My 2 cents worth and I may be overcharging at that...
I think there is a greater chance of a Octogon Barrel shifting around.  Looking at a Cross Section on a Round Barrel from the center point the Wall Thickness is consistent, not so on an Octogon.  Starting off round and milling the flats as well as turning the taper changes the set of the steel.  My preference would be to have the barrel profiled before any Bore work is done.  Good luck with your Rifle, sounds like it will be fine.  David

I'm just not sure myself, as I've not seen enough data or testing of round vs. octagon where the round diameter was the same as the octagon across the flats. I would think since the octagon takes a larger blank to be equal across the flats, it might give a little edge, but just can't say for certain.
I saw an old letter from the 1870's written by Major Fulton of the US Creedmoor team, where he wrote C. Sharps company to discuss just this thing. It was Fulton's feeling that round barrels were inherently more accurate than any other shape of octagon or half octagon.

Thanks for sharing that Ken. 
I have pretty much always shot a 1/2 Octagon to Round 28 inch #3 Steven's weight Douglas barrel off the bench and I still do so.

IIRC Sellers in his Borchardt book included a statement from Sharps claiming round barrels were superior.   I suspect they were looking at lowering cost?

Sellers also referenced a statement in the end of the line of the business for Sharps, that they had proven to their satisfaction that round barrels were superior.  They also said that as a result of that testing that they would only offer full round barrels other than special orders (presumably at extra cost).

Rich

One of the things on the action that I will try to check is the threads and action face in relation to the breech block. I’m not sure what I should use for the datum, maybe the back rails that takes the thrust from the case. Maybe fixture the action on the surface plate and sweep both sides. It will be a bit tricky as I understand that the breech block rises on an angle. Also I’ll thread a barrel stub, that will quickly show if the action face is perpendicular to the threads. I’ll smoke the face of the breech block and run it up and see how it fits to the barrel end. By guess and by gosh is the plan.
Any ideas welcome.
Richard

Thanks Chuckster. I need to figure out a way to find out if the receiver threads are perpendicular to the bore.
Cheers Richard

Chuck, thanks for the answer to my question. Not knowing how the action was fixtures during manufacturing. What I understand you saying is the action was fixtures action face up and the receiver face was milled flat in the x, & y axis and then the receiver was bored and threaded in the z axis. 
Cheers Richard

Richard, That would work if you have threading capability on the mill.
Generally use the lathe as shown in the pictures.
If the action is pretty well complete, might look at Greg's (GT) fixture to hold the action.
Chuck

calledflyer wrote on Nov 30^th, 2021 at 8:52am:

Why is that? Extensive testing of 22RF ammo has shown that anything over 18" is primarily for looks and longer sight-picture as it has no relation to accuracy. If using a scope there is no reason for a longer barrel. 
That said, my shortest 22RF barrel is 23". I like the looks of the longer barrels. 

Even if I lost velocity, or had a scope, I'd still not own a .22RF single shot with a barrel less than 24", and most of mine have 28"-30" barrels. Beyond the better looks, and sight radius, I like the balance also.
I have a full size Rolling block that came with a new 22" half octagon barrel, and shot very well. But looked hideous with 11" of octagon, and 11" of round on the full sized action. We turned down the round portion, and used a 19" piece of old .45 barrel to press over it, and but to the octagon. It now looks and balances wonderful, and still shoots like it did before. 
If I ever built another .22 single shot I'd use a 22" barrel and the bloop tube extension to do this same thing again.

I’m going to open a can of worms here about the length of a 22rf barrel longer than 18” benefiting more than just aesthetics and sight radius.  I have built 14 rifles now for this little cartridge – in single shots, a few more in gate latch and semi-autos and nearly twice that many more in the works, (it’s a disease) all but a couple have barrels that exceed 28”.  Several have 30” RKS barrels, most of these are gaintwist, and several are left hand… a personal preference…  I shoot mostly open sights but that isn’t the sole reason for the extra length.  Once the barrel has been either seasoned, (about 2K rounds fired through it to smooth the bore) or it has had approximately one round for every inch of barrel fired through it before data is collected, I’m not seeing validity to the rumor of a longer barrel slowing down the projectile… My average velocities have increased as a rule -due to the consistent distribution of lube?  I did experiment with pre-lubing the barrel but trying to match the Mfg.’s lube is challenging.   I've had some intense objections about the velocity comparisons I've done because the barrels all differ, but the longer barrels have averaged higher velocities than the shorter barrels using the same ammo when these procedures are followed.   

The other real interesting note with the longer barrel – the velocity Standard Deviation dropped in almost every case.  This is using a LabRadar for data collection.  Several of these rifles have produced consistent single digit SD’s and the vertical string at further distances (250 - 400-yard experiments) has coincided with these results.  Head and tailwind contribute to vertical but in my case, I have a severe obstruction at my range that creates a consistent variance from the 3 o'clock position so my horizontal is generally frustrating at these distances.    
   
There are a few of us locally building/built with these features, there’s more experimenting happening with different chamber configurations and most recently, some changes in twist rates.

I picked up a Pope-Ballard that has a 28” barrel that may have instigated this pursuit, not that I wasn’t already on this path, but the ODG’s had an idea I think.   This rifle gave me some data that made me pursue the subject further, several different brands of ammo gave impressive groups and most gave very low single digit SD’s.   It may be just a Pope thing – but interesting to say the least. 
My 2¢
Greg

All I will say here is that Merkava has little understanding of fishing equipment. Or .22 shooters. 
I don't get involved with the competitive rimfire guys because the 'serious' centerfire crowd is pretty intense itself, and the double-deuce contingent is about rabid. Carry on fellows. I'm done here.

No understanding of fishing equipment or 22 shooters.
Send me your resume.
If you were sitting in a restaurant, with a bunch of fellows discussing 
.22 rifles or fishing equipment, would you talk to them the same way 
You do here??
I think not.
Hide behind the computer screen.
You are a legend in your own mind.

I don't take bait- how's that for fishing expertise?

I like round or octagonal barrels. I also like half round and half octagonal barrels. I also like fluted barrels. 

I'm not a good enough shot to tell the difference as to which shape is more accurate so I don't think about it.

Don't think I would like a barrel that was half round and half octagon if the round half were at the breech end. Well,,, maybe I would. Never had one like that.   
This thread really drifted bad.

I think that a round would be the most accurate.   On the outside, tapering from small at the chamber to larger at the muzzle would be the best.  Cutting a flat side on a round blank? Eight of them? what does that really do to the insides when you think about it? In theory it should take the bore out of round, does that make eight grooves and lands better if you are going with an octagon barrel?

Sorry for the earlier drift.

Actually all about the conditions and learning to compensate.

The bottom line is if you can hit what you're shooting at, at what ever distance you're shooting, that is all that matters. Some are better at it than others. 
Lee Gibbs Pres.ASSRA

Totally agree Dellet it takes both.