Question on Pre-boring while chambering

I've been researching before I chamber my first barrel and I have a question about Pre-boring and using a piloted reamer. When people say they prebore or step bore to .010 less than shoulder diameter, I'm assuming when they start reaming, the pilot won't reach the bore. Does it correct any minor deviation once it gets deep enough to start in the bore, or is the theory that the pre-bore quides it in perfectly straight? I don't have a rougher, so I was thinking about Pre-boring only .300 deep at a time, so my pilot was always engaged. My other option is to cut the entire chamber with the finisher.
Thanks, Derrick D

When I was doing it with a floating holder, I'd bore as deep as I could while still allowing the pilot to make purchase in the bore. On the typical cartridges I do, that's about 1.1"-1.2" deep.

Now I'm holding my reamer rigid so I bore as deep as I can reach.

Ksracer said:

I've been researching before I chamber my first barrel and I have a question about Pre-boring and using a piloted reamer. When people say they prebore or step bore to .010 less than shoulder diameter, I'm assuming when they start reaming, the pilot won't reach the bore. Does it correct any minor deviation once it gets deep enough to start in the bore, or is the theory that the pre-bore quides it in perfectly straight? I don't have a rougher, so I was thinking about Pre-boring only .300 deep at a time, so my pilot was always engaged. My other option is to cut the entire chamber with the finisher.
Thanks, Derrick D

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It's pretty common to use a finish reamer to do the whole chamber without pre-boring.

I've also Wondered about pre-boring too. What stops the reamer from wandering before the pilot gets to the bore of the barrel? I see it like a drill bit and with my experience they don't always go straight. And then the tailstock, it has to be perfectly inline with the spindle then, virtually and horizontally. And it can't be moved while in the process of reaming. And also the clearance/play of the tailstock quill.

The idea of preboring as I understand it is to taper bore to fit the taper of the reamer body. The bore job has to be concentric with the barrel bore. That way the reamer is guided by the hole you machined and the pilot can be loose fitting. Obviously you have to calculate depths carefully so the reamer finishes with a full cut everywhere. Done right the pilot bushing reaches inside the bore before the reamer starts cutting.

It's a 260ai that has .010 difference in diameter between shoulder and base. That means the difference between not even starting in the bore, and not getting a full cleanup is .005 per side. I don't think I feel comfortable trying to bore it close enough to get the pilot started. I think I like the idea of boring just deep enough to keep the pilot engaged, even if it means boring twice.

Ksracer said:

It's a 260ai that has .010 difference in diameter between shoulder and base. That means the difference between not even starting in the bore, and not getting a full cleanup is .005 per side. I don't think I feel comfortable trying to bore it close enough to get the pilot started. I think I like the idea of boring just deep enough to keep the pilot engaged, even if it means boring twice.

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If you pre-bore the neck as well before you taper bore the chamber it is entirely possible to make the bush engage with the bore before the reamer starts to cut anywhere, then as the reamer engages it starts to cut pretty much everywhere at the same time . Ive just done a 6.5x47 last week, I pre drilled for the chamber then further drilled the neck another 0.390" using a drill that was 0.020" over reamer neck diameter. I then taper bored until my tool touched the shoulder left by the drill to a diameter that was under the finished size but allowed the reamer to reach in half way along its required length before it tightened up i the taper, this was then enough for the pilot to engage in the bore and away you go.

It takes a little thinking about at first as to exactly how much and where exactly you pre-bore but once you have it worked out and written down its a fairly simple task to work to your drawing, the problem is each cartridge is different so it can take a bit of thinking about and experimenting first. You could of course use some 1" Alu in say 3" lengths to pre-bore then ream dummy chambers until you are happy what your doing is right, 3" of Alu costs much less than a barrel and wont wear your reamer out.

Springer said:

If you pre-bore the neck as well before you taper bore the chamber it is entirely possible to make the bush engage with the bore before the reamer starts to cut anywhere, then as the reamer engages it starts to cut pretty much everywhere at the same time . Ive just done a 6.5x47 last week, I pre drilled for the chamber then further drilled the neck another 0.390" using a drill that was 0.020" over reamer neck diameter. I then taper bored until my tool touched the shoulder left by the drill to a diameter that was under the finished size but allowed the reamer to reach in half way along its required length before it tightened up i the taper, this was then enough for the pilot to engage in the bore and away you go.

