Dilaing in setbacks

JonathanK

JonathanK

New member
I am wanting to dial in a barrel I have about 700 rounds down...when dialing it in I find the the lands to be .001 or so different from land to land...I was wondering how others dial in barrels that have the lands worn a bit. I usually dial in off the lands but it seems in this case it may be better to dial in off the grooves
Thanks,
Jonathank
 
JonathanK said:
I am wanting to dial in a barrel I have about 700 rounds down...when dialing it in I find the the lands to be .001 or so different from land to land...I was wondering how others dial in barrels that have the lands worn a bit. I usually dial in off the lands but it seems in this case it may be better to dial in off the grooves
Thanks,
Jonathank
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Are the lands different? Or are the grooves different?

Fitch
 
Reach up in there a far as you can and indicate the grooves, not the lands. That's what you should indicate off of anyway, IMO.
 
Wayne Shaw said:
Reach up in there a far as you can and indicate the grooves, not the lands. That's what you should indicate off of anyway, IMO.
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I've read that before but try as I might I can't figure out a reason why it's true. Please explain the physics leading to that conclusion. Seriously, I really don't know why the grooves would be better than the reamed hole that is the top of the lands, but I'm willing to learn.

Thanks
Fitch
 
On new barrels, where the lands and groovers are supposed to be an equal depth, it really does not matter which you indicate..

I set barrels back on a regular basis, so yes I do have experience with this. But I chamber my barrels with the intent that I will do this. I simply set the barrel up exactly the way I did originolly, indicating the chamber dead true. The lands and grooves, howerver, never show that much wear.

In all honesty, if the barrel shows that much difference between the lands and grooves, you might be wasting your time. In barrel, talk, .001 is a lot. How did it get this bad?........jackie
 
frwillia said:
I've read that before but try as I might I can't figure out a reason why it's true. Please explain the physics leading to that conclusion. Seriously, I really don't know why the grooves would be better than the reamed hole that is the top of the lands, but I'm willing to learn.

Thanks
Fitch
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I fire-lapped a couple of factory barels with the NECO kit. Guess where you coud see the most polishing. Yup, the lands. So I say that's where the bullet is riding. Could be wrong . . .
 
If you chamber with a snug guide bushing I suggest indicating the lands. Thats where the bushing rides. If you do not use a snug bushing why bother to order a reamer with a bushing nose or why bother using a bushing at all?

As Jackie said, 0.001" is a bunch. With that much difference in the lands and grooves you are not going to get an accurate chamber/barrel mergance regardless what you indicate!!! You might want to measure again.
 
JerrySharrett said:
If you chamber with a snug guide bushing I suggest indicating the lands. Thats where the bushing rides.
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I was thinking much the same thing. Indicator, range, and Grizzly rods all rely on a bushing that fits in the lands, so does a piloted reamer, or any piloted tool. I thought about that and decided I liked the lands as the reference because they constitute the surface that is most precisely controlled during the manufacturing process - it's gun drilled, then reamed. Reaming is a precision process.

The rifling processes, to the limited extent I understand them, are not as precise as a reamer. If there is chatter in a barrel, it was put there during rifling, not boring and reaming.

If the barrel is lapped, then it will be smoother and more uniform, but I'm not sure how that would turn the bottom of the grooves into the most accurate reference surface.

Finally, I've also fire-lapped one barrel as a desperation measure (it didn't do any good). Looking in it with the borescope after each 5 rounds the lands were where the vast majority of the wear took place, not the bottom of the grooves.

Fitch
 
Definitely 2 schools of thought. If you rely on the piloted tooling, then use the lands. If you indicate, drill and pre-bore, the piloted reamer isn't critical. The groove diameter is the bullet diameter. The groove surface area is much greater than the land surface and will have the biggest influence on the bullet by far. It is THE center of the barrel/bore, so to speak.

I think the reason a fire lapped barrel shows more wear on the lands is because they have much greater pressure on them from embedding into the bullet.

The lands spin the bullet, the groove diameter guides the bullet.
 
Wayne Shaw said:
Definitely 2 schools of thought. If you rely on the piloted tooling, then use the lands. If you indicate, drill and pre-bore, the piloted reamer isn't critical. The groove diameter is the bullet diameter. The groove surface area is much greater than the land surface and will have the biggest influence on the bullet by far. It is THE center of the barrel/bore, so to speak.

I think the reason a fire lapped barrel shows more wear on the lands is because they have much greater pressure on them from embedding into the bullet.

The lands spin the bullet, the groove diameter guides the bullet.
Click to expand...

Hmmm.

I think one vertical side of each land imparts the tangential force to accelerate the bullets rotation - the friction force exerted by the bore surface of the land and groove is always exactly opposite the direction of motion and retards spin. I think normal forces from both the land surface and the groove surface act to guide the bullet. It can't escape pressure from either one. Different rifling designs may influence their relative roles. In my Glock with its little ribs, clearly the groove diameter, to the extend one can call a surface that covers nearly all of the bore a groove, is the one doing the guidence. I need to go look again but I think there is quite a bit of variation in my rifle bores with regard to surface area split between land and groove.

