Bore straightness, important?

[VA_XTC_Shooter]

I know most barrels have some curvature to the bore, but I was curious what, if any, is detrimental to accuracy. A long sweeping curve ok, it a tight curve bad? I’m thinking of those 6” bore straightness gages. Typically they are about 0.004” smaller than bore diameter.

 

[Alex Wheeler]

Not if chambered right. I have seen zero correlation between accuracy and bore straightness.

 

[boltfluter]

Not if chambered right. I have seen zero correlation between accuracy and bore straightness.

What Alex said. Some will scare you when looking down the barrel bore spinning in a lathe!

Paul

 

[VA_XTC_Shooter]

Thanks.

I am curious if there is a limit to that though. As I mentioned the PTG gages are quite a bit smaller than the bore. If that gets stuck, it seems there would be quite the "bend".

Then again, I guess as long as the bullet starts off in the bore centered, it will follow the roller coaster just fine.

 

[DaveTooley]

What Alex said.
If barrels had to be perfectly straight to perform at a high level or at any level, there would not be a firearms industry.
Consider that the barrel only contacts the bullet the length of the bearing surface. A distance so short that we can't measure the runout difference in that short a distance. Also cup and core bullets are very malleable. They have to be to deform and engrave the rifling.
What I like is the OD being concentric with the bore. Other than finish and uniformity I'm not the least bit concerned about what happens between the chamber and the muzzle.
The government has laser equipment to measure all things internally. They can't get accurate numbers as to diameters or straightness. So why should I even try.
The target tells all.

 

[DaveTooley]

Thanks.

I am curious if there is a limit to that though. As I mentioned the PTG gages are quite a bit smaller than the bore. If that gets stuck, it seems there would be quite the "bend".

Then again, I guess as long as the bullet starts off in the bore centered, it will follow the roller coaster just fine.

What gages are you referring to?
Just read your post again.
There are minor variations in bore diameter in barrels. Much depends on the manufacturing techniques. Cut VS button. Many variables that mostly are just something to talk about.

I'll add I would never ever run a steel gage/bushing down a barrel.

 

[Alex Wheeler]

Thanks.

I am curious if there is a limit to that though. As I mentioned the PTG gages are quite a bit smaller than the bore. If that gets stuck, it seems there would be quite the "bend".

Then again, I guess as long as the bullet starts off in the bore centered, it will follow the roller coaster just fine.

I like to test things and form my own opinions. When I come across an outlier, either and extremely straight bore or an extremely crooked one I always make a point to follow up on how its shooting. I usually wont say anything to the customer so you dont put anything in their head about it. One of the straightest barrels I have seen went to one of my best shooters, multiple record holder. Hes also had one of the worst I have seen. Those two barrels shot very similar. There are many things that do matter, this is just not one of them. In fact a friend who tracks every one of his ppc barrels would choose one with a little bore wander than a perfectly straight one given the choice. Certain chambering methods may give less desirable results with a barrel that has a lot of bore wander though.

 

[VA_XTC_Shooter]

Since the bore straightness gage was developed by the Army for the M16 and to evaluate damaged (bent) barrels, I am wondering if that is all it's for. Not really an applicable test for target guns. But I am curious what the threshold of a bend in a barrel would be before it causes issues. A 6" straightness gage may not be a big deal, but if you had a 1" gage get stuck, I'd think that would be problematic.

 

[VA_XTC_Shooter]

What gages are you referring to?
Just read your post again.
There are minor variations in bore diameter in barrels. Much depends on the manufacturing techniques. Cut VS button. Many variables that mostly are just something to talk about.

I'll add I would never ever run a steel gage/bushing down a barrel.

PTG makes a pile of them

Bore Straightness Gauge - Pacific Tool and Gauge

Bore Straightness Gauge at Pacific Tool and Gauge

pacifictoolandgauge.com

 

[VA_XTC_Shooter]

Also, I think we are talking about slightly different things. A bore that is not perfectly straight, but "wanders" but over short distances, still is fairly straight.
Versus, a barrel where a short section is so not straight, a 6" or shorter rod would get stuck.

