installed my KAC RAS in about 5 minutes time, without silly handguard removal tools, and that included reading the instructions that came with the rail from Knight's.

I guess good local gunsmiths are harder to find than good car mechanics.

I also just installed my first kac ras for my carbine the bottom rail seems to have so much play and the top rail was a pita to put on

A quick youtube search on how to install it would have revealed dozens of videos showing exactly how to install it.