Asumetrical Barrel Cross section question for machinists

[frogwalking]

Gentlemen,

I recently received a  fowler barrel in the mail. It was a replacement for one received long ago that was very much out-sized.  This one was only slightly too large cross section but I noted something odd while draw filing it down to size.  

It appeared that something was not symmetrical when I noted that a couple of the flats "ran onto" the wedding band where the barrel transitions to round.  Others appeared to stand proud of the wedding band.  It appeared that some of the flats were wider than others.  My vernier caliper confirmed that some are .05 inch smaller than the adjacent flat at the same crossectional spot.  As would follow, the cross section is .02 larger across some of the flats than across others again, at the same location.  None of this is of practical matter to me as I doubt that originals were anywhere near to being a regular octagon.  I have drawfiled this one to fit the stock just fine.  My question is therefore accedemic.  

The question for the machinists:  The seller of this barrel told me it was made on a computer controlled machine.  I assume they mean a CNC machine.  How likely  is this to be true, considering the lack of precision apparent in the piece?  A pistol barrel I bought recently has swirl marks on the flats that I assume came from an end mill.  The fowler barrel has fine parallel longitudinal machine marks on the flats.  What kind of machine would have made these?  Thanks for the information.  

[Jim Kibler]

CNC machines are only as good as the set-up and operators.  Sub par work can certainly be produced on CNC as well as manual machines.

[Jim Cook]

And Jim, I can make a lot of "Sub par work" faster on the CNC machine. 

I wonder if the questionable barrel was the "first piece" meant for inspection and since it was not up to par the program was tweaked and the remainder of the barrels were acceptable.

Perhaps the "Sub par" piece was put in the "good" box rather than the scrap box by mistake. Or maybe the "Sub par" piece was sold anyway. 

CNC machines are only as good as the set-up and operators.  Sub par work can certainly be produced on CNC as well as manual machines.

[Acer Saccharum]

A most likely finish for a CNC milled flat would be longitudinal marks, very smooth, if the cutter was sharp. If dull, the finish could have some longitudinal scuffing and tearing. A cutting tool typically would be an end mill, and used in a vertical mill. That's where the spindle is vertical in relation to the work table. The bit looks somewhat like a router bit, with spiral flutes on the sides of the tool. In a CNC you can mill very smoothly with the side of the tool.

If the flats are different, there can be many reasons. One might be that the part did not get clamped the same every time after indexing. Or that the fixture wasn't bolted down well, and it moved under cutting pressure. Many more reasons can be invented.

[Karl Kunkel]

Limited experience speaking here.  I'm working on my second rifle, both with Rice 50 Cal, C Wt, 38 Inch Transitional. 

The first barrel was so smooth, all I had to do was sand my rusty finger prints and scratches off of it, prior to degreasing and browning.

The second barrel had the "longitudinal machine marks" running the length of each flat.  You could feel them with your finger nail, when you drug it across a flat.  Simple to draw file the marks out.

Different runs with different tool heads I guess.

[Acer Saccharum]

Some makers use a planer, which uses a tool like a lathe bit that is drawn along the length of the barrel, peeling great curls off the stock. Others use a horizontal mill, which has a horizontal rotating spindle, upon which a spiral milling cutter is mounted. This can slab off a lot of metal at once, and leave a very nice finish.

[Don Getz]

The longitudinal marks would be that of a planer.    As for the octagon to round barrel.....assuming the bore was straight
before it was milled, it should have come out OK.   If it would have been straight, and supported several inches in front
of the octagon section, it would probably have come out properly...........it would appear these things were not done.
Somebody that didn't know what he was doing did this, but, on the other hand, if it was from a normal barrel supplier it
should not have been sent out to a customer, and should have gone into the "second/blem" barrel...........Don