Pre-boring long chambers.

C

Curious

New member
I am very happy with the results Im getting from pre-boring before reaming. On short cases like PPC and BR I now parallel bore to around 0.020" less than the reamer shoulder diameter and deep enough so the bush engages along half its length before the shoulders start to cut. Im consistently getting runout of 0.0005" or less.

When it comes to longer cases I tend to taper bore so that again the bush makes reasonable contact before and reamer flanks make contact with the undersize chamber mouth and before the throat/neck starts to cut. The problem with this though is you have to get the tapered pre-bore very close to the correct size or the neck starts to cut before the flanks are supported. How much do you think that really matters?

I have been wondering what the best way to do longer cases like 6.5x47 or .308? I was considering only pre-boring to maybe half case depth which would be as short as say a BR and then parallel boring again to 0.020" under. Im thinking that with cases like .308 it would be like doing a BR but just continuing in deeper. I appreciate about half of the chamber depth will then be cut with the full shoulder of the reamer but Im thinking the first half pre-bore will have set everything on a true course.

What are your thoughts on this guys?
 
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I drill out most of the chamber to .100" shorter than the go gage, then straight bore the hole .050" shorter than what was drilled. The start of the chamber will be bored to reamer shoulder diameter for about .050" to start the reamer. The reamer is pushed with a dead center to support the reamer at the rear until most of the chamber is cut. For about the last .2" of the chamber I push the reamer with a floating pusher. That's how I do .338 Lapua's, Nosler magnums, that kind of thing. For most of the chamber the pilot isn't in the bore and is why I push it with the dead center. On my lathe switching to a floating pushes to finish the chamber results in a tighter chamber closer to reamer diameter. It's very easy to make a reamer cut oversize especially if everything isn't absolutely in line. Anything you can do to get it to cut as close to reamer diameter as possible is a good thing. As always standard caveat, if it shoots well and functions properly then you've done a good job, no matter how you got there.
 
Thanks for your reply Mike, some very useful info there, a couple of further related questions if I may. :)

Mike Bryant said:
I drill out most of the chamber to .100" shorter than the go gage, then straight bore the hole .050" shorter than what was drilled.
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When you have straight bored the hole how much under shoulder diameter is it? Does this ever change depending on the caliber?


Mike Bryant said:
The reamer is pushed with a dead center to support the reamer at the rear until most of the chamber is cut. For about the last .2" of the chamber I push the reamer with a floating pusher.
Click to expand...

Have you ever measured the runout at the point before you go to the pusher? does the pusher ever correct/reduce runout that was there over the remaining 0.2" of travel?

Mike Bryant said:
On my lathe switching to a floating pushes to finish the chamber results in a tighter chamber closer to reamer diameter.
Click to expand...

My back centre is well aligned and for the last 20 or so barrels I haven't went with the floating pusher at all and have had runout no greater than 0.0005" just using the dead centre, often less. I have it marked my dead centre so it goes into the tailstock the same each time and I feed with the quill lock and tailstock both torqued to the same settings each time. I mentioned this on a thread a while back and it was questioned, I have no reason to lie and it is what was measured.

Does the method you describe consistently keep you below 0.0005" in all areas?
 
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My tailstock is very well aligned. You can't feel any wiggle of the reamer in the chamber when pushed by the dead center when you pull the dead center out of the reamer and try moving the reamer in the chamber sideways. A few years ago I was chambering two barrels for my 6 PPC benchrest rifle. With one barrel I pushed the whole chamber with the dead center. The other I pushed with the floating pusher. What I found when I was finished with both of them for two different identical rifles. The brass fired in the chamber that was done with the pusher was smaller in diameter than the one done with the dead center. I could chamber fired brass in either chamber from the one that was chambered with the pusher. But, fired brass from the one chambered with the dead center would only fit in the chamber that it was fired in. Meaning that it had cut very slightly oversized from the pusher chambered barrel. A reamer cutting oversize will not necessarily have more runout than one cutting close to size. This actually didn't make any difference as I full length size for my benchrest rifles and brass fired in either chamber would fit in either barrel after full length sizing. But, what it did show me is that the floating pusher was the better way to go with my lathe. My PPC chambers typically have from zero runout to no more than .0002" runout. I chamber the large chambers the same way I do a PPC, just hog out more of the chamber. I bore to about .020" under shoulder diameter no matter what the diameter of the reamer is. The only problem I've seen from doing this is that you wind up doing a lot of cutting of the chamber with the outside leading angle of the shoulder of the reamer instead of spreading out the cutting all across the shoulder. This can cause some uneven wear on the shoulder and may make you have to send the reamer back to have sharpened on a reamer that is used a lot. You just have to watch that with a borescope. For reamers such as a 6 BR that you use a lot, it doesn't hurt to have more than one of the same reamer just so you can finish a chamber if you happen to have a cutting problem. I've been chambering barrels like this since 1991 and have only had one reamer that developed a problem on the shoulder caused by uneven wear on the shoulder of the reamer from preboring of the chamber. So it's not much of a problem. I drill and prebore mainly though because it gives me better chambers with less runout than when I used to ream the whole chamber with the reamer. One thing about chambering, you'll find that there are lots of people doing things differently from one to the other. It doesn't mean they are wrong, just different. It's always nice when you check your runout when you're done and the indicator needle doesn't wiggle. The barrel may not shoot, but at least you feel like you've done your part in giving it your best shot at shooting well.
 
