Hypothetical question - long reamer pilot?

The notion came to me the other day looking at reamers for some of the older metric rounds like the 8x57 or 9.3x62 with the super long leades that what would be some of the problems with extra long pilots on reamers?

Theoretically, the longer the pilot, the better the alignment with the bore. I realize that theory is often disrupted by actual practice and imagine reamers have evolved to offer the best compromise but they do seem rather short.

 

Reamers will flex, if you are not in perfect bore alignment. I don't think a long lead on your reamer will help keep you in perfect alignment.

 

With the bore running true, cutting forces will position the reamer concentric to the bore. Long pilot bushings as in fixed or reamers where the bushing is way out front run the risk of forcing the reamer off center. Most barrels are straighter than they used to be but......
I never use a snug fitting bushing. Barrel setup and a concentric roughed chamber are the key.

 

The pilot will be the same no matter how long the freebore- it will be farther from the case mouth (like a weatherby) but still use the same pilot.

 

If you've got a barrel crooked enough to worry about pilot length it's probably the least of your worries...

 

It's been my experience that crooked barrels shoot just fine. Look at the BR records over the past 4 decades. Many were with button rifled barrels that were anything but straight. The goal is to not introduce any misalignment in the throat. A tight fitting bushing/pilot that is anything other than the bare minimum in front of the throat can and on occasion will cause a less than concentric throat.
Been there done that.

 

Hmmm, never thought of it but it makes sense that reamers flex and to add to that, the piloting end is the most flexible part...

 

If you have the section of bore that the bushing will be in dialed in then it wont make a difference either way. If you dial the throat and muzzle you dont want the bushing doing a thing and you sure dont want it even deeper into the bore, as there will be some runout up there with that method most of the time.

 

So, forgive my pestering, is it more critical (and perhaps harder) to achieve a concentric throat than a chamber that is parallel (enough) with the bore?

My initial though was that a long pilot, by matching up with more of the bore, would tend to center and align the reamer better than a shorter pilot.

 

Early in my career I thought the pilot would overcome my limited understanding of the process of cutting a concentric chamber. We didn't have the internet back then so over time I developed my own technique for chambering. I won't go into how I do it as it's a commonly used process these days. One of two schools on how to set up a barrel. That is the crux of getting a concentric chamber. Lets start with the barrel setup, dialed in, tenon turned and threaded. I rough out chambers so I have a concentric hole to start with. When the reamer enters it can only go one place, to the center. All the cutting forces applied to the reamer push it to the center of the bore. Only we, the smith, can force it off center. Either with a pilot/bushing far enough down the bore that it follows the curve of the barrel. Thereby forcing the reamer, the throat area, off line. I have done that before.
The other scenario is cutting the entire chamber with the finishing reamer in a floating holder expecting the floating holder to achieve perfect alignment. If there is any wobble in the reamer shank protruding from the chamber there is a good chance concentricity will be compromised. You can not force a reamer to cut a dead perfect chamber. It does it all on its own. That's the simplicity of cutting a good chamber. Don't make it more complicated than it needs to be. Every once in awhile after cutting a chamber, without removing the reamer, I'll put an indicator on the outboard end of the reamer. Run out is almost always under .0005" TIR. So between the throat and the end of the shank, maybe a distance of 4"-5" there's less than .00025" axial misalignment. I consider that pretty good.
Reamers can and do flex, some. When a heavy cutting load is applied they can and will twist, some more than others, changing every angle the reamer maker ground. Usually cutting forces are very light. You can hold a reamer with simple tap wrench in your hand and cut a chamber. I worked, AKA bitched, with one manufacturer decades ago to thicken the flutes and increase the core diameter of his reamers. Much much better. If your worried about bending the reamer then you need to rethink your process. Something is very wrong.

 

Button rifle barrels aren't straight Humm

 

Dave, thanks for the insights. FWIW, the method I’ve settled on is basically the first method you described. For one thing, my lathe is small and the bed is too short to fit a barrel between centers and chamber in a steady rest anyway.

I use an outboard spider and either a four jaw with a copper ground wire ring in the jaws or a D&T’d faceplate with brass tipped screws at the inboard end. I use a combination of Gritter’s Rods and a long stem Mitutoyo to dial in the breech end as close as I can and let the muzzle run where it ends up. I try to start where the throat will be located and go forward from there as I don’t care what the bore is doing toward the tailstock as it will become chamber.

I then put the reamer in my Manson holder (being careful to ensure the floating surfaces are in full contact) and then set the compound to follow the body taper on a reamer flute. I then run in a twist drill to hog material and follow that with a carbide mini boring bar to establish the chamber taper with at least four passes to account for flex. I then run the reamer in to complete the chamber.

My lathe is small, light, and, frankly, not very precise or rigid. A heavy nudge on the chuck will upset the dialing-in process although it does seem to gravitate toward a consistent center. This is where my pondering upon pilots comes from and why I use a floating reamer holder. I agree, the reamer will want to cut concentric and only does otherwise if forced to do so, but I can’t help but wish there was a simple way to ensure perfect alignment every time.

 

Stan
I wasn't condemning button rifled barrel manufacturers. They just sometimes have a personality all their own. Not a bad thing but I've seen more odd twists and turns with button rifled barrels. Nothing wrong with that, they all shot fine.

 

My experiences mirror Dave's, barrel straightness has no correlation to accuracy IF chambered well. I have tracked some of the straightest ones to see if they had any better performance, that also goes for some of the least straight.

 

Dave
I had to rattle your cage LOL
Shucks in 2008 I shot at least 4 heavy gun screamers with a 6mm barrel that our guy was going to throw away because it had a little wiggle in it probably the best I ever had.

 

The best advice I can give you is don't hold your reamer. Just push it. That allows it to go to the center with no influence from a holder. I started doing this probably 30 years ago. I told Mike Bryant about it and as far as I know he still pushes the reamer. I pinched this pic from his website. I use drill chuck adapters in the tailstock that I drilled by holding the drill in the chuck. Over size the hole maybe .020" over. Shortens up the OAL of the tailstock a lot. Simple and it works.

 

Im my opinion a longer reamer pilot will just make it easier to scratch the lands further out