detrimental effects of uneven leade?

[lrgoodger]

I have more than one barrel that will engrave one or two lands on the bullet ahead of the rest when determining optimum bullet seating. I have been unable to determine whether this affects accuracy. I know better than to seat the bullet so that it touches only one or two lands, but when I seat it completely off or positively touching all lands (whichever works best), the barrels seem to shoot fine.

Does anyone know of any testing of barrels like this that leads to a definite conclusion that it is detrimental to accuracy regardless of seating depth?

 

[Dusty Stevens]

Sounds like horrible chambering runout or a real bad barrel

 

[Dusty Stevens]

I suppose it could be runout in the loaded round too

 

[lpreddick]

A borescope will answer that question. Have friend who bought a factory rifle that would not shoot. Borescope the pos and it was easy to see the variation in freebore before each land began. 6 different distances. Engraved a bullet and we could see where each land engraved at different locations. Quality control was not a priority! I would not expect very good accuracy if jumping bullets into such lands, while a good all contact jam might be acceptable. How does it shoot?

 

[Eddie Harren]

"How does it shoot?" This is what matters.

 

['Freak]

He’s talking mostly about factory T/C barrels, but same applies to all.
http://www.bellmtcs.com/store/index.php?cid=156 Chamber Throats, [his] tirade on the subject
http://www.bellmtcs.com/store/index.php?cid=169 Chamber Cast Library
http://www.bellmtcs.com/store/index.php?cid=440 More Chamber Misalignment

 

[Webster]

In Tony Boyers book “The Book of Rifle Accuracy” he covers finding jam and just touch (pgs. 131-134). If your determining jam with a new barrel recheck it after about 30-40 shots because of machining burrs. If you are determining touch, he goes over bullet misalignment or the ejector pushing the round to the side. Only one or two rifling marks does not necessarily mean you have a poorly machined chamber. A little more detail as to interpreting marks in the book. Comments on the marks opposite or next to each other. Midway USA had reasonable shipping charges for the book.

 

[Webster]

He’s talking mostly about factory T/C barrels, but same applies to all.
http://www.bellmtcs.com/store/index.php?cid=156 Chamber Throats, [his] tirade on the subject
http://www.bellmtcs.com/store/index.php?cid=169 Chamber Cast Library
http://www.bellmtcs.com/store/index.php?cid=440 More Chamber Misalignment

Thanks, Great articles. Very nice chamber casting photo's.

 

[JarheadAZ]

I have had several opportunities to inspect my rifle bores with a bore scope. While it is very informative about throat migration, tool marks, chamber misalignment, and fouling conditions; it can often indicate issues in barrels that shoot very well.

In the end, my most important instructional/diagnostic tool is the target.

BTW, the bore scope also demonstrated to me that Outer's Gunslick Bore Cleaning foam literally does get it all out. Whether or not that's such a great thing seems to be a subject for discussion over the past few years.

A tool I use for checking rifling engraving is a simple hook style paint can opener. Leaving the bolt open, the dummy is inserted into the chamber and lightly pushed to engagement. Using the hook to catch the extraction groove, the dummy is gently extracted far enough to remove from the chamber with my fingers.

BTW, I only resize enough of the neck length to ensure the last 1/16" of the neck directly above the shoulder junction is not sized. This may prevent donuts from interfering with bullet seating, and also may serve to better center the cartridge in the neck.

Greg Langelius

 

[lrgoodger]

Dusty,

I forgot to mention that I always use loaded rounds with perfect concentricity when measuring to the lands. You are correct that a load with runout will engrave as described even in an evenly cut chamber.

LP,

A borescope doesn't answer the accuracy question. It merely confirms the uneven leade. Yes, on some of them I can see the difference in distance to the leade with the borescope. That is what prompted the original question, which if you read carefully you will see that I said they still shoot good.

Webster,

I have Tony's book and am well aware of the points you mention. As stated, I always use concentric rounds for measuring to the leade, and the ejectors have been removed from all my bolt guns.

