Tag Archives: Gunsmithing Student Handbook Series

Excerpt from “Gunsmithing Student Handbook Series”

At the time of this writing there are three books in the Gunsmithing Student Handbook Series.  In early 2018 “Chambering Rifles for Accuracy” was released.  In its first week on Amazon.com it was #1 in two categories.  Gordy Gritters, famed benchrest gunsmith & Fred Zeglin well known wildcat and hunting rifle maker joined forces to create this comprehensive title.  It covers the subject of installing and chambering barrels from simple rechamber jobs, to hunting rifles and the last half of the book is all about benchrest quality gunsmithing.

The excerpt here gives you an idea of how careful the authors are to cover details often overlooked in such books.  Enjoy:

Tolerance Stacking; 
                Everything You Need to Know About Pilots?

Chamber reamers come with two styles of pilots, solid or removable bushing (the later sometimes called a floating pilot).  One is evil and one is practically perfection.  But which is which?  I can tell you that in renting tools to gunsmiths I have found the industry is split on this question about 50/50.  In other words, about half demand solid pilot reamers exclusively and the other half will not touch those nasty solid pilots with a ten foot pole.

It is clear that there is a trend toward the removable pilot reamers.  This is because barrel makers are not all holding to the same production standards.  Some have the idea that a tighter bore is better, while other makers hold close to the “standard” bore dimensions.  Example, .308 bores are .300” on the bore and .308” on the groove.  Custom barrel makers have tightened the bore to say .298”  This will required a smaller pilot as the standard pilots are normally .299” with tolerances of + 0 to – .0005”. removable pilot reamer 

Removable pilot reamer.

The reason for the tighter bore?  In short, it is believed that it produces better accuracy.  This is only true as compared to a loose bore that is oversized, i.e. a groove on a 30 caliber of .309 or .310 will produce lower pressures and potentially be less accurate because the bullet is not fully engaged until pressure bumps the bullet up to match the bore.

For a pilot to work correctly it should be .001” smaller than the actual bore dimension.  In other words, it needs to be a close slip fit.  If a pilot is too tight it will bind and likely break the reamer, and possibly damage the bore.  If a pilot is too loose it will promote chatter.  It is possible to run the pilot on a removable pilot reamer closer to the bore diameter (.0005” under bore diameter is ideal), but it must still slip easily in and out to avoid damage to the barrel or the tools.

A little history at this point might be interesting:  Red Elliot was and still is legendary with old timer gunsmiths as the absolute best reamer maker of the last century.  Near as I can tell he was the first to offer removable pilots on his reamers.  Why did he do this?  Well, he found that there were enough different barrel makers in his day that the dimensions of the bore diameter (where the pilot rides) varied a fair amount. 

So, this problem of bore dimensions changing a little is nothing new.  What about SAAMI standards you say?  I will address that in just a moment, for now lets talk about how Red Elliot handled bushing pilots.

I have seen several of Red’s reamers with bushing type pilots, what we sometimes call floating pilots today.  Red held very tight tolerances on his bushings so that it required a little pressure to slide them onto the reamer, held in place by a screw mounted in the end of the reamer the bushing would not turn once the screw was tightened.  This is contrary to the bushing pilots we see commonly used today, where the bushing is a slip fit with about .0005” tolerance internally.  This tolerance is added for manufacturing ease. Tolerance stacking is not usually mentioned in conjunction with floating pilot reamers, but we are going to take a closer look at it here.

Another source of tolerance issues is the fact that the pilot receiver on the reamer must be concentric (round), and in line with the reamer.  If either of these conditions is not correct there will be problems with the reamer cutting oversized or out of alignment with the bore.  Admittedly, this is not much of an issue with today’s cnc machines.  So long as the operator does not make an error, and no chips get caught in the set-up.  One other possible source of trouble would be a warped reamer (not common).

Now for SAAMI, their standards are voluntary, so obviously any barrel maker can decide whether or not to hold solid to the standards.  Industry standard is plus or minus a half thousandth (+ or – 0.0005”) on the bore diameter.  The bore diameter is the smallest diameter of the barrel, also referred to by shooters as “across the lands”.  The same tolerance applies to the groove of the barrel.  I will leave the discussion of groove depth as we are talking about bore diameter as it relates to chambering tools, groove depth does not affect these dimensions.

Admittedly barrels considered “match” grade or “air gaged” are supposed to be held to a tolerance of .0003” or less total variance, end to end of the barrel.  This does not indicate the actual bore diameter, we are left to assume that it is the standard diameter for caliber.  In the case of a 30 calibers we would be talking about a .300” bore.  What if the maker decides to simply use a gage that works with the bore diameter they are making, say .2995” and it air gages as above.  You have a match grade barrel but the bore is at the minimum size according to industry standards. 

Are you starting to see how bores can vary and still be within standards?

