Airgun triggers, springs, bolts and metallic hardness
by B.B. Pelletier
Brian Saade has delivered another great technical guest blog for us. This time, he’s going to discuss metal hardness and how that affects various airgun parts. I can already see the flurry of questions and comments!
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Brian Saade (aka Brian in Idaho)
Now there’s a catchy topic. Hope you had that extra cup of coffee!
Yes, I’m the same guy who recently wrote about the virtues of modern plastics in airguns, so how is it that I’m now writing about the virtues or properties of metallic parts in those same airguns? Truth is, both materials have their place in modern airgun designs and there are many parts inside and outside of our cherished airguns that (still) require metallic designs, not plastics.
Certainly, a rifle barrel is an obvious item that requires fine, high-carbon steel to withstand the rifling process as well as the strain put upon it (think elasticity) during the cocking cycle of a breakbarrel airgun? But, there are other metal parts that we more frequently tinker with or modify on our airguns, and those are most often found in the trigger modules and trigger components.
In both firearms and airguns, we have all heard the terminology associated with certain metal moving parts. Terms such as hardened sear, case-hardened bolt and precision-tempered spring (I’m still baffled by that one, as tempering of steel is as much an art as it is precision). What do all these terms and buzzwords really mean to the average airgun tinkerer or DIY airgunsmith? Let’s explore some metallurgical basics first, and we can then see how they apply to our main interests…argues!
The Rockwell hardness scale
In the good old USA, we most often measure surface hardness of steel metals using the Rockwell “C” scale (aka Rc). As an example, a common framing nail nearly falls off the bottom of the scale at a lowly 12 to 15 Rc. Conversely, a finely made steel cutting tool or knife blade could range between 50 and 60 Rc or higher. Drill bits and other high-speed tools range even higher on the Rc scale. I’ve broken many a drill bit and know that the very high hardness also induces brittleness or lack of elasticity and ductility in the metal. Metal parts that bear upon each other or that must withstand wear and strain will typically require some form and level of hardening or hardness.
As a kid, I was always in awe of fine pocketknives that claimed to be case-hardened. Not sure why, it just sounded cool. To imagine the case of the metal, think of a piece of round steel bar, say 2 inches in diameter and one foot long, that was case hardened. The surface of that bar or the case is now harder than the interior portion of the bar. This could be up to .125 inches deep from the surface. Case-hardened metals are often used where surface wear is an issue, but the part must also retain inner elastic properties to a greater or lesser degree. In everyday terms, it needs to resist wear and also flex or move within its design purpose. On the opposite end of case-hardened steel is through-hardened. As the term implies, the raw material or finished part is hardened throughout the cross-section of the part, not just on the surface or some depth away from the surface. There are algorithms and Jominy tables and other methods of determining how this is done, but that’s another 10 or 12 pages of rather dry stuff that won’t help you polish that trigger sear in your Crosman 2240! (Yes, there was actually a Dr. Jominy, Ph.D., who designed the Jominy tables for hardening metals. Go figure.)
We all recognize this term, whether in metals or in life experiences. “Temper your remarks young man,” our sixth-grade teacher might have said to us back in the day. She was actually saying “restrain yourself.” So, think of tempering (as in tempered springs) as restraint in the metal, or the metal not easily letting loose of its potential, elasticity or memory. Tempering can be achieved through many methods. Quenching in oil or water, air- or vacuum-controlled increases in heat followed by cooling, and hybrids of all the above. Ever heat a tempered spring with a propane torch to try to reshape it or expand the length of a coil? Oops! It’s not very springy anymore! Why? The spring lost its temper (pun intended) when you heated it enough to move the metal. That’s called annealing of the metal or softening and is just the opposite of tempering. That topic would be another 10 or 12 pages and a full pot of coffee, so let’s move on!
So, we now have a few of the basic terms and definitions in grasp, so what about those parts in the trigger we want to tinker with to improve those 10-meter groups at the range?
Ah, yes, the often maligned trigger sear, the object of our Dremel tool and much debate on the forums and blogs. The part of the trigger set that lets loose of the mainspring or the hammer and sends the pellet on its way. The surface hardness of well-made trigger sears should be in the 45 to 50 Rc range. This is not an empirical statement, as the mating parts in the trigger set require complimentary hardness and the hardness could be lower, but, in higher-quality trigger sets this hardness matching or design of mating part hardness is a key design criterion (e.g., the Rekord triggers found in Weihrauch guns, such as the HW97).