The whole situation goes against the logic of getting the bullet started into the barrel as straight as possible for best accuracy. I'm beginning to think it is not as critical as most believe. I just read this article very carefully.

http://www.geoffrey-kolbe.com/articles/rimfire_accuracy/tuning_a_barrel.htm

This indicates that there is more to the tuning process we all go through to accomplish our good groups than what most think. From all my reading on this site (accurateshooter), I was under the impression that getting ES down so that velocities were consistent and tuning so that bullets exit on a "node" (high or low point on the barrel vibration sine wave) was the only barrel vibration component of the good groups. According to the article, that is only part of the equation.

The article indicates that the direction and rate of change of muzzle position during the vibration cycle is also important. The 'positive compensation' component of the analysis indicates that with a correct rate of change, the best accuracy point on the sine wave will be just ahead of the upper node on a barrel upswing and just behind the lower node on the upswing. If the rate of change is correct, varying velocity is 'positively compensated' by barrel angle so that all the bullets impact closer to the same height. For the slower bullet to hit at the same point as the faster bullet, the muzzle has to come up slightly, so the barrel must be on an upswing to accomplish 'positive compensation'.

 

[Webster]

RE: LRGOODGER

Nice article. I don't shoot in competition. I have an accurate GH rifle. It's hard to apply a lot of the techy info even though I like reading it. Tony Boyer says he makes his decisions based on looking at the target. That's what I do to keep life simple. My 6BR barrel is five years old and it still shoots under 0.400" Good enough for me (2800 rounds). I really liked the article. You need to have a very good rifle and exceptional shooting skills to see small improvements. I always thought of the barrel as the final swagging operation to make a bullet whatever that means? The SuperShoot is near where I live. I have gone to many matches as a viewer. The top shooters must spend at least $20,000 a year for equipment and traveling around the country. Some of the top short range BR guys buy as many as 15-20 barrels a year looking for one that might win a big match. A large % of the competitors have large campers or motor homes. Most of the competitors look like they are over 50 and self employed.

 

[stansrifles]

The chamber in question how many down it when you observed this uneven rifling contact.
Stan

 

[jsthntn247]

My ftr 308 and 223 both seem to get this after 1500 or so rounds and about .030 erosion. I seem to get horizontal dispersion in my groups about the same time I see this start to happen.

 

[lrgoodger]

The chamber in question how many down it when you observed this uneven rifling contact.
Stan

That's the thing. I have seen it come on gradually on several barrels (which still shoot accurately, by the way). That's not a big surprise since one could not expect each land to erode in exactly the same manner. But the barrel that prompted this thread has one land that seems to have a hump on the leade. It makes a mark that is even with the other three, then there is a very small space and another mark behind the first one. This rear mark is always there on this particular land and is the last to disappear as I gradually back the bullet out. It doesn't seem to be affecting accuracy. The gun is shooting 1/4 MOA at 300 yards.

Add: It is a new barrel and has less than 200 rounds on it.

 

[Dusty Stevens]

Let us know if you figure something out

 

[gunsandgunsmithing]

Just something to consider...A 1.0000" peg does not fit into a 1.0000" hole...they are the same...it takes clearance for the two to "fit". Also, there is no such thing as perfectly concentric ammo...everything has tolerances...including our very nice custom actions and barrels, as well as chambering work.


My point is, if the freebore is say, .2435 and the bullet is .2435", you will get a mark behind the lands. The fact that it shows up first on one side, is not soley dependent upon a perfectly aligned throat and concentric ammo. Even IF the ammo were truly perfect, you still have the stacking of tolerances between chamber and brass...and ANY at the rear means it's even more up front. People on these forums often toss around "tenths" (.0000") like everyone should be able to work within them and that everyone even understands just how little that is...neither of which is often the case.


True, if everything were "perfect", the bullet would mark the same all the way around...but remember what I said about the one inch peg not fitting a one inch hole? With some amount of force to overcome the interference, it can be made to go in....but will it leave a mark?

 

[gilmillan1]

I think the issue is moot if the rifle shoots 1/4 moa. I wouldn't worry about it if the rifle is doing what it is supposed to do.