Fit, Finish and Tolerance: What’s the Difference?

When folks discuss the differences in firearms and how they function, they are discussing how the pistols are manufactured, “put together,” and how they operate.

The same is true to a certain extent in ammunition and holster selection. The high-end product may be built to closer tolerances. As regards, tolerance ambiguity is something we must not allow.

A dimension is one thing, but tolerance is another. A lack of understanding of these critical elements may slow or stop production and results in greater cost.

All About Finish

Many makers outsource parts. Springfield now makes their 1911 handguns in-house, but at one time received rough castings and finished them in their facility.

A recent start-up company failed, in part, because their supplier delivered parts that did not fit properly. Manufacturing drawings must be exact.

Tolerances may meet interference from a tighter-than-projected fit and the parts do work properly together. System performance is degraded.

As an example, during World War II, inspectors in various plants roamed the floor with gauges to ensure 1911 handguns were manufactured to the proper specifications.

Colt, Remington Rand and Ithaca parts fit the other handguns. The tolerances may have been looser than commercial guns, but as long as the locking lugs were a good fit, the pistol would be accurate.

It doesn’t make sense to demand tighter tolerances that require more machine work and more attention to detail and greater time and expense when reliability is the primary demand.

On the other hand, at the top end of manufacture, some handguns are built to very tight tolerances. The result is less eccentric wear and a longer life span, while the less tight the pistol is, it is often subject to sloppy wear.

The engineer must be certain to specify the tolerances of manufacture.

As an example, some machine jobs feel fine with .005 inches (plus or minus) tolerances, but true custom design work demands shaving another decimal point or two.

Parts are machined from larger parts of metal unless you use modern machine injected molding (MIM) parts made from a mix of powder and glue.

That is another thing altogether. MIM can be precise and works well for mass manufacture if not for high round count competition and tactical use.

All About Fit

You may learn a lot about machining and tolerances starting with the lowest common denominator.

As an example, about two decades ago I helped a young fellow who worked at a municipal agency build up a 1911 for competition.

His uncle had given him a Llama 1911, possibly the worst choice possible. Just the same, I helped him fit a Bar-Sto barrel and bushing and eventually we added Wilson Combat internals.

There was a lot of fitting as the Llama was not Mil-Spec, but the owner and I learned about fitting. The pistol actually performed well for more than 3,000 rounds in competition.

The shooter learned about shooting and building guns on the cheap, and while he never won a match, he learned some things.

When the locking lugs of the soft steel of the Llama finally sheared off, he bought a Springfield and kept going. We were able to fit the Bar-Sto barrel into the Springfield as well.

All About Tolerance

Frame to slide fit may be tightened in a fixture if you must, although this may be expensive. Locking lugs are another matter and sometimes it takes a lot of work.

After all, you fit the barrel to the gun, not the other way around.

A family member building an AR-type rifle tells me had has found pin holes within .010 inches and others within .002 of specs, and settled on Aero as the best build for the money.

The tighter the fit, the more reliant on the positioning of the tool. It doesn’t hurt to begin a career as a home hobbyist with a looser-fitting part.

There is a great deal of difference between fit and tolerance in a wartime Garand and an English double rifle, and you must understand the differences.

Also, the same firearm has different tolerances for different parts. A dovetail may be tight. Barrels are another subject.

Some years ago, a respected maker introduced a concealed carry pistol, one of the first slimline 9mm types, and advertised a match-grade chamber, and it was tight.

Sometimes they called it a NATO chamber, whatever that means. That is great in a pistol like a SIG P210.

This little 9mm was more accurate than most, but it suffered malfunctions with cheap bulk ammo, and not all of it was steel cased. The 9mm cartridge case is notoriously inconsistent.

A Browning Hi-Power with its heavy hammer spring will ram the cartridge into the chamber and ignite it. The little modern 9mm simply did not chamber some rounds properly. Some misfired.

This was a case of misapplication of tolerances. Recently, a new introduction featured a barrel in 9mm that was larger than the nominal .355-inch bore, but as large as .365.

Accuracy was astoundingly poor and one wonders how this happened.

Working in the industry, I also learned that some CNC machines are capable of .0025 inches and that’s it, you have to spend more money to get .001-inch tolerances.

When the M1 Garand and M14 were being, manufactured .01 was considered tight and they were very good rifles. We do better today.

The tighter the tolerances, the more parts are scrapped by the inspector. The tighter the gun, conceivably the greater warranty issues as the firearms may not tolerate grit and powder ash as well.

I am familiar with a number of warranty departments and folks at gun shops that have guns come back. Remember, clearance is the distance between two mating parts.

Tolerance is the allowance made for departure from these clearances. The single greatest problem is that the firearms have not been cleaned, followed by a lack of lubrication.

Few other issues are as common.

Tolerances in Revolvers

In revolvers, the single most tolerance point that makes or breaks accuracy is the barrel-cylinder gap. Some revolvers are a sloppy 0.12-inch, rather than the preferred .006 to .007.

Velocity escapes, or rather pressure escapes, lowering velocity when the barrel-cylinder gap is too generous. Not only that, ejected lead may be dangerous.

During the days of black powder, the barrel-cylinder gap was more generous. Black powder ash built up and would cause the cylinder to stick and sometimes fail to rotate.

Loose tolerances were a necessity. A few decades ago, Uberti introduced their copies of the Colt Single Action Army into the United States.

These revolvers are sometimes made to sell at a certain price point and the brass frames and matte finish reflect this. Some of the revolvers are among the finest examples of the gunmaker’s art.

These revolvers were often very accurate, among the most accurate single-action revolvers ever made. However, tolerances were so tight they would begin to bind after less than a full box of ammunition.

This occurred even though modern smokeless-powder ammunition is relatively clean. Adjustments were made and the modern revolvers are quite tight and accurate, but may be fired a lot without any problem.

Among the greatest examples of close tolerances are the 1911 handguns from Les Baer, Guncrafter and Wilson Combat. Slide to frame fit is excellent.

These handguns are fitted part by part in-house and the result is a handgun as accurate as possible. These handguns are among the finest ever built, regardless of price.

Yet, as a shooter, I realize that there are many firearms that are fine performers and about three-quarters as accurate as the high-end pistols for a lot less money.

Conclusion: Fit, Finish and Tolerance

In the end, you pay a lot for every movement of that decimal point.

How important are tight tolerances to you? Let us know in the comments below!