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Indicating a bore

For those of you who indicate a bore when chambering, whether just at the front or reindicating after drilling and boring, do you indicate the bore diameter or the groove diameter to true the barrel for chambering?
 
From a "leading authority" in the field, and a bit of emergency gunsmithing last week at Trinidad, it's not too unusual to have something amiss. So, question remains, indicate the bore diameter or indicate the groove diameter?
 
If working the chamber dial in the bore. The pilot of the reamer will follow the bore, it has no other choice. On the other end I dial in the bore diameter. I like to add a small chamfer to my crown and this insures that the chamfer will be concentric to the largest diameter and the actual "seal" around the bullet as it exits, or so I believe. However Speedy thinks a crown should be sharp so that imperfections in the groove depths won't have any effect on the bullets exit. I see his point. I think however that sling shooting and the high round count and thorough cleaning regime at the end of the day can be pretty hard on a sharp crown and that it will need to be renewed ocassionally whereas a slight chamfer can be forgotten about. I guess I had noticed the differences in concentricity between bore and groove diameter but never thought that much about it.
 
I give the groove the highest priority over the bore and single point the chamber with a small amount to finnish with the chamber reamer.
 
wapiti25 said:
I give the groove the highest priority over the bore and single point the chamber with a small amount to finnish with the chamber reamer.

Do you use a pilot on your chamber reamer?
 
The pilot ride on top of the lands.
Preacher is right though indicate both to make sure the barrel is a good one.
As to the crown there are several ideas to that one.
Benchrest shooters like it straight across some like it counter bored to protect the muzzels and some with an 11 degrees crown.
They all work. If you brush the barrel with a back and forth motion a 11 degree chamfer is best.
 
I have used reamers both with and without the pilots. You need to remember there is clearence on the bore of the pilot and between the pilot and the land. My therory is that the bullet upsets into the groove and is mostly controlled by by it. I complained to Nosler a few years ago about some of their bullets that were undersize to what I thought they should be (the accepted groove size of the caliber) They said the bullet will upset. I had some older lot bullets that shot better with a larger dia. so I didn't really buy their answer. In my thinking why put the "pressure ring" on a bullet other than to seal the base in the groove and help control/protect the bullet base from blow by. Dr. Mann found that the base was the most important part of the bullet. Cast lead bullets really depend on upset and seal in the groove to shoot well.
 
OK, I'll admit to just fishing a little bit here. I had a problem similar to wapiti at Spirit of America whereby I had some undersize bullets (.3076) and was trying to shoot them in a barrel that was getting a bit long in the tooth. I had planned on replacing the barrel this winter but thought I could get another match out of it. I probably could have if not for the undersize bullets. A friend loaned me some of his ammo the last day after he made sure I was "good and whupped" and his bullets worked fine :) I would be remiss here if I didn't mention all the help received from Speedy and one of his students Nathaniel Skoug at the Trinidad Gunsmithing School who was set up at S.O.A. with a bore scope and giving his services free of charge. Thank you both, great job guys! When Nathaniel scoped my barrel it showed that one of the grooves had some wear and was showing blowby near the crown. Since I had nothing to lose I gave him my barrelled action and he took it back to the shop that night, recessed the crown instead of cutting the barrel off and brought it back to me the next morning ready to begin the next days match. It didn't solve my problem but I thought it was a great effort by young Nathaniel anyway and he is to be commended for it. But the conversation ensued about indicating and that he wanted to cut the square crown instead of the small bevel that I usually put on the muzzle because Speedy was telling them to do that because the groove depths wasn't always equal in barrels. Now, I've done lots of barrels and indicated lots of bores and I'll go ahead and admit that I haven't noticed a lot of difference in groove depth vs. bore depth concentricity. But I wanted to fish around a little bit to see if any of you have. With all the .0001 indicators that are being used to indicate bores somebody must have noticed something, or maybe we should be more observant, I know I will on the next barrel I install, which will of course be my new Palma rifle barrel :)
 
The tools used today, to rifle barrels, come off of CNC tool grinders, whether it be that carbide button (for button rifling) or the single point cutter (for cut rifling). Not that those CNC machines are not capable of making poor products, they are (garbage in - garbage out). But , the tooling , in general, is better made and to tighter tolerances than ever before. With the competition between custom barrel makers being what it is today, they are all looking for the best steel they can get and many make their own cutters/ buttons in shop to get the tools that are as close to perfect as possible. At this point, groove diameters any more than a couple of .0001" (or less!) becomes a moot point. If you've got problems with groove diameters, and you're looking for 'bench rest' accuracy, you might need to change barrel makers. just my 2 cents
 
i have tried both and as in the last post the i have found no advantage . so i do the lands . i believe the depth you can get into the bore makes for a better chamber . i start at around 4" and come back to the ojive and where the neck will finsh . The Rifler
 
I have a rifle that shoots very well that I made the chamber .005" off center at the breech end of the chamber.

