Chamber Pre-Boring Thoughts

[Jud96]

I am down with Covid and have spent the last several hours reading on chambering methods and I am bored. I am one to try many different things in a pursuit of finding what works best for me. In an effort to better understand and to ask questions I haven't found answers to, I wanted to start yet another chambering thread. However, I have a few specific questions regarding pre-drilling/pre-boring. I don't want this to turn into an argument on how to chamber barrels, I would like my questions answered by those that have been there and done that and a discussion on these particular questions.

My first question is, has anyone ever used a solid carbide endmill to rough bore the chamber? My thoughts were it wouldn't walk like a drill and it should cut a clean concentric hole for the reamer to follow. You could use for instance, a 7/16" endmill to rough bore a 308 chamber and that would leave roughly .008 per side under finished shoulder diameter. This would alleviate having to drill then bore the hole true and bore out the drill tip angle.

My next question, couldn't one simply use a boring bar to bore the chamber .010-0.020 under shoulder diameter and avoid the two step process of drilling and boring?

Next question, I dial the first few inches of the barrel in so the throat and everything behind it is dialed in true. Could you plunge the reamer in until you get full shoulder diameter, then rough drill the bulk of the chamber and then go back with the reamer and the reamer would pickup on it's previous established shoulder?

Final question, when you are rough boring/drilling, how deep do you go? I've heard people that almost go the full depth right up to the shoulder, and I have heard of people only going so deep so that the pilot is never not engaged in the bore.

Thank you! I am really looking forward to this one!

 

[LVLAaron]

used a solid carbide endmill to rough bore the chamber?

No. But how much faith do you have in your tailstock? Set up a barrel stub, run an endmill into it the depth of your case, and then measure the runout. I bet it's not awesome. If it's perfect... try it again... if it's still perfect and you trust it, rock on. But I find a drill easy to use and much faster.

My next question, couldn't one simply use a boring bar to bore the chamber .010-0.020 under shoulder diameter and avoid the two step process of drilling and boring?

Sure. The drill is for speed. You could cut the whole chamber with a small boring bar, and you can clean your entire shop floor with a toothbrush, but it might not be the fastest route to a goal.

Next question, I dial the first few inches of the barrel in so the throat and everything behind it is dialed in true. Could you plunge the reamer in until you get full shoulder diameter, then rough drill the bulk of the chamber and then go back with the reamer and the reamer would pickup on it's previous established shoulder?

Final question, when you are rough boring/drilling, how deep do you go? I've heard people that almost go the full depth right up to the shoulder, and I have heard of people only going so deep so that the pilot is never not engaged in the bore.

I go almost full depth. I understand why people always want the pilot engaged. I find the pilot completely unnecessary if your setup is right. This is an entire debate all by itself. But again, if the setup is right, the pilot shouldn't technically be doing anything anyway. If the pilot is guiding the reamer around, somethin ain't right.

 

[TaperPin]

The principle you’re fighting against is rough boring accuracy is never as good as the light finish pass. If you’re using a fancy cnc lathe milling center and a proper milling head is plunging a solid carbide mill into the spinning barrel, it would still take a second light skim pass. Rough boring a hole with a boring bar is followed by a finish pass. Drilled hole needs a single point skim to straighten it out. Rough boring with a mill would still need a light finish pass.

Hope you get over your Covid soon

 

[LVLAaron]

Next question, I dial the first few inches of the barrel in so the throat and everything behind it is dialed in true. Could you plunge the reamer in until you get full shoulder diameter, then rough drill the bulk of the chamber and then go back with the reamer and the reamer would pickup on it's previous established shoulder?

Do you mean, get the shoulder cut, then go in with a smaller diameter drill, then finish ream? You could... but you'd be switching tools back and forth, which seems annoying... and you've still got an uneven hole to content with... how much of a difference will it make? Try it and tell us, but you'd still be cutting unevenly.... but you do have the shoulder there to help keep it going straight. I get why you thought of it.. but I dont like it.

Drill, prebore, chamber. Occams razor

 

[Walt Krafft]

If you have read some of the posts on this subject, you will notice there are a lot of ways to chamber barrels. None are wrong if the final result is good. Try doing it your way, if it works for you great! If not, try it different next time. I have my way, but try others at times to see if they work better, FOR me.

 

[LVLAaron]

On top of what walt said... Try things. Shot out barrels are a dime a dozen around any group that shoots regular matches. You could try a dozen things on one used F open barrel.

Try it.. measure it.

I always over analyized the bushing size... until I had a buddies tight bore .308 in the lathe and didn't have a bushing... so thought through how the whole thing works... and just skipped it completely.

 

[Jud96]

Thanks guys for the input! Sometimes it takes another mind to point out the obvious faults in an idea. I plan to do some testing with an old reamer and a shot out barrel. I just thought I'd pick you alls brains before wasting time.

@LVLAaron, what is your chambering setup if you don't mind sharing? Sounds like you have a solid system. Also, do you machine away the drill point with the boring to have a square shoulder opposed to the drill tip angle?

 

[jackieschmidt]

As a machinist, I look at things from the standpoint of what you are trying to accomplish working within the parameters of what you have to work with.

in this case, you are wanting to establish a chamber in a barrel where the chamber is true with the first thing the bullet encounters as it leaves the neck of the case. You also want the chamber to be dimensionally correct.

All while cognizant of the fact that the ID of a barrel is not straight with it’s own self.

Always remember, when dealing with a ID that is not straight, all you can do with the truing process is indicate two points. So the trick is to establish a third true point with to predetermined points that you will indicate true with the spindle bearings of the lathe.

The only affective way to accomplish this is with single point boring for the third surface, which will be the chamber.

