Moulding plane for barrel channel

[Nhgrants]

Given a situation where the wood grain requires  that plane travel from breech to muzzle, I would think that the
Rear portion would need to have clearance below it.  The plane I'm using is a 3/4" half round wooden moulding plane.
The portion of the plane behind the cutter is about 3" long.  Do I just need to rough out the breech so the Rear of the plane has clearance under it?  My goal is to modify another plane to make it octagonal but I need figure out the above.
Thanks.

[rich pierce]

In my experience, yes, you need that clearance.  Not a lot, but some.

[Ian Pratt]

If a blank won't allow me to plane back toward the breech even by making thinner cuts, I rough out the breech end the length of the whole plane body + a little extra. Have long thought about making a plane for this job with the iron set toward the back and a dog leg handle that would jump over the top of the wrist. 

[Elnathan]

This might be a model to guide you:
http://s3.amazonaws.com/finewoodworking.s3.tauntoncloud.com/app/uploads/2016/09/06085150/Leopold-main.jpg

Coach-maker's plane. I think the squirrel-tail handle would be a good design for such an application.

I'm not sure that an octagon shape would be all that much of a benefit, though - at least one person I know who tried it said that he couldn't get it to work and that he ended up re-modifying his octagon barrel plane a kind of tooth plane which worked much better, IIRC. He didn't say why, but I suspect that the (unsharpened) sides tended to bind up once the three bottom edges cut down into the wood.

[Nhgrants]

This got me thinking. Should plane with a half round
Cutter? I made a octagon scraper from a file that works
Well.  I could rough in most of it with the plane and then
Switch to the scraper. I intend to do an initial cut with a router.

[Mark Elliott]

I thought that I would mention that there is a gun stocking plane different from a moulding plane.   A round moulding plane has a side that is sloped in.   A gun stocking plane is round and has two parallel sides.  I have an original  3/4"  gun stocking plane that I use to even out a channel initially cut by gouge.   When the channel is planned to depth,   I use octagon scrapers to square it up.   I do have to cut the last few inches of the breech with gouges regardless of which way I plane; usually from the muzzle to the breech.

[Nhgrants]

Thanks all, your replies have me going in a good direction