I have been dialing in over a length of pin gauge in the bore, to get the chamber parallel with the lathe.

I realize now that dialing in is not that important.
 
Why would you intentionally cut one .005 off center? Dunno!? ??? Installed my new Palma barrel and checked a few things that we have been discussing here in this section of the forum while doing so. On indicating the bore I tried the indirect method described and used by Gordy Gritters using the range rod held in a chuck in the tail stock and I used the tapered rod and bushing I got from JGS. What I can say is that there is no replacement for actually putting the indicator in the bore and taking a direct reading to satisfy myself that my bore is running exactly true. And by the way, the bore diameter and groove diameter were NOT exactly concentric. Did it make a difference in how the barrel shot? No! My guess is that it may have to do with cut rifling tolerances and hand lapping. But we're only talking about one or two tenths which could be the fault of my equipment too. You'd really need lab equipment to determine it for sure.
 
yes i tried a pile of methods for bore ind. and have settled on direct bore contact . i don't like the rod method it seems if you remove the rod you get a different reading .direct to the bore is more reliable. T.R.
 
I use the steady rest method so dont indicate the bore at all.
Instead i use piloted centre drills and form a 60 degree chamfer in the bore.
Firstly i hold the breech in a 4 jaw, indicate the OD true and support the barrel with the steady rest.
Using the piloted centre drill in a floting reamer holder i form the 60 degree chamfer, remove the steady and support the barrel with a live centre.
I then turn a chucking piece concentric to the bore to be removed later.
The muzzle is then held in a collet chuck indicated to zero runout and the same operation repeated on the breach end.
With the breech supported by the live centre, i turn the thread tenon and take a truing cut on the barrel shank concentric to the bore.
While still supported by the live centre i single point cut the thread, again true to the bore.
The steady rest is then re-employed supporting the barrel on the shank checking for concentricity.
The breech counter-bore is then turned and after its completed the chamber is ready for reaming using a suitably sized piloted reamer held in the floating reamer holder.
concentricity checks taken after chambering is complete show a max runout of .0003" usually less when i use this method

Ian.
 
I bought a $35 take off Rem700 7mmRemMag barrel off Ebay, when they sold them.
I was building an expensive 7mmRemMag on a VZ24 action, so I was going to put the Rem barrel on a 1908 Brazilian Mauser at the same time to get tuned up with my new PM1236 lathe.

I dialed in the old Rem barrel on the 4 jaw with respect to the chamber.
I dialed in the spider so the barrel would tip in the 4 jaw gimbal and the chamber would dial in at more than one depth.
I cut off the Rem threads with a parting tool.
I cut Mauser threads.
I put the 7mmRemMag reamer in the breech without de burring the hole.
The reamer started wobbling.
It wobbled for the whole ~ .625" + deep cut until the chamber headpaced with the 1908 receiver.
I measured the run out of the newly cut chamber with respect to the lathe; 0.0050", which is about how much the reamer had been wobbling.
I put the action in and old cut down VZ24 stock and bedded it.
I went to the range and sighted in at 50 yards with 162 gr SST 2950 fps.
I shot one 3 shot group at 100 yards: 0.75"

Had that chamber measured 0.0000" run out, I would have not expected that group.
This calls into question the relationship between dialing in a barrel and group size.
We know there is a mathematical relationship between eccentric ammo and group size, and that has been verified.
But the eccentricity of chambers does not have that same relationship, because of constant rotational orientation.
I do not expect old barrel dialing in habits to change based on facts.
 

Attachments

  • Rem700 barrel on 1908 Braz Mauser 7mmRemMag 162 gr 100yards tiny cropped for 6mmBR.jpg
    Rem700 barrel on 1908 Braz Mauser 7mmRemMag 162 gr 100yards tiny cropped for 6mmBR.jpg
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  • 1908 Mauser Rem700 take off barrel 7mmRemMag.jpg
    1908 Mauser Rem700 take off barrel 7mmRemMag.jpg
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