question