Of course there are those makers who operate outside the standards and make perfectly good barrels.  The point being; different size pilots will be needed to chamber these barrels as was recognized back in the 1950’s and 60’s by Red Elliot.  It’s pretty obvious by now that removable pilots are necessary tools in dealing with variations in bore dimensions.  It should be clear by now that variations in bore diameter of plus or minus .001” or even more, is not that unusual, even though such dimensions do not follow the voluntary standards set by SAAMI.

Solid pilot reamers offer certain advantages over the floating pilot.  First and most obvious there is no built in tolerance between the bushing and the reamer, because the there is no bushing.fixed pilot

 

 Solid Pilot Reamer

Since most barrel makers today are making barrels by the button rifled method dimensions tend to remain pretty steady for a given maker as buttons last a long time if properly cared for.  So if you deal with the same barrel maker all the time chances are a solid pilot reamer will fit the same from barrel to barrel. 

There are other factors that play into the bore and groove dimensions, but that is for a discussion for another book.

One limitation of a solid pilot reamer is that it cannot be changed to deal with variations in bore diameters.  Of course you can have the pilot ground down if necessary to fit a tight bore, but then you would probably need a second or even a third reamer to deal with various diameter bores. 

Everything in life is a trade-off.  Because of the expense of multiple reamers for the same caliber removable pilots are a cost effective answer to the problem.  $10 for a bushing beats $100 or more for another reamer.  There are shops that stock bushings in 0.0002” steps for the popular calibers.  This allows them to match the bushing to the bore every time.

pilot bushingsPilot bushings can be a big investment.

To make the use of removable pilots efficient and accurate, the gunsmith should invest in a set of pin gauges.  These are precision ground pins that can be used to gauge the bore and insure that the correct bushing is selected.  Using pin gauges allows the gunsmith to know what bore diameter the barrel maker is really supplying.

Now keep in mind the pilot has to slip into the bore, so in mechanical terms the pilot has to be about 0.0004” smaller than the bore to slip in without any interference. In most shops the pilot is figured at 0.001” smaller than the bore and rightly so.  Too tight a fit can gall and or leave marks in the bore or stress the reamer and break it during the reaming process.

What happens if the pilot is too loose?

Ninety-Nine times out of a hundred when a reamer chatters (vibrates) in use, it is because the pilot to bore fit is too loose. 

The lack of support when the pilot is too small allows the reamer to move around in the bore, as the tool tries to bite into the steel it grabs hard and because even tool steel is flexible you get chatter as the tool loads and releases tension.  This is the reason that some gunsmith’s insist on having a set of pilots that cover the possible variations in .0002” (That’s 2/10,000 of an inch.) increments.  Keeping the pilot as close to bore dimensions as possible will help eliminate chatter and promote a more precise chamber. 

If you have a pilot that is a perfect match for the bore but is too loose on the inside where it rides on the reamer then the advantage of a close fitting pilot is negated.  To pull the whole concept together…  If you have a .0002” tolerance on your bushing to barrel fit and the same on the pilot to reamer fit, you end up with .0004” total slop on the pilot. 

I can tell you that most people do not grasp this or understand why these tolerances  matter.  I base that statement on 30 years of talking to gunsmithing customers, and the people who call to rent tools.  The comments that shooters and gunsmiths make during our conversations indicate their level of understanding in a hurry.

In general if the total pilot run-out is under .001” then all will work fine and there should be no worries.  This rule holds true for solid pilot or removable pilot reamers.  Long ago I lost track of how many rechamber and barrel jobs I have done.  I can tell you that

Chambering Rilfes for Accuracy, cover

 it is possible to get an accurate job from either type of reamer.  In fact, if pressed for a choice I would say that solid pilot reamers are more accurate on average.  Especially for inexperienced gunsmiths.

I do not make this statement lightly, as I own hundreds of reamers of both types.  This goes back to the understanding of how the tools relate to the barrel.  To reiterate, the one caveat would be that for best accuracy the pilot of the reamer must meet the tolerances of less than .001” run out verses the bore, for all this to hold true.

There is another major factor in how well a reamer cuts and how accurate the gun will be…  The gunsmith must do a good job on the set up for machining.  If the threads are not true to the bore, or the chamber is crooked or oversized, or the throat of the chamber ends up off center, accuracy will be elusive to say the least.

Use of a floating reamer holder is a great way to insure an accurate chamber.  This tool allows the reamer to follow the hole in the barrel without any side pressure that might be caused by minor misalignment of the tail stock to the bore of the lathe.

Whether you plan to build accurate hunting rifles or top quality competitive benchrest and long range guns this handbook has detail descriptions and plenty of clear photos to make the subject easy to understand.  The other titles in the series deal with headspace, the vital counterpart to chambering, check them out here.  More titles are planned for the series.  These are college level training manuals that a gunsmith at any level of experience will value.

Another post of interest on this subject.

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