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How I Barrel A Precision Custom Rifle - Long Rifles, LLC

 
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  #1  
Old 06-01-2010, 01:31 PM
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How I Barrel A Precision Custom Rifle - Long Rifles, LLC

I like sharing this kind of stuff as it seems to generate interest and lots of commentary/discussion. In the interest of keeping things interesting around here I thought I'd share how I plumb a barrel on a receiver.

1. Paperwork.

Documentation is a big deal for me. I build every rifle from a sheet that I've conjured up to record all the critical dimensions. This is nothing more than getting some numbers to work with off the action. I verify the thread pitch, measure the height from the receiver face to the bolt, barrel serial number, caliber, twist, the cartridge, etc.









Now its onto setting the barrel up in the turning center. This is the machine I use for all my barrel work. A Doosan Mecatec 10 tool slant bed CNC turning center. I've modified the machine a bit to work for barreling up actions. It's been equipped it with a Dunham 16C collet spindle nose that's able to be indicated just like a four jaw chuck. I like the idea of collets because it offers much more rigidity which means the potential for improved surface finishes. This is the "baby" of the shop.



Time to indicate the barrel and get to work! First we have to set up a special tool I made for doing this. Indicating a barrel is nothing more than getting the hole that the bullet travels down to rotate on the same imaginary center that the spindle of the machine rotates on. We want them to be concentric with one another. I do this a little differently. Since all barrels are "bent" to some degree I decided I wanted to be able to pick the place where I make the bore concentric with the machine. I figure where the bullet is going to be introduced to the lands is the best place. So I built a tool that allows me to do this. It has different adapters for each caliber and pivots on a gimbled base that fits in my reamer holder. Here's the tool being set up for this particular caliber. My method for determining how far to stick this up the barrel is pretty "scientific". I lay the reamer next to it and mark the shaft with a black marker. It's a visual "guess" where the freebore transitions to the throat. Paring them up I mark the stylus where the scribe mark is on the reamer. Then we cram the contraption up the bore like a rectal thermometer.
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Old 06-01-2010, 01:31 PM
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re: How I Barrel A Precision Custom Rifle - Long Rifles, LLC

Here's the tool installed in my reamer holder in preparation of truing things up. The plumb weight is just to ensure the tool follows the bottom of the bore. Nothing here actually rotates, it just bobs up and down as the barrel flops around. Once the bore is concentric it'll quit moving.



Now I dial back the hydraulic pressure being exerted on the chuck so that I can move things around without stripping the heads of the allen set screws.



Barrel is all trued up now after some fiddling with the set screws and careful use of a craftsman deadblow mallet. A few taps makes pretty quick work of all this.

This particular action is new for me so it requires I write a new program for the lathe. I start with a simple CAD model of the tennon that I use to write a program with. This is where the "cheat sheet" comes in handy as all my dimensions are recorded. I write all my lathe code longhand so the model is just a convenient visual to pull dimensions from. With actions that I've used before I apply the same numbers from my "cheat sheet" and just edit values in a master program for that particular brand/type of receiver. Much easier/quicker that way.




Once done I'll load the program into the machine's control via the RS232 cable.

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Old 06-01-2010, 01:33 PM
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re: How I Barrel A Precision Custom Rifle - Long Rifles, LLC

Now it's matter of setting the chuck pressure, the tool work offset, and letting the program do it's thing. I start by contouring the tennon and qualifying the diameter with my 1"-2" Mitutoyo Mic.







Threading to fit the action starts with a generic G92 threading cycle. It gets the thread "close". Close means the bulk of the material is out of the way but not so much as to allow the action to actually thread on. From here it just cycle through additional thread cycles written to only take .001" at a time. Once I get to where the action will go onto the barrel just enough to not stick a thread I skip through the rest of the passes and move onto boring the breech/chamber to get it concentric for the reamer.

(I forgot to take this photo for the actual barrel I'm working on so I'm substituting this one just to illustrate.)



Barrel is now threaded for the action and its time (yet again) to do some inspection. I like to set breech clearances at .015" as it gives enough room for funk without running the risk of rupturing cases and what not. Since the web is still well inside the bore and fully supported I'm not too concerned about a case "sneezing" due to lack of support from the chamber. I like to keep inspection as simple and dumb as I can as they seem to work the best. In this case I leave the fancy math to the school teachers and stick a piece of soft lead solder to the lug face of the bolt. I'll hold it with a dab of grease. From there I'll just insert it in the action and rotate into battery. The camming action squishes the solder and when removed provides a no question dimension. I measure with a pair of calipers and record it. Easy stuff. . .



In this case I was off a bit. My clearance is only .003". No worries as I'll just edit the code and reface the tennon an additional .012" to get the .015" I'm looking for.



