Hadn’t seen this thread but have one touching upon this very subject in the Reproduction section.
Speaking for myself, I’m still on the fence about opening up a choke on a 'spare' set of barrels. Although I cannot stand reloading, just might do myself some learning using fiber wads etc as this is where I just do not possess fundamental experience in manipulating patterns.
If you care to read my ramblings, here’s a long winded post on the subject of opening chokes ~ proper.
—
The photo of that suave fella w/a set of barrels mounted in a Cylindrical Grinder is about as good a setup as you’re going to find, by every measure it is the ‘right’ way to do it.
The benefits of using a cylindrical grinder are many.
First, the barrels are in an unrestrained condition and the grinding will result in perfectly round holes provided the barrel set has been afforded stabilization following join.
As is most often true in a machine shop, set-up is everything and often it is the most time consuming part of the process.
A good setup will have three key elements; Rigidity, repeatability, and geometric control.
I’m going to go out on a limb and proclaim that a perfectly round hole at the end of shotgun barrels (sxs ~ o/u) is rare.
There is likely to be some degree of oblong condition due to the rib. (unless ground as final finish in the manner as shown, including cigar of course).
A significant benefit of setting up on a cylindrical grinder for fine tuning is that the barrels can be fixtured or jigged proper so that they can be ground, removed, tested for pattern, and reinstalled precisely in the same exact location for more adjustment until the desired pattern and point of convergence is achieved. I would personally jig the fixture but that’s a whole different subject entirely. (Jigging would ensure precision in follow up finishing steps.)
Another benefit to a set up such as this is the fact that the holding fixture can be adjusted very easily in order to grind a directional change about the axis in any direction if poi is off at the desired distance. If poi is off on one barrel set, it is a simple matter of trigging out the adjustment required, adjusting the fixture, and grinding..this sort of setup facilitates that very easily.
Of course you have to have the material to grind away in order to do so. Can you say..way to go Mr. Skeuse on those uber full chokes
The only downside to the use of a cylindrical grinder is the fact that the resultant grain flow as finished would be perpendicular to the axis & in theory, that’s not ‘correct’. (Broaching would literally be the only way around this however that's just not feasible)
Although this is fact, this could be remedied quite easily with a quick swipe or two while barrels are in hand over some crocus cloth or lapping compound applied to a sufficiently long expanding mandrel made out of oil impregnated bronze (oilite)
The extension shaft being relatively close to the bore size & working from the breech end of course..although a bit overkill..this would be the exact sort of customized tooling found on the shop floor so the barrel finishers can do their thing.
Experienced hands would do this well and this is the exact sort of thing that is being lost to the ages,
sadly.
—
Hand reaming:
Barrel sets are likely being jacked up for one of two reasons.
First..if the reaming is done from the muzzle end..run and run fast as this will destroy your barrel, I think this is generally understood and doesn't warrant a horse whipping.
The right way to ream proper is what has been alluded to already with one important caveat.
First and foremost, I personally would use an adjustable reamer and I would creep into my final dimension while also patterning.
The reamer should be inserted from the breech and the extension shaft should be a good slip fit into some machined for size bronze bushings which are closely fitted to the inside bore diameter along the axis. (About a .002 slip fit and bushing length of about 1.25 would be just about right)
Here is where a lot of folks go wrong..the last 10-12” of the extension just behind the cutter should
NOT BE SUPPORTED by bushing. Let the reamer follow the hole that’s already there.
Think path of least resistance..the reamer is going to want to follow the existing hole but if it is too rigid up near the cutter..the extension/shaft is working against you and wedging against one of the chokes side walls ..if it’s too rigid it is more likely to cut unevenly and result in a finished hole that lacks concentricity to the outside diameter. Let the reamer float, within reason.
Always use at least a four flute reamer so that each cutter is properly supported during the cut..
always.
Use a goodly amount of cutting oil -
not WD-40 for goodness sakes
(Cool Tool is a good steel cutting oil)
Go slow and steady in a single forward cutting action start to finish, if you stop rotation, do not reverse direction because you want that initial bite to maintain its bite throughout the entire cut..let the tool cut and do not push it. If you take several ‘bites’ w/the reamer - this action will imprint itself into the final finish leaving little pock marks behind.
This may seem a bit overkill, but as they say, the devil’s in the details & these are just the sort of things that separate the amateur from the skilled..this requires feel.
Here’s a tip..often a fresh cutter is just a shade too sharp & it will want to grab instead of cutting cleanly unless all cutters are in perfect alignment and uber sharp - when they are, it’s like butter and feels as such.
Although this is not the time to pinch pennies on cheap tooling, good tooling aint cheap so with that being said.
To take a keen edge off of non premium tooling, make up a mock pc or two and run the reamer through it before cutting the chokes while using plenty of oil..this will knock down the cutting edge just enough as to avoid vibration. It warrants mention..a reamers cutting edge is only at the leading edge just behind the lead in chamfer of the cutter..the rest of the material behind it is for support, nothing more (it’s not an end mill).
*As an aside since we’re talking reamers..if you ream a hole in a drill press/milling machine..run the reamer through at low speed & a higher feed rate than what you would think..nice and steady in one continuous cut all the way through. Once reamer is through the material, stop the press and extract the reamer back through the hole while the tool remains stationary, unlike a drill.
Even by hand this same rule applies, no spinning reamers back out.
For this application, take it a step further..once the reamer is passed through,
do not pull it back through the freshly cut choke for extraction because it will leave witness marks from each cutter in the finish along the axis.
Do one of two things - either adjust the reamer to a size smaller than the choke bore & then back it out or remove it from the extension altogether. This is what I would do in order to maintain the size as last reamed making it easier to make further adjustments if necessary.
Did I mention cutting oil..as in lots of cutting oil..cutting oil be good.
If you have a choke that is not concentric to the o.d.…a reamer will not fix it, don’t bother trying unless using a cylindrical grinder. You could do your best to hone one side in hopes to regain concentricity but…good luck w/that.
A poor mans cylindrical grinder can be fashioned up by using a tool post grinder so long as you have the bed length on your lathe and a precise rigid method of securing the barrel(s).
When grinding, the axis of the bores must be fixtured perfectly perpendicular to the barrels breech face or positioned 'as found' geometrically on a set of barrels as already fitted, otherwise - they are toast.
Me thinks this is not for the garage mechanic unless thoroughly understood.
If one insists upon such an adventure..get yourself some cheap choke tubes & a threaded barrel to practice on first.
Whole lot cheaper than learning on a set of sxs barrels.