 

[DaveTooley]

Also, I think we are talking about slightly different things. A bore that is not perfectly straight, but "wanders" but over short distances, still is fairly straight.
Versus, a barrel where a short section is so not straight, a 6" or shorter rod would get stuck.

The stuck part is my concern. Not in regards to straightness but surface finish. Precision barrel makers go to great lengths to achieve the finish they want as well as uniformity. When stuck shows up you just marred the finish on top of the lands. The area that has the highest bullet contact pressure. You're probably assuming the radius on top of the land closely matches the diameter of the stuck gage. I would venture to say that most of the time it doesn't. Cut barrels are very close but that's not the case in button rifled barrels. So your hardened and ground gage impacted on a small area deforming metal. Again we're talking about the length of the bearing surface on the bullet. The bullet knows no other part of the barrel except where it is. Any angular variations in that distance can't even be measured.

 

[AccelR8]

Since the bore straightness gage was developed by the Army for the M16 and to evaluate damaged (bent) barrels, I am wondering if that is all it's for. Not really an applicable test for target guns. But I am curious what the threshold of a bend in a barrel would be before it causes issues. A 6" straightness gage may not be a big deal, but if you had a 1" gage get stuck, I'd think that would be problematic.

I agree.

I think the bore straightness gauges were originally employed with the M16s because the barrels were getting bent forward of the hand guard/ front sight post and then the rifle would not shoot where the sights were aimed. How they were getting bent might be open to discussion So in other words I think the gauges were used to check for problems caused during use in the field and not by manufacturing.

A couple years ago a guy brought me an A1 rifle built from an all original parts kit on a semi-auto lower. There was not enough adjustment in the sights to get it on paper. It took another barrel barrel to fix it.

 

[Alex Wheeler]

I think we are talking about different things. I have no experiance in barrels that have been actually bent in the way your talking about.

 

[urbanrifleman]

What Alex said.
If barrels had to be perfectly straight to perform at a high level or at any level, there would not be a firearms industry.
Consider that the barrel only contacts the bullet the length of the bearing surface. A distance so short that we can't measure the runout difference in that short a distance. Also cup and core bullets are very malleable. They have to be to deform and engrave the rifling.
What I like is the OD being concentric with the bore. Other than finish and uniformity I'm not the least bit concerned about what happens between the chamber and the muzzle.
The government has laser equipment to measure all things internally. They can't get accurate numbers as to diameters or straightness. So why should I even try.
The target tells all.

Soo.... if a barrel curves... let's say 30 thou over 30 inches... what is the angle of runout?

 

[AccelR8]

I think we are talking about different things. I have no experiance in barrels that have been actually bent in the way your talking about.

Yes, I guess my point was I don't think using a bore straightness gauge on a new barrel is a worthwhile thing to do.

 

[Alex Wheeler]

I would agree.

 

[VA_XTC_Shooter]

Yeah, it seems like it was designed (ARmy) as a maintenance check.
I guess, on a new barrel, you could use one to verify there are no tight spots or burrs?

I think bore straightness pigeon holes this gage, but it could be useful for other purposes.

 

[DaveTooley]

Soo.... if a barrel curves... let's say 30 thou over 30 inches... what is the angle of runout?

How is the 30 thou determined? What are reference points used.

 

[urbanrifleman]

How is the 30 thou determined? What are reference points used.

Well, correct!!!

Say, you took the first six inches and dialed it to zero, and then dialed the other end to see the wiggle. That would be a good representation of total runout.

In a practical way.

 

[jackieschmidt]

I can’t imagine any circumstance where I would stick a hardenned pin down a barrel.

They are what they are. As was mentioned before, some look downright scary when you look through the ID as the barrel is turning in the lathe.

years ago, I thought I could use my machinist skills to ascertain which barrels were good. With the cut rifled barrels I shoot, Kriegers and Bartliens, they hold the ID groove dimensions extremely close, so close it is an exercise in futility trying to figure out if there actually is any.

I buy them, chamber them, and shoot them.