I have been shooting (for fun) my 7mm-08 with aperture sights while my eyes are still good enough.

The high power rules 3.1

"(e) Attachments which do not cause any portion of the rifle to exceed the weight or dimensions as listed herein are permitted provided they do not extend past the end of the barrel. Sighting devices are not considered "attachments".

"(g) The barrel may be no longer than 30 inches including any sleeve or attachment, as measured from the face of the closed bolt to the end of the barrel."

Is a barrel extension like a "Houle Tube" that is meant to increase the sight radius considered a "sleeve or attachment" or is it part of the "sighting device"?

I believe you have mounted your "sighting device" On a "tube or attachment". If your front sight cantilevered out past 30 inches you would be good. If you put it on a tube for support, not so good. A bloop tube is a tube or attachment.

Over the weekend I asked this question to two other match directors I know and both would consider it to be part of the sighting device as it serves no other purpose but to extend the sight radius. One of the arguments was if the front sight body and extension tube were machined out of a single piece of material would it not be the same thing?

I would consider shooting SB or HP silhouette with aperture sights a disadvantage over a scope so I don't see this as a big issue. Certainly not at club matches but for a state or national match it might be an issue.

The rules refer to attachments for various things but do not define what an attachment is.

Needing more clarity I contacted the NRA and Greg could not answer the question so he is forwarding it to the rules committee.

At the nationals, your rifle has to fit into a jig. If the overall length is too long, you don't shoot. It really doesn't mater what you call it, if it is too long you are out. We really don't need to lawyer up to shoot in our sport. The rules seem plain to most of us.