Weihrauch Rekord triggers achieve this in part with hardened steel pins in the trigger that act as bearing surfaces against levers and the sear. In this way, the aluminum trigger does not require the higher hardness level, and the pins can be replaced if needed. Clever Weihrauch guys! When polishing or forming a shape in a trigger sear, use a good stone and oil, much as you would on a fine knife blade. Resist the urge to use the Dremel tool or bench grinder, no matter how fine the grit or wheel. Heat builds up very quickly on these small, steel parts and that heat is the enemy of the surface hardness of that sear. Ever cut down a 10-32 steel screw with a Dremel wheel and (sadly) put your finger on it before it cooled? You get my point, a lot of heat! Likewise, be very cautious in any reshaping of the sear or engagement features. What you may consider as polishing may actually be a relief that lets the sear release prematurely and without notice. Last, as to low-end airguns and some of those of Chinese origin, I can’t speak to their metallurgical skills or desires in making trigger components so, as always, buyer beware.
Unlike the trigger components in our airguns, the bolt in an airgun sees little friction or wear, particularly where it meets the very soft lead pellets. In firearms, the bolt is a more complex device with actuators and inner workings and, more importantly, it sees huge pressures and strain from magnum loads. In firearms, the hardening of bolt faces and components is one of the most critical design features in the weapon. For airgunners, the bolt just needs to be hard enough to not wear out, but also not so hard that it wears a groove in that plastic breech that I wrote about a few months ago. However, an airgun bolt such as those found in CO2 guns will have a connection to the cocking mechanism and some bearing surface that rides over or engages the trigger assembly. My old Benji/Sheridan AS392 .22 cal bolt is an example of this type part that rides inside the bolt and requires some level of hardness on its face.
Bolt from my Benjamin AS392
Temper, temper! This is one component that I seldom home-brew or practice DIY surgery on. With the wealth of spring types, sizes, wire alloys and configurations available to hobbyists like us, it seldom pays off to modify springs other than cutting and cold-forming the ends square and perpendicular. I’m speaking mostly about coiled springs and especially powerplant springs in airguns. Also, keep in mind that heat (above 200-deg. F) is the enemy of the small and usually thin, tempered spring we might find in trigger sets. Careful cold bending at shallow angles or shearing (not grinding) to length or shape is the method of choice for these spring modifications or repairs. As for other types of single- and double-loop springs with tails or flat wire springs found in trigger sets, finding replacement or
alternative springs is not nearly as easy as it is with their coil cousins. However, there are many industrial spring makers out there and a search on the web will produce interesting results for replacements or alternative types of springs. You’ll be surprised at how common that weird looking spring in your trigger may be. Not Home Depot common, but common among industrial designs and spring makers. Keep safety in mind though, as the guy who designed your trigger also calculated the loads and weight of return necessary to actuate the gun and still be safe. Springs are also levers, and as Archimedes said, “give me a lever and…” anyway, always practice safety first in spring replacement or modifications.
Pop quiz: No, but maybe a little summary is in order?
When polishing or improving the finish on trigger sears and parts, keep heat to a minimum and always maintain the flat surfaces and basic shapes that you started with. Use clean, flat stones and fine wet/dry papers and oils to polish. Resist using power tools and rounding off edges.
Cutting or modifying springs: “Don’t lose your temper” is all you need to remember. Temper is lost when heat (or energy induced during stretching) overcomes the elastic memory of the metal. This is very easy to do in small, delicate trigger springs. Cold working and patience is the key.
Safety rules the day. Always think your project through to the end result before starting down the DIY road. Don’t do as I have done in my earlier days and modify everything in sight with no way to tell which mod improved what. Worse yet, having one mod offsetting the benefit of the others and possibly making the gun unsafe. Take incremental steps, especially inside the trigger group and mainspring components or modules.
Hopefully, I’ve armed you with a few bits of new or refreshed knowledge in this report that will help you apply some additional science and a little more precision to your next trigger or spring modification/repair project. Till next time, shoot well and shoot safe!