Now I'm ready to chamber. My reamer holder never changes position. It stays in the machine and I've checked it enough times to know that it repeats. (It better for what the machine cost). One thing I did forget to mention is the spindle of this machine. When starting one of these jobs on a cold machine I'll let the spindle run in at 1000 rpm for about 15 minutes. The casting that the bearing cassettes run in warm up and pick up about .003" from ice cold. This is an important step for the chambering part since we are using a tool that depends on a reliable center position. If the tools used in turning/threading are above/below the part a bit its not nearly so critical since that can be compensated for with some easy edits in the tool wear offset page. Machine is all warmed up by the time the chambering part comes.
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Old 06-01-2010, 01:33 PM
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re: How I Barrel A Precision Custom Rifle - Long Rifles, LLC

I start by selecting the right pilot size. (I own every pilot size Pacific Tool makes from 17 to 338 caliber) From there it's a matter of establishing a start point, setting the RPM of the spindle, and then hosing the tool down with the home brew cutting fluid I use. A 50/50 mix of Castol Moly D tapping fluid and Marvel Mystery oil. Our chamber today is a 6mm 22-250 Remington Ackley Improved with a .274"ND and .098"FB. Easy stuff...

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Old 06-01-2010, 01:36 PM
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re: How I Barrel A Precision Custom Rifle - Long Rifles, LLC

Chamber is now cut to depth. Again the dumb/simple process is what I resort to. When I get close to my depth I'll pull the tool out of the way, screw the action onto the barrel partially and then drop the bolt on the GO gauge. I'll then screw the action on until the gauge hits the chamber/bolt face. Then just measure the gap between the receiver/shoulder and add that to my last depth. I do however have to compensate a couple thousandths for crush when the barrel gets pulled up tight on the action. I torque all my 1.0625" OD tennons to 125lbs. That nets about .002-.0025 of crush. I'll add an additional .0035" to my final reamer depth which should give me a "GO+.001" chamber depth once I'm all done. It's very, very rare that I have any issues with chamber surface finish so I just use some 320 emery and green scotchbrite to buff the chamber up when I'm done. This ensures I'm not altering the internal dimensions or rolling corners in the shoulder/case body/neck area. The other part of this to pay attention to is surface finish. I personally don't advocate a chamber polished to a mirror finish. they look pretty and guys get all hot/bothered by it, but in my experience its actually hurting things (brass) a little bit. Reason is the flow of material. Brass wants to typically move towards the neck/shoulder over a period of time. The resizing process does it and so does firing. I like to have a sort of cross hatch pattern typical of an engine's cylinder so that the little scratches "bite" the brass and hold it. It's a fine line though as going too coarse means that the brass ends up looking frosty after being fired. I also use a shot of Kroil or WD40 as it keeps the chips/abrasive in suspension and helps prevent it from loading up in the chamber. 1K RPM and some quick strokes back and forth.

Once it's all done it looks like these:

6mm 22-250 Remington Ackley Improved:


300 Remington Ultra Magnum:



Now we move onto the inspection process. Are things the way I want them?
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Old 06-01-2010, 01:36 PM
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re: How I Barrel A Precision Custom Rifle - Long Rifles, LLC

Inspection is a process of checking the total indicated runout (TIR) of the chamber and the barrel tennon. I check the chamber in two locations. Up by the shoulder/body junction and back at the web just in front of the breech. I check the tennon at behind the shoulder where there's no threads. (recoil lug sits here)

I use a Brown and Sharp "best test" indicator with a .00005" resolution.

Lets see how this one came out.

Case web area looks to be .00005" TIR. (I can live with that)



Close up view:



Now onto the body/shoulder junction:

Zeroing the indicator since it moved due to the taper in the case:





Here's our number, .00015". Most chambers are considered "good" if TIR is held to within .001". In this case its about 6 1/2 times inside of that dimension.

Now the tennon. .00015" again is the number.



With everything checked and completed now its onto flipping her around and getting the crown tuned up.
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Old 06-01-2010, 01:37 PM
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re: How I Barrel A Precision Custom Rifle - Long Rifles, LLC

I basically use two different crown styles. A true 11* target crown and a recessed 11* crown. Application drives what gets used. In this case my customer is an Avid BR shooter who likes to use cartridges that aren't "mainstream". I'll be cutting an 11* target crown for this barrel.





Set up is essentially identical to chambering only now I'll just use the indicator to fine tune. no need for the tool since I know where the bullet is going to be when it exits the barrel. I'll chop the barrel to length with a parting tool, use a chamfering cutter held in my hand to nick the burrs off and then go about getting the bore centered up.

From there its touch the tool work offset again, load the right program, and let her rip.



Now that were essentially done all that's left is polishing up the barrel and stickin her into the CNC mill to engrave the caliber and the shop name.

.

Finished!

Here's a sample photo from an older job. I run my engraving in batches of 4-5 so our little 6mm guy will get shelved till I get a few more done.





Thanks for looking. Enjoy your Memorial Day gents.


Chad
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