Mike Bryant said:
My tailstock is very well aligned. You can't feel any wiggle of the reamer in the chamber when pushed by the dead center when you pull the dead center out of the reamer and try moving the reamer in the chamber sideways. A few years ago I was chambering two barrels for my 6 PPC benchrest rifle. With one barrel I pushed the whole chamber with the dead center. The other I pushed with the floating pusher. What I found when I was finished with both of them for two different identical rifles. The brass fired in the chamber that was done with the pusher was smaller in diameter than the one done with the dead center. I could chamber fired brass in either chamber from the one that was chambered with the pusher. But, fired brass from the one chambered with the dead center would only fit in the chamber that it was fired in. Meaning that it had cut very slightly oversized from the pusher chambered barrel. A reamer cutting oversize will not necessarily have more runout than one cutting close to size. This actually didn't make any difference as I full length size for my benchrest rifles and brass fired in either chamber would fit in either barrel after full length sizing. But, what it did show me is that the floating pusher was the better way to go with my lathe. My PPC chambers typically have from zero runout to no more than .0002" runout. I chamber the large chambers the same way I do a PPC, just hog out more of the chamber. I bore to about .020" under shoulder diameter no matter what the diameter of the reamer is. The only problem I've seen from doing this is that you wind up doing a lot of cutting of the chamber with the outside leading angle of the shoulder of the reamer instead of spreading out the cutting all across the shoulder. This can cause some uneven wear on the shoulder and may make you have to send the reamer back to have sharpened on a reamer that is used a lot. You just have to watch that with a borescope. For reamers such as a 6 BR that you use a lot, it doesn't hurt to have more than one of the same reamer just so you can finish a chamber if you happen to have a cutting problem. I've been chambering barrels like this since 1991 and have only had one reamer that developed a problem on the shoulder caused by uneven wear on the shoulder of the reamer from preboring of the chamber. So it's not much of a problem. I drill and prebore mainly though because it gives me better chambers with less runout than when I used to ream the whole chamber with the reamer. One thing about chambering, you'll find that there are lots of people doing things differently from one to the other. It doesn't mean they are wrong, just different. It's always nice when you check your runout when you're done and the indicator needle doesn't wiggle. The barrel may not shoot, but at least you feel like you've done your part in giving it your best shot at shooting well.
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Thank you for the detailed reply, much appreciated Mike.

When I started chambering everything I read about pre-boring (which I liked the idea of) seemed to be about the importance of the pusher. I made a pusher and I was never really happy with using it. I didn't like the fact it could move when I knew my tailstock was accurately aligned and my dead centre wouldn't, I can turn parallel over 18" to within 2-3 tenths and it is very close on centre height.

The thing I haven't tried yet is relying on the trued up pre-bore with a start to guide the reamer without the bush first being engaged, I have some barrel stubs I will try this out on for myself. I dont need brass interchangeability between chambers, Im just looking for individual concentricity so if a chamber is minutely oversize its irrelevant so long as everything is concentric to within 0.0005" or less. I too have had more than a couple of chambers with between zero and 0.0002" TIR using just the dead centre. Im not sure the rifles shoot any better than once with 0.0003"-0.0005" but it makes me happy to have measured this and I feel I have done a good job. :)

This discussion has also got me thinking just how important the bush really is if more than half a chamber is to be cut without it?

Do you think its engagement towards the latter part of the chamber really helps to stabilise or bring anything back into line? Without a bush my oil feed goes a little crazy in that it has much less pressure than it does with a bush but I guess it has a greater flow - what are your thoughts on this?
 
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When the pilot isn't in the bore, it seemed better pushing the reamer in with the dead center which gave the reamer support at both the shoulder and the rear of the reamer. With just the pusher, the reamer only has support where it's just starting into the chamber giving one point of support for the reamer and not two points of support. Once the reamer starts cutting on the taper, then it doesn't make much difference how the reamer is pushed at the rear. I switched to the pusher just because it cuts a smaller chamber on my lathe. It probably doesn't make a bit of difference, but when your goal is to cut a chamber as close to reamer diameter as you can, then may as well use whatever method helps you get there. If a chamber is a thousandth oversize from another, it probably doesn't make any difference in how the barrel will shoot. That's just my take on it. Use what works for you, discard what doesn't.
 
Mike Bryant said:
When the pilot isn't in the bore, it seemed better pushing the reamer in with the dead center which gave the reamer support at both the shoulder and the rear of the reamer. With just the pusher, the reamer only has support where it's just starting into the chamber giving one point of support for the reamer and not two points of support. Once the reamer starts cutting on the taper, then it doesn't make much difference how the reamer is pushed at the rear. I switched to the pusher just because it cuts a smaller chamber on my lathe. It probably doesn't make a bit of difference, but when your goal is to cut a chamber as close to reamer diameter as you can, then may as well use whatever method helps you get there. If a chamber is a thousandth oversize from another, it probably doesn't make any difference in how the barrel will shoot. That's just my take on it. Use what works for you, discard what doesn't.
Click to expand...

I tried the pre-bore method you suggested this morning Mike but I pushed all the way with my dead centre, the result was a chamber with 0.0003" TIR at the chamber mouth and 0.0001" in the throat. Thank you very much for taking the time to engage with me on this, it was very useful to me to be able to talk this through with someone who gave me answers rather than more questions.

God bless you :)
 
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