by Tom Gaylord
Writing as B.B. Pelletier
Announcement: Pyramyd Air has asked me to announce their Memorial Day Madness sale, which features some sizable price reductions.
This report covers:
- Design parameters and constraints
- Pivot joint
- Coiled steel spring items
- Spring guides
- Gas-spring items
- Piston bore
- Trigger and safety
- Scope base
- You do the rest
I used to teach a subject called Value Engineering to Department of Defense procurement personnel. Value Engineering was a U.S. Army initiative from World War II, where a design was examined by not just engineers, but by all the disciplines that dealt with the product. The goal was to create the function of an item at the lowest cost.
They discovered, for example, that a maintenance man could make a small change that saved the Army millions of dollars by either making the item easier to maintain or making it so it didn’t require maintenance at all. On the other hand, a production manager might make a change in the design that dropped the cost to produce the item from $2000 to $3.00 by simply changing the way it was produced.
What I’m going to do today is start a multidisciplinary design review of an inexpensive breakbarrel spring rifle — the most popular airgun sold in America today. Blog readers can participate through the comments. I can assure you of one thing — nearly all airgun manufacturers have at least one person reading this blog every day. So, let’s give them some food for thought.
Design parameters and constraints
- The rifle we design has to cost very little to manufacture. Nothing that costs extra money will be included in the design.
- We’re building a rifle for the broadest possible use. That means it must be accurate, easy to cock, easy to maintain, have a good trigger and good sights, and a stock that adapts to the broadest possible shooter base. Power will be secondary to all other constraints, but we won’t do anything to limit the power of the gun. It’ll be whatever it must be — in light of all aspects of design.
- Fewer parts means a simpler design.
- The broader the appeal, the more we’ll sell. The more that sell, the lower the development costs per gun.
- Maintainability builds customer confidence.
- Accuracy is the most important aspect.
- Smoothness of the firing cycle takes precedence over everything except accuracy.
The rifle must have a good barrel, capable of producing good accuracy. That would be 10 shots grouping in less than one inch at 25 yards. We have learned in this blog that good barrels don’t have to cost a lot of money. Look at the accuracy we got from a thin-walled steel barrel in the $100 PCP — (read Part 4).
Because this is a springer, we can keep the barrel short. Ten inches is all that’s needed. That keeps the cost down, because short barrels take less time to rifle. The rest of the tube that appears to be the barrel can be hollow. We need the extra length for cocking and separation of the open sights.
The barrel needs to have a good, clean crown, so special attention will be paid to each crown produced. The barrel needs to be mounted solidly in the jacket or shroud so there’s no chance for movement.
The pivot joint must be a bolt or other means of attachment whose tension can be adjusted when the barrel becomes loose. A locknut is required unless the pivot bolt can be designed to not loosen with use.
The pivot joint shall have a design that reduces friction to the maximum extent. Washers on either side of the baseblock are in common use for this today, but they’re not mandated.
Coiled steel spring items
The following items apply if a coiled steel mainspring is used in the design.
The piston must have a seal that’s easily replaceable when needed. The piston must also serve to guide the mainspring to reduce vibration. The piston stroke should be long to give maximum power with the lightest spring tension.
Some method of guiding the mainspring shall fit the mainspring tight to reduce friction. It should be incorporated into the design of the piston without adding additional parts.
There shall be a means of eliminating the rotational torque from the mainspring at both ends. A plain washer is sufficient for this. The finish of the washer shall be as smooth as is economically feasible in a high-rate production scenario.
The mainspring shall be a strong yet light coiled wire spring. It should be under minimum load when at rest. Before installation, it shall be lubricated inside and out with a low-friction, high-viscosity grease.
If a gas spring is used, it should be very easy to cock. Use of a long piston stroke allows for lower gas pressure in the spring and also reduces the cocking effort through optimum cocking linkage design. The piston weight should be kept as low as feasible with longevity to reduce forward recoil with the shot.
This applies equally to steel mainspring and gas-spring guns. The piston bore size should be as small as possible to keep the overall size of the rifle in check. Use piston stroke rather than bore diameter to generate power.
Trigger and safety
A two-stage trigger with an adjustable first stage should be used. It should be possible to adjust out the first stage — making the trigger single-stage.
The trigger should work through geometric design (over-center release) rather than sear slippage. If a sear is used, sear contact should not be adjustable.
There should be a manual safety that blocks the trigger.
The stock should have an adjustable length of pull that ranges from 12-1/2 inches to 14-1/2-inches. The stock line should be as straight as possible, negating the need for an adjustable comb.
The sights should be adjustable in the rear and a hooded post on a ramp up front. The adjustments should have a means of determining where the rear sight notch is and where is is being adjusted. There shall be markings on the sight to indicate which direction to turn the adjustments to move the strike of the pellet.
A Weaver-style scope base should be permanently attached to the spring tube.
The rifle shall be designed with ease of disassembly in mind. The mainspring will not be under great tension, so a mainspring compressor shall not be needed for this.
Try to apply any one of these design features to a rifle that’s currently being made, and you incur considerable cost. Therefore, this rifle must be designed from the ground up. Airgun companies today are fearful of a clean sheet of paper because of the engineering time they think it entails. However, if they would just hold a couple preliminary design reviews of these features, they would save thousands of man-hours by finding the simplest solutions to every feature.
This is a process in which you “make haste slowly,” as Benjamin Franklin once said. You put the descriptions of the features up on the walls of the conference room and let them soak into everyones’ pores over time. You don’t set deadlines in the beginning, like, “We want to roll this out at next year’s SHOT Show!” In the world of software design, there’s a saying: “You want it bad? It’s bad right now!”
The company comptroller is just as welcome to contribute as the lead engineer. Remember that actress Hedy Lamarr co-invented the spread spectrum and frequency hopping communications technology that was first used to control torpedoes in WWII. For her efforts, she was inducted into the Inventor’s Hall of Fame in 2014.
Avoid absolutes in the beginning. Saying the gun should retail for $175, or weigh 6 lbs. or achieve 900 f.p.s. in .177 caliber eliminates 98 percent of the design possibilities.
You do the rest
That’s my contribution. Now, you do your part and help design this spring rifle farther.