Tolerance Stacking, or, Which Came First the Reamer or the Barrel?

ZeglinThumbnailwith Fred Zeglin
AGI Instructor and Owner of 4D Reamer Rentals

My résumé has exposed me to some issues that most gunsmiths only experience from a very limited point of view. First, I rent reamers and headspace gauges to the gunsmithing fraternity. Naturally I hear everyone’s theory of what works and what matters, and I hear very clearly what clients want. Second, I am approaching thirty years in the gunsmithing business full time. Finally, I spent a little more than a year and a half as production manager for a large barrel making company.

I can hear it already, “Yea, yea, yea, so now you’re the expert!?” said with a tone of disdain. Sorry for tooting my own horn, but there are two things to remember about that, first who else will do it? Second, I need to tell you why I am qualified to make the comments you will read in this article, not just some schmoe spouting his “beliefs”. Because, after all, there is an entire Internet full of that.


Removable pilot held by “C” clip

Chamber reamers come with two styles of pilots, solid or removable bushing. 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. In the last 5 or 10 years some 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”.

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.

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.


Floating pilot with screw retained bushing

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 that 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, as long as the operator does not make an error, and no chips get caught in the set-up.


Bushings (pilots) are available from the reamer manufactures in any size desired

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 to another article since we are talking about bore diameter as it relates to chambering tools in this article.

Admittedly barrels considered “match” grade or “air gauged” 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 .30 calibers we would be talking about a .300” bore. What if the maker decides to simply use a gauge that works with the bore diameter they are making, say .2995” and it air gauges 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 1950s and 60s 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” are not that unusual, even though such dimensions do not follow the voluntary standards set by SAAMI.


Solid pilot reamer

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 there is no bushing.

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 another article.

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 tradeoff. 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 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.

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 smith 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?

Nine times out of ten when a reamer chatters (vibrates) in use it is because the pilot is too loose. Because even tool steel is flexible, the lack of support to the pilot allows the reamer to move side to side in the bore causing chatter. 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 pilot 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 runout 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 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 a tolerance of less than .001” run out verses the bore.

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.

The advent of bore scopes have been invaluable to the gunsmith’s ability to diagnose problems inside the bore of the barrel or in the chamber area. There is a clear down side to this tool however. The most popular brand of bore scope has about a sixteen power magnification. This allows us to see details that do not actually play into accuracy, so the untrained eye will see minor flaws in the barrel and then worry about them. I have seen barrels that were shooting well under minute of angle come back because somebody looked through a bore scope and saw a shiny spot or a burr from the manufacturing process.

To become educated on the use of a bore scope, look at lots of barrels. So that you can recognize what is acceptable and what is a real concern. Many end users think that the bore must appear perfect to produce accuracy. To this I would simply say, prove it at the range!

Why the side trip into bore scopes? Because solid pilot reamers will sometimes burnish, or polish a small area ahead of the throat where they rub. If this burnishing does not dimensionally change the bore, and in most cases it does not, then it’s a nonissue. In fact, it will be gone in most cases within 100 shots. And, 100 shots is about the normal amount for a breakin period on the barrel. If the marks caused by the pilot are deeper than that, then the pilot is too large for the bore, so it should not be used with that barrel in the first place.

To summarize, if your pilot is properly matched to the bore, i.e. .001” smaller than the bore, then you have the best relationship between the bore and the tool. Such a close fit will help eliminate potential chatter. This close fit will also insure that the throat and chamber are properly aligned with the bore. If the bullet cannot enter the bore properly aligned it will never be accurate.

So if you ask which is better, a solid pilot or a removable pilot? I would have to say, that depends on the barrel you’re using and your experience with the tools. Either can produce amazing accuracy, it’s up to the gunsmith to set the job up correctly to get the most out of the barrel. But now you know, there are no evil reamers.

Fred is a graduate of the Lassen College Gunsmithing program, a professional gunsmith and custom rifle builder for over 30 years, a former production manager for a major barrel making company, designer of the acclaimed Hawk series of cartridges, free lance author, wildcat cartridge designer, and AGI instructor.

5 Responses to Tolerance Stacking, or, Which Came First the Reamer or the Barrel?

  1. I’m with Brian Deezar.

    EXCELLENT article Fred!

    Thanks a bunch for taking the time to share this here. Hope all’s well with you Sir. Best to you,


  2. Many thanks for this excellent write up from an expert in the field of chamber reamers, as has been flagged up in this article there must be many that finish a chamber reaming job with the reamer held rigidly in a fixed tailstock, thinking that will give them an accurate concentric chamber, an axiomatic (floating) chuck is essential for this operation, otherwise it comes down to pure chance, not something I like to rely on given the amount of work that goes into this job.

  3. Excellent article! Most informative on past and modern understanding of chamber reaming tech. While I may never ream a barrel as a hobbiest, the information will be most helpful in understanding the issues involved as gun tech study. Thanks for this superb effort.

  4. Fred,
    An excellent write up on a very controversial subject. As a smith for the past 20 years I have found that there is no place for improper tools. Your article is an excellent example of this fact. The correct tool for the job will in 99.9% of cases produce the superior results. One thing you didn’t mention was lubricants and flushing methods which I find vary from gunsmith to gunsmith. Perhaps you could do an article on this in the future.

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