It takes a little thinking about at first as to exactly how much and where exactly you pre-bore but once you have it worked out and written down its a fairly simple task to work to your drawing, the problem is each cartridge is different so it can take a bit of thinking about and experimenting first. You could of course use some 1" Alu in say 3" lengths to pre-bore then ream dummy chambers until you are happy what your doing is right, 3" of Alu costs much less than a barrel and wont wear your reamer out.

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How far are you pre-boring and how much farther (if any) are you reaming than your pre-bore?

Thanks,
Andy

I read that a couple times. He's got to be reaming at least .390 past the prebore just to clean up the throat. Not sure how drilling the throat helps keep the pilot engaged either. I decided I'm just going to prebore .010 under shoulder diameter, .6 deep at a time. I just started and it's my first 1, so if it takes awhile, that's OK

Don't worry about boring/reaming/boring in multiple steps. Simply parallel bore .010"-.020" under shoulder diameter to a depth that the pilot still makes purchase in the bore. Probably about 1.1"-1.2" on a .260 size reamer.

300sniper said:

Don't worry about boring/reaming/boring in multiple steps. Simply parallel bore .010"-.020" under shoulder diameter to a depth that the pilot still makes purchase in the bore. Probably about 1.1"-1.2" on a .260 size reamer.

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I've tried pre-boring(on two rifles) and it did not turn out as good as just reaming the whole thing with a finish reamer. The difference was the amount of run-out. .001-.0015 with a pre-bore and less than .0001 to .0002 with just using the finish reamer. Same results when I have set back a chamber and it was re-chambered with the same reamer I chambered it with originally.

I use a floating reamer holder so maybe that is the issue either way I'd like to figure it out.

Andy, where are you measuring the run-out? I prebored and reamed .600 deep last night. It cut incredibly easy. I was surprised that unless I was continuously advancing the tailstock, after a few seconds the reamer handle would almost dangle and spin free. I think I'd better check my work so far. If all looks good, I'm going to prebore to within .200 of the shoulder, or until the pilot won't engage, and finish it.

Ksracer said:

Andy, where are you measuring the run-out? I prebored and reamed .600 deep last night. It cut incredibly easy. I was surprised that unless I was continuously advancing the tailstock, after a few seconds the reamer handle would almost dangle and spin free. I think I'd better check my work so far. If all looks good, I'm going to prebore to within .200 of the shoulder, or until the pilot won't engage, and finish it.

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At the end of the chamber (outside end).

I did verify it was just the chamber by checking the outside of the tennon.

Andy221 said:

How far are you pre-boring and how much farther (if any) are you reaming than your pre-bore?

Thanks,
Andy

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As Ksracer points out I needed to ream just over 0.390" to clean up the neck, this also brings the chamber out to finished size and reaming 0.390" is a lot better for tool life and accuracy in my opinion than poking a finish reamer the full length of the chamber using it as little more than a drill that relies heavily on the pilot bush. My pre-bore goes down to full chamber length and shows zero to 0.0002" run out over the entire chamber length before the reamer gets anywhere near, with a well fitting bush it doesn't give any room for anything to move and my chambers show good concentricity or at least good enough to make me happy.

There are lots of ways to chamber and this is mine, I dont expect everyone to want to do it this way. For me it uses the reamer as a reamer much more than a piloted drill/form cutter and keeps its reliance on the bush for good concentricity to a minimum as there is a large and accurate surface area along the chamber walls to help keep the reamer centralised. With this method its about cutting the chamber after the tenon has been cut to within 0.020" of finished size, once Im happy with the chamber I will clean the tenon up and thread it and I set my headspace by working back on the shoulder and chamber face using a sleeve and depth mic.

Personally I can't see much point in doing it as 300sniper suggests (no offence meant) although Im sure it works well for him. I come from a toolmaking background where reamers were used to finish holes rather than drill them so I tend to use mine that way, I dont like the idea of the reamer self entering on the shoulder when it can do it much better on its tapered flanks.

Accurate rifles are built using lots of different methods so Im not looking for a ruck here over which method is best. At the end of the day when Im behind the trigger I want to feel like I did things as well as I could and as things stand Im happy that my method works well, this gives me great confidence in the rifle and the groups confirm this.

Thanks for clarifying that, Springer. It makes alot of sense, if I wasn't scared to screw the prebore up, I'd give it a shot

If you're not reaming a bunch of chambers, why screw with pre-boring? Just use the finish reamer. All pre-boring or roughing reamers do is save wear on the finish reamer.

I just buy the rougher and finisher for chambers when I know I'll be doing a bunch of them.