First, let me just state plainly this isn't the least bit confrontational. It's a discussion, nothing more.
I think the big answer to your question(s) is in your own statement.
The precision action manufacturers of course don't do it because their shit is (presumably) dead nuts out of the machine. It is my understanding that Remington (because you brought it up) does not use a tap. I see no evidence of it in their actions. The thread is effective from start to finish. You only get that by single pointing. It was told to me once (by a vetted person who'd know as he works for them at a high level) that Remington has a work holding method that locates on the outside features of the action in a turning center.
Regarding the mandrel topic:
If I take a 1/2" carbide endmill from any well known tooling vendor and I put it in a spindle to make a pass on a piece of stock, If I am more than say a diameter or two in depth of cut I can count on some tool deflection on just about any grade of steel or aluminum. Now, there are other factors that play into this. Feedrate, percentage of tool engagement, coolant, flute count, flute helix, tool holder, etc... The point is carbide is ground, it's harder than wood pecker lips, and it's very, very rigid.
It will yield from tool pressure.
By comparison a ground piece of tool steel supported by a pair of wedding bands is a wet noodle. It will deflect if a lateral load is applied to it. You are relying on the action itself to support the very tool doing the work. How many articles are on the internet about heavy barrels and poorly fitted scope rails causing receivers to hunch up like a cat being petted? How many exist about a guy with a chassis that snugs up the rear tang screw a bit too much and finds out that the bolt now sticks or the gun slam fires?
But now suddenly the action is more than capable of supporting tooling for machine work? Corner, paint brush, wet floor....
Now, lets address the tooling:
Split this down like this: I'm guessing here and being very respectful to the vendors making this stuff.
Mandrel: .500" dia +0.00/-.0003"
Bushing: .4998" dia +0.00/-.0002"
Tap: .5005" dia +0.00/-.0003"
Fitment of bushing to receiver bore:
Did you ream or hone the action prior if for no other reason than to qualify the surface so the bushings aren't biased by an inclusion?
Add up the tolerance stacking from figures above. It's small and most small shops lack the resources to really qualify it. It takes BIG money to do that. As I said, the values I chose are a guess and they are very small. Assuming I am right, they are very, very well controlled diameters.
Next,
- is the mandrel straight?
- Will it stay straight?
- Does it have any taper?
- How much and where?
- Is it hard enough to mitigate deflection when the lead tooth of the tap initially engages material prior to the one 180* from catching the thread lead?
If taps are good enough when fitted to a mandrel, then I would reference back to your italicized quote above. The "boutique" shops making actions are not using the process for a reason. They aren't even using rigid tapping in the CNC which should/would be far more reliable if the part is in fact fixtured, located, and has all the relevant work done in that one setup.
A business in the firearms industry is profit driven just like any other business. If there is a better/cheaper way to do something then that is all the incentive needed for pursuing it.
I didn't liquidate my life savings to buy the equipment I have just for the hell of it. I did it because if I am going to do this work I feel compelled to do it as best as I possibly can. The tooling manufacturers can offer a counter argument if they wish. They are in the business of selling a product. The business we are in sells a
service. A tap will make a very qualified hole. No argument there. Taps however don't do as good a job at holding position. Not when compared to the single point method. I agree 100% that its absurd to try and sell the notion that one must lap a barrel tennon to the receiver in order to wring out the accuracy potential of a rifle. A 60* thread form is self aligning. All that means is the barrel will attempt to naturally align itself concentric to the effective thread form of the receiver. It does nothing to address whether or not the threads in the action are concentric, parallel, and free of taper/bellmouth when the datum point is the hole that the bolt registers from.
Since the GB of 2013 we've knocked out roughly 3000 receivers. For about a 6 month period I kept a log of the machine's work offset values. This is nothing more than documenting the machine coordinates where the zero point is set after setting up an action. Probing makes this not a big deal. Remington actions over the full gamut of production history hover around about a .017" "box" in the machine. Because I use the same collet, the same fixture, the same platter, and the stuff never comes off the machine, and the control on this machine relies on battery backup power when turned off, the home position never changes all that much. So, the +/- "whatever" that a Kitamura is able to position to is really, really good.
Conclusion, the +/- .017" value is pretty darn solid.
The only way I can ensure the threads are located off this common center point is by first internally boring the ID of the minor followed by single pointing the thread form with a thread mill. There's a number of other points in this discussion that I am ignoring for the moment just because of time constraints. If anyone experienced in this wishes to chime in feel free to fill in the gaps.
Last:
Take a premium barrel fitted by someone who can do this kind of work. Fit it to an action that has had zero work done to it, you can expect increased performance. It's been done for decades. If you go splitting hairs by finessing all the critical surfaces on the action, the gains you achieve are quite small. Stab the original "mater stake" factory barrel back on it and you'll likely never see it. Its usually not relevant until all the "I's" are dotted and the "T's" are crossed.
It's like stuffing a blower on a smallblock. You instantly net a big gain that vastly overshadows the 20-50hp your missing because you didn't do the detail work to the block, crank, heads, etc...
The detail work is what we are supposed to be selling when we offer this to the community. I don't work barrels with a Dewalt. I am of the opinion that receivers should not be either.
C.
