Gunsmithing bore diameter variation?

[Winny94]

Has anyone here with better measuring resources than I have ever measured bore diameter variance from barrel to barrel for a given caliber?
Reason I ask is I have a 7 Sherman, but it's always slower than other very similar setups (there's only 2 reamers for this round, only 1 option for brass, very similar loads, etc).
I also found I get very little copper fouling. Just curious if I have a slightly larger bore diameter.

 

[Praeger]

Slug the bore. By pushing an oversize lead slug through the bore, it will form to the narrowest diameter of the bore.


Alternatively, you can use pin gauges, but those will only give you the bore diameter (land to land) at the muzzle or breach.

 

[RTH1800]

Calling @Frank Green.

 

[DAVETOOLEY]

I would not recommend pushing hardened steel of any nature through the barrel. As soon as size or straightness stops the pin gage damage has probably already been done. Slugging shouldn't be tried by everyone.
I don't know who made your barrels but very seldom do two barrels do exactly the same thing even if made by the same manufacturer at the same time. One of the mysteries of life. We know cut barrels are closer to nominal dimensions than some button barrels.

 

[Praeger]

I would not recommend pushing hardened steel of any nature through the barrel. As soon as size or straightness stops the pin gage damage has probably already been done. Slugging shouldn't be tried by everyone.
I don't know who made your barrels but very seldom do two barrels do exactly the same thing even if made by the same manufacturer at the same time. One of the mysteries of life. We know cut barrels are closer to nominal dimensions than some button barrels.

Hi Dave. I wasn't suggesting pushing a pin gauge through the barrel. Just use it as you would any hole, the first inch of muzzle or breach end.

 

[DAVETOOLEY]

Hi Dave. I wasn't suggesting pushing a pin gauge through the barrel. Just use it as you would any hole, the first inch of muzzle or breach end.

That would work. I do it with pilot bushings when I get curious. Obviously that's only part of the story but it's a start. I know of people that have made adapters for their cleaning rods and push bushings through the barrel. I cringe when I hear that. What could go wrong? I sometimes slug barrels but they are usually factory barrels with an additional process added. We should be able to trust a premium grade barrel.

 

[Wannashootit]

We should be able to trust a premium grade barrel.

Bingo, this is what it comes down to.
These are air gauged, and out-of-tolerance barrels are rejected (or, in the case of Shilen and perhaps others- they're "graded"). No way to measure uniformity without this special equipment. Same as chamber reamers- we can't measure/check them so reputation is everything here.

 

[RTH1800]

I will add that there is no reason I can think of to worry or measure. Proof is in the pudding. Shoot the barrel. The bullet will tell the story.

 

[308pirate]

Bingo, this is what it comes down to.
These are air gauged, and out-of-tolerance barrels are rejected (or, in the case of Shilen and perhaps others- they're "graded"). No way to measure uniformity without this special equipment. Same as chamber reamers- we can't measure/check them so reputation is everything here.

I will add that there is no reason I can think of to worry or measure. Proof is in the pudding. Shoot the barrel. The bullet will tell the story.

Agree with both

These academic questions, by people who have neither the equipment to do these measurements nor the knowledge to do anything with the data, are exasperating because all they end up feeding is useless and sometimes harmful speculation.

 

[308pirate]

Has anyone here with better measuring resources than I have ever measured bore diameter variance from barrel to barrel for a given caliber?
Reason I ask is I have a 7 Sherman, but it's always slower than other very similar setups (there's only 2 reamers for this round, only 1 option for brass, very similar loads, etc).
I also found I get very little copper fouling. Just curious if I have a slightly larger bore diameter.

Why would you assume a larger bore diameter if your barrel fouls very little and is slower than others?

I would expect it be the other way around. A tighter fit between the bore and the bullet would create additional friction. More friction with the same amount of force behind the bullet = slower speed.

 

[RTH1800]

In my opinion this is a risk/rewards scenario. There is potential bore damage with steel pin insertion. Unlike many, I am not opposed to bore scopes. I recently have had an issue with a Bartlien barrel chambered by a highly regarded shop. The chamber and throat are simply not up to standard. It was a rare mistake that can and will be corrected. It was not feeding properly and the bore scope told the tale. They have a use at times.

 

[Winny94]

Why would you assume a larger bore diameter if your barrel fouls very little and is slower than others?

I would expect it be the other way around. A tighter fit between the bore and the bullet would create additional friction. More friction with the same amount of force behind the bullet = slower speed.

Logically (in my mind) a smaller bore would increase pressure = more velocity. A larger bore would be less friction = less copper fouling.

 

[308pirate]

Logically (in my mind) a smaller bore would increase pressure = more velocity.

It's also more friction. Which one dominates? Neither you nor I have the facilities to determine that.

Also, even if you knew why your barrel is slow and fouls more there is nothing you can about the bore's dimensions to fix the problem.

If it's too slow and you don't feel safe pushing up the load, the only alternative is to replace it.

If it fouls too much for you, you can:

• Fire lap it
• Change bullets
• Clean it more often
• Do nothing and clean when accuracy falls off
• Replace the barrel

I make my living in manufacturing. One thing I learned a long time ago is to not bother collecting data that you can't use to take action.

 

[RTH1800]

^^^^^^This:

.308
This is the most logical thing you have posted in a long time!

 

[Winny94]

It's also more friction. Which one dominates? Neither you nor I have the facilities to determine that.

Also, even if you knew why your barrel is slow and fouls more there is nothing you can about the bore's dimensions to fix the problem.

If it's too slow and you don't feel safe pushing up the load, the only alternative is to replace it.

If it fouls too much for you, you can:

• Fire lap it
• Change bullets
• Clean it more often
• Do nothing and clean when accuracy falls off
• Replace the barrel

I make my living in manufacturing. One thing I learned a long time ago is to not bother collecting data that you can't use to take action.

There is less fouling, not more.
I never said there is a problem to fix. Just a variance to identify.

 

[308pirate]

^^^^^^This:

.308
This is the most logical thing you have posted in a long time!

I'm trying

 

[308pirate]

There is less fouling, not more.

Whatever

I never said there is a problem to fix. Just a variance to identify.

I don't waste my time investigating something that isn't a problem. But you do you.

 

[Steel head]

Shilen sorts it’s barrels with an air gauge.
I believe it measures to the ten thousandths with certain limits for both class of barrels.
I’ve had both classes and can’t say I saw a difference.

Really if your buying quality it’s a non issue to me as I expect to to get a good product and so far I have.

 

[DAVETOOLEY]

For a cup and core bullets gilding metal and lead are very mallable. The bullet obturates to fill the bore. That happens in the length of the bearing surface of the bullet as chamber pressure is increasing. From then on the bullet matches the bore and groove profile. It's difficult to see any difference with barrels that measure +/- a few tenths. A .001" undersize on the bore like the Palma shooters used to use you can see differences.

 

[LeftyJason]

As a user of air gaging at work (aerospace) using anything other than air gaging you won't see a difference. I'm not sure the magnification range the manufacturers use but they can get ludicrously tight. Ones I normally use are the 2500:1 magnification. Each tick line is 0.00005"(50 millionths) measuring +/-0.0015 on the face. Others I've used are the 8000:1. 0.00001" (ten millionth) ticks measuring +/-0.0007 (7 tenths) on the face.

We also have some like in the picture. Internet picture. For scale the body is 5" wide.