The loupe magnifies the circle created by the cylinder section at the end of the free bore. Say for instance you have a reamer that has the free bore .0005" over nominal bullet diameter. The shadow at the intersection of the bore and free bore appear much larger than that. Example the shadow cast by the same height object at 1PM will be much shorter that at 6PM as the sun is setting. That's the difference between a bore scope and loupe for inspecting chambers. You can actually see misalignment in terms of tenths. Also it's easy to see the surface finish of the throat-lead and tell if you have a problem with the reamer.
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Gunsmithing Help un-dick my chambering method
- Thread starter Rprecision
- Start date
The loupe magnifies the circle created by the cylinder section at the end of the free bore. Say for instance you have a reamer that has the free bore .0005" over nominal bullet diameter. The shadow at the intersection of the bore and free bore appear much larger than that. Example the shadow cast by the same height object at 1PM will be much shorter that at 6PM as the sun is setting. That's the difference between a bore scope and loupe for inspecting chambers. You can actually see misalignment in terms of tenths. Also it's easy to see the surface finish of the throat-lead and tell if you have a problem with the reamer.
I'm a home shop gunsmith, I have had some professional machining education, but don't be fooled this is a hobby for me. I have chambered a total of about 6 guns up until this one all short action chambers. I have had good results. All of my rigs out shoot me by a long stretch, most when I do my part are better than 1/2 moa guns.
Two years ago I started a .300 Win Mag project based on a Stainless Factory 700. I trued the action, and went with a 1 piece PTG bolt that got NP3 treated.
When I chambered the magnum I ended up with some run out in the chamber. Best I could tell it was around .005-007" ish. I was disappointed in how the project turned out, that combined with the lack of components (Still!) put the project in mothball mode. Initial testing was inconclusive as I shot approximately 30 rounds which all printed 1 moa ish. Well now I want to get it back up and running.
If I were to rebarrel the gun, I need to learn what I did wrong the first go around. Let me describe my process. I want to improve my process as much as I can.
1. Warm up lathe for approx 20 minutes
2. Align barrel in lathe with PTG Range Rods zeroing both ends to within .0005" I chamber through the headstock. I use copper penny's between the jaws and barrel and spider.
3. Machine shoulder, threads, counter bore.
4. Cut chamber with PTG Tac Match .300 Finish Reamer. I use the bushing model which I select a bushing to be within .0002" or so.
5. I push the reamer with a floating holder which allows for up to .025" of misalignment.
6. The reamer is held by a modified tap wrench which is square to the above floating holder.
7. I cut the Chamber .025-.050" at a time withdraw the reamer and clean everything. Re-oil and repeat.
8. I cut at around 45-60 rpm
9. I cut this until I get within .025 of finish depth. I will then cut .010 and the final few thousands, making sure to clean really good between the last cuts.
10. Double check headspace
11. Flip it around, crown, thread, Etc
Here's a few shots
Please help trouble shoot my process, methods, etc
I have been thinking it's one thing to do this with a short .308 style cartridge. But for this big of a chamber I could probably drill, true that with a boring bar and then use the finish reamer. Thing is I would have to cut that on a taper to align the reamer and how does the bushing work if it's not in contact until after its cutting the body.
Please be brutally honest, if I'm being a tool here, throw me a bone and school me!
Thanks
Assuming those are 1moa grids on your targets, you look to be getting about .4 to .7 min groups. Considering how far out of concentric your chamber is, I'm pretty impressed. The majority of people who can't shoot well enough or reload well enough to get that kind of performance even out of a perfect chamber!
To be honest, it kinda makes me wonder how concentric a chamber actually needs to be. I'd have never expected a rifle can shoot .4 - .7 groups with a chamber that's .004 out of concentric with the bore.
I got nothing! To be entirely fare in working up a load the lower charge weights were shooting 1.5 MOA, those pictured were the very best.
To your point, I don't know. What I do know is through reloading, through component selection, etc. we work to avoid run out. To go to all that work and stuff it in a rifle that's out of concentric is foolish. This is something nobody really discusses in great depth. For a matter of comparison I chucked up a old savage barrel and checked the same measurements just to get a feel for what the factory spits out. Chamber run out to bore was .0009" (Call it .001") Considering that................
my 'go-to' chambering method is to put the blank between centers, dead in the spindle and live in the tail stock and take a light cut on the chamber and muzzle ends to true them up. I set the compound to a slight taper, a fraction of a degree, and after i pass the point i plan to cut the barrel length to, i taper my concentricity cut into the barrels contour.
Then I put the muzzle end in a 4 jaw, with the chamber end still in a tailstock mounted live center and dial the muzzle end as close to .0000 as I can get, usually about .0002-.0004ish. Then with the chamber end still in the live center I mount up a steady rest and snug it up, then remove the live center.
Then I cut the tenon, counterbore, and chamber in the steady. After chambering, I flip the barrel around and put it between centers again, recheck both ends with an indicator to make sure everything is still good. If it indicates good, and it always does, i put the chamber in the 4 jaw and indicated as close to .0000 as i can get, then snug the steady up on the muzzle end, then remove the live center. Then I finish to length, crown and thread if the job calls for it.
Every barrel i've chambered with this method has shot as good or better than its owner was capable of. Its pretty easy once you get good and dialing in a 4 jaw. I know it really relies heavily on an accurate live center but it works for me.
Groups are a great way to blame a chamber for an almost infinite number of other potential issues.
1. I see vertical in the shot plots. First thing(s) that come to mind:
-can the shooter hold elevation properly?
-did he set the parallax correctly?
-reticle in focus?
-mirage on the barrel?
-is the ammunition holding a good SD? -not really that big an issue at 100 yds for most cartridges.
-guard screws tight?
-mag box bound up?
-jewel trigger? Is it touching the floor metal? -they do that alot.
-all else fails. Dick with your seating depths. It can make a big change.
-recoil. Magnums can beat your ass. Make sure your fundamentals are spot on. anticipating recoil, etc will cause this stuff as well.
Notice that chamber work isn't mentioned here anywhere. I say this only because from the coaching perspective there's more to look at.
Next, the cartridge.
300 Winchester Magnums are generally pretty forgiving, but they can also be a Diva from time to time. A lot of folks subscribe to the practice of jamming bullets into lands cause they see it talked about all the time. Magnum cartridges can often work better by getting a run at the throat. Why?
-Big powder columns.
-Heavy bullets.
-Heavy bullets generally means lots of bearing surface contacting the bore.
All of this can be summarized to mean that the bullet can act "bigger" than it actually is. What I mean is if we look at a 220 Sierra compared to a 220 Berger we can easily see that the Sierra has a great deal more surface bearing contact than the Berger does. This means alot more friction to over come when all hell is breaking loose during the point of ignition.
If you spread out that chaos by even a few milliseconds the gun will often (not always, but most times) respond very well.
Put a school bus against a speed bump. Now back it up 10 feet and get a run at it. We can see what's easier to do.
Now flatten the tires. Bus still weighs the same right? Your going to be blowing gaskets in your ass trying to push it though. Why? More friction -just like the differences between a Sierra and a Berger.
That's all I'm trying to articulate here.
In summary:
Before OP casts himself on the sacrificial alter for a "botched" chamber job, try tinkering with some of this stuff and see if the thing settles down. You might be pleasantly surprised.
Good luck.
C.
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