Interchangeable Parts
The subject is Part & Component interchangeability in shotguns.
This is a subject that seems to stress everyone. The apparent contradiction issues presented when parts and components are required to be interchangeable and yet give the esthetic looks of a conventionally made gun.
Lets be specific and look at the obvious problems with a high quality Hand Made shotgun to a Interchangeable shotgun of the John Wilkes design.
Think about the Trigger Plate for a moment shall we. Okay, convention tells us that this part has a tongue of some length, with some shoulders on either side. The shoulders as it turns out are used to pull against. In
other words the gun stock is being pulled to the action by the trigger plate. That means that these shoulders are going to be stressed quite a bit and when one uses soft metal 8620 case hardening material any stress will open a gap quickly. So what do your do?
Well first the trigger plate needs to be larger than the space it is to be fit into. The sides therefore need to have a negative camber or draft. A wedge if you will so the part will begin to fit. Because the metal of the Action and the Trigger Plate is soft, there cannot be a close tolerance and the two parts are well, “Hammered together” so to speak giving a zero gap fit. With the excessive metal that remains, it is just filed away until smooth.
What about the stress of being pulled? This then is accommodated by a screw drilled right through the Trigger Plate into the Action and the part is staked in place.
Having understood the Hand Made process, the question of interchangeability with those components certainly remains no mystery. And it does challenge an interchangeable part on many levels to resemble that of a hand made part. What has to be appreciated is a departure from convention is necessary to achieve similarity. Lets look at that thought process and then come back to a comparison or end result.
First lets assume the two types of Trigger Plates are similar in shape as before. I.E. a long tongue and a couple of shoulders to fit its position. In order for a high tolerance part to be used in an Action slot to receive it, the Action as well must have a high tolerance. That means BOTH must be made of the finest material available, that when heated or hardened, it will retain its tolerance. Remember we are not hand fitting these much if any.
Second, for interchangeability and fit, the negative draft is not necessary and in fact might be a detriment. A negative draft is a “fit all” adjustment. But if this is true, what about the stress on the Trigger Plate when pulled by the stock clamping it to the action?
Well, we could put in a screw? In fact if a part is purchased from us likely it will as convention remains.
An Extra Finish John Wilkes shotgun in progress.
Okay now we have two trigger plates down in position in their respective actions. The Hand Made one is filed off nicely and blends perfectly. It IS beautiful.
The second trigger plate, call it “IP” for interchangeable part, it fits nicely too but there is (or can be) given all the tolerances some visible gap. You might select from several to find a best fit. What about the lever or height difference of the two metal surfaces of the Action and IP? Tolerance will be close to .050 -.100 microns or .002 - .004 inches. Not bad and quite acceptable really considering a human hair is .090 microns in diameter.
“IP” Trigger PLate supplied to John Wilkes Gun & Rifle Makers, London.
So, that is the difference in interchangeability in a nut shell. One IP can be switched to a double trigger or changed with another that has preferred engraving. Or maybe that trigger plate has been updated internally with a new trigger system and can changed out in a few minutes like on the John Wilkes. Is that a desirable feature? You can’t do that with a Hand Made part.
Fine, you are an old style gunmaker and like the look and fit of a hand filed part. Why not just install an IP and file it smooth and call it a day in 30 minutes, not 3 days!!
At John Wilkes (got to get in a plug here but the only example we can use really) its guns are IP. It also has another nomenclature called “Extra Finish”. And extra finish gun is one where the IP parts are filed and surfaced together in the conventional fashion. The only difference is that one looses interchangeability in favor of a marriage of both worlds. So a customer actually rules in how the gun is made and finished. If the long term interchangeability features seem to be the best benefit, he goes with tight fitting IP parts like in a Perazzi trigger plate. Or, strikes the surface making the engraver happy and looses all the benefits technology has offered to his uniqueness and originality. The cost difference is only labor isn’t it? And major time!
But, let’s not forget about the remainder of the shotgun with IP parts and components. Wow, those just drop in and still maintain close tolerances for wear and replacement. That’s really good. And if one buys on to the Brazier process for wood it is suddenly interchangeable and can be replaced as you grow and age, or just because you take fancy to a nicer piece of wood. That’s a good thing.
What about barrels? We talked about tolerances etc., do those just “fall on”?
Yes and no.
Do they just “fall on?” Well they will and the simpler the design the better. But given all the tolerances involved, they do have to be fit in a complex gun like say the Wilkes, Boss or Purdey. There are locking systems to be considered there and where the metal meets the Action, perhaps some marriage sanding. But they do largely “Interchange”. The overriding factor is the long term life of the IP barrel in “SolidSolid”™ and “MonoSolid” ™ with the homogeneous components of ribs and lugs not being soldered on. Also the increased pressure and tensile strength the superior metals in them offers.
Depending on the level of close fit then, interchangeable or IP parts require a lower level of fitting. Many maker today are using very sophisticated machines but IP is a mindset that encompasses and entire design. Rolex watch company stepped away with a complex system which today is largely made by machine with IP inside. Leaders like Perazzi paved the way for the IP thought introducing a shooting system. And of course John Wilkes which push high quality guns to the limit of engineering today.
Karl C. Lippard, Managing Director