I favor indicating the muzzle end, and the chamber end. Then rough drill within about 1/16 inch of the final depth. Then, using an appropriate .0001 indicator, you reach in to where the throat will be established and now indicate that spot dead true. You now have two points true, keeping in mind that the pre drilled portion of the chamber needs to be single point bored so it then runs true with the other two points. The reamer will follow this bored hole.

I go so far as to pre bore the chamber on about the same taper as the case. The reamer goes in about 40percent of the way before it cuts.

Here is a picture of the drill and the boring bare I use to rough drill and prebore.
You now have the chamber, the throat, and the muzzle running dead true with one another. You do not concern yourself with everything in between.

Now, notice I did not answer your inquiry about using an end mill because it is not the same as single point tooling.

Also, notice I do not use the “Gordy” method to indicate. My barrels always point dead straight, doing away with all of that clocking stuff that is a result of using an indicating method that results in the muzzle pointing in an unwanted direction.

 

[DShortt]

Interesting subject and perhaps I've been overthinking this so I'd appreciate thoughts from others.

I only chamber my own barrels so it matters not to me how long a process takes. I use one of the few reamer holders that I am aware of that corrects for misalignment in all 3 axis. Not a pusher, a holder. I have an old SouthBend Heavy 10 that's seen a lot of use. It's not terrible but it's worn.

I also use a double spider off the spindle end to chamber through the head stock because there's not much to get ahold of on the tail end on this lathe.

I've always used a piloted finish reamer to cut the entire chamber. I'll never cut very many anyway and I've yet to notice a problem with wear. But again I don't cut many chambers.

So given this situation I've been reluctant to prebore because I'm not convinced that the reamer will follow the bore axis.

Am I missing something? I tend to think given this particular situation this approach makes the most sense. Doesn't mean I'm right.

 

[LVLAaron]

Thanks guys for the input! Sometimes it takes another mind to point out the obvious faults in an idea. I plan to do some testing with an old reamer and a shot out barrel. I just thought I'd pick you alls brains before wasting time.

@LVLAaron, what is your chambering setup if you don't mind sharing? Sounds like you have a solid system. Also, do you machine away the drill point with the boring to have a square shoulder opposed to the drill tip angle?

I leave the drill point.

Nothing special about my setup. I just know my lathe is leveled correctly, tailstock is aligned, and I dial the barrel correctly for the method I use.

 

[LVLAaron]

@DShortt - Give it a shot. If you D&PB - your reamer has an even cutting surface when it enters your bore... it will center itself... Try it on a barrel stub to see if it works with your machine. Bore it out... cut 50 thou... measure... go a little more... measure.

Even if I was only doing my own, I sure wouldn't want to be clearing chips off a reamer all afternoon

 

[LVLAaron]

results in the muzzle pointing in an unwanted direction.

up?

 

[Jud96]

I leave the drill point.

Nothing special about my setup. I just know my lathe is leveled correctly, tailstock is aligned, and I dial the barrel correctly for the method I use.

What are you using to hold the reamer if you're not concerned with a bushing on the reamer?

 

[jackieschmidt]

up?

Think about all the things you have to do to get it up.
Straight is easy.

if you could, go through the steps involved in getting the barrel indicated and the the muzzle pointing up.

 

[DShortt]

@DShortt - Give it a shot. If you D&PB - your reamer has an even cutting surface when it enters your bore... it will center itself... Try it on a barrel stub to see if it works with your machine. Bore it out... cut 50 thou... measure... go a little more... measure.

Even if I was only doing my own, I sure wouldn't want to be clearing chips off a reamer all afternoon

Good point. I don't know why I didn't think about checking it myself but that's absolutely the best way to know if something works for you or not. And yeah, clearing the chips is tedious to say the least.

I appreciate it!

 

[Walt Krafft]

I will thow this out for people to ponder. I ran a wire edm for 25 years in a mold shop. Half my time was spent straightening out reamed holes. There is nothing guaranteed about a reamer following a hole. It can be like playing Russian roulet with a 6 fluted boring bar, which flute will cut this time? One dull edge will push the load to the opposite flute.

 

[LVLAaron]

Think about all the things you have to do to get it up.
Straight is easy.

if you could, go through the steps involved in getting the barrel indicated and the the muzzle pointing up.

* Dial it in the "Gordy way"
* thread tenon

Here are the extra steps
* see where the high spot on the muzzle end is, mark "top" on the chuck
* see how far off your action is from that top mark....
* do some math and recut the shoulder/breech end.

Takes 10 minutes "extra" if I'm also petting my dog.

Jackie, I know you're a BR guy so probably dealing with short(er) barrels. I'm doing stuff that's 10-15 inches longer, 30-32 inches. Everyone likes nice even land engagement, the "gordy" method gets you there.

If you have a 32 inch barrel that's 100 thou out from end to end, it makes sense to take a couple extra minutes and get the chamber aligned to the bore, vs the barrel aligned to the stock.

 

[LVLAaron]

I will thow this out for people to ponder. I ran a wire edm for 25 years in a mold shop. Half my time was spent straightening out reamed holes. There is nothing guaranteed about a reamer following a hole. It can be like playing Russian roulet with a 6 fluted boring bar, which flute will cut this time? One dull edge will push the load to the opposite flute.

The one flute oval chamber wonders from PTG have taken the stage.

 

[LVLAaron]

Good point. I don't know why I didn't think about checking it myself but that's absolutely the best way to know if something works for you or not. And yeah, clearing the chips is tedious to say the least.

I appreciate it!

Try stuff! And if it works, ask yourself why it worked... if it didn't work, ask yourself why not? When you understand why things aren't working... it's a lot easier to get them working the way you want to.

 

[Judd]

@jackieschmidt where do you source those bits?