To the extent the two surfaces conflict, the bullet gets massaged, deformed, during it's passage through the bore - hard to say which surface has the most influence - it can't escape pressure from either of them - and it is surrounded by both. The forces are large enough that the bullet responds like stiff putty, and the barrel itself deforms radially though by a much smaller amount, as the bullet makes it's passage. The bullet experiences pressure from the rear and resistance to motion from all the contact surface with the bore, which I think, will cause it to expand to follow the surfaces of both the land and groove. The land and groove surfaces have minor variations, the variations are smaller in good barrels, expensive barrels, which is what one pays for when buying a high end barrel.

I personally don't think it matters which one is indicated on to tell the truth - and it probably matters least in benchrest quality barrels. If they give a significantly different result (more than the allowable tolerance adopted by the one doing the work), return the barrel blank - it has problems. But I was curious to see what the various rational might be and what they are based on.

Thanks
Fitch
 
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Barrel wear and set backs

In most rifling the difference between the land and grove may only be .007-.008. With the smaller calibers it may ony be as much as .004.

700 rounds for a .308 or 6mmBR would not be much but 700 rounds in a .243 or a 22-250 would be considerable.

Once barrel errosion begins it grows at a faster rate with each additional shot.

Nat Lambeth
 
Wayne Shaw said:
Definitely 2 schools of thought. If you rely on the piloted tooling, then use the lands.
If you indicate, drill and pre-bore, the piloted reamer isn't critical.
The lands spin the bullet, the groove diameter guides the bullet.
Click to expand...

Wayne, I have found that to get a really good merging chamber you need to indicate, pre-drill, pre-bore, then use the piloted reamer. Its not an either or, IMO.
 
Jerry,
I have to agree with Wayne. Fitch, I indicate the groove and bore to that dimension. I use a loose fitting bushing in order that it does not influence the reamer path.
Yeah Jerry, I have considered ordering without bushings, but if I have to sell my reamers one day some folks may pay more for the bushing type.
Butch
 
Butch Lambert said:
Jerry,
I have to agree with Wayne. Fitch, I indicate the groove and bore to that dimension. I use a loose fitting bushing in order that it does not influence the reamer path.
Yeah Jerry, I have considered ordering without bushings, but if I have to sell my reamers one day some folks may pay more for the bushing type.
Butch
Click to expand...

If you used a snug bushing and it did influence your reamer what does that tell you?? It tells you the chamber would not be true with the barrel bore to start with!!
 
You're wrong Jerry. I'm boring to the point in the grooves that I indicated. If it wanders beyond that point and the bushing follows it your chamber will be a larger diameter at the base.
Butch
 
Wayne Shaw said:
I think the reason a fire lapped barrel shows more wear on the lands is because they have much greater pressure on them from embedding into the bullet.
Click to expand...
If that is true, and I'm not claiming it isn't, you would expect to see the wear even out toward the muzzle. I can't remember.

From column B -- Suppose you're right about this. "Guide" versus "spin" aren't separate phenomena in my book. It would be fun to see 10 barrels set up on the lands shoot against 10 set up on the groves -- except there are so many variables, even if there did turn out to be a difference, it would probably take over 100 chamberings each way to spot it.
 
After drilling and boring I went in deeper, a little past the throat and remeasured. Its was more like .0001-.0002 runout on the lands with it zeroed off the grooves....I know I have seen runout of .0005 on the lands when zeroed off the grooves, and I remember feeling one land kind of grab the reamer when it came around, this may have been a fluke. I plan on dialing in off the grooves from now on as this number doesnt seem to change and I should be able to establish od easily on setbacks IMO
Thanks for all the help
JonathanK
 
I agree with Jerry. In my opinion as the tooling pilots off the lands, then use the lands as your reference. Also, I feel the lands are a more consistent reference point, especially depending on how the barrel was manufactured. With single point cut rifled barrels would it be easier to have a groove depth error, than say with a button rifled barrel? It really comes down to how well the barrel maker did their job at the end of the day. As gunsmiths we just have to do the best job possible we can with what they supply, or send it back if it's not up to scratch.

If you can't use a bushing on your reamer within 0.0002" of bore size without getting reamer bind/twisting/etc. problems whilst reaming the chamber to full depth, then you have barrel set-up/alignment issues. I don't care what my tooling is worth when i'm dead, i'm just interested in doing the best work possible now!
 
K Smith,
Regardless of the fit of your bushing, what happens as the bore moves? You do know I'm sure that a bushing has a .0002 clearance on the reamer. It also has at least .0002 clearance in the bore as you know. I know in your own mind you are cutting the perfect chamber. There are no perfect chambers because there are no perfect barrels. As Jackie and others have stated, you do the best you can with what you are given.
I do care what my possessions are worth when I die because I want my family to have the extra dollar if it is there. We really ought to be smart enough to quit a little early and not leave them the burden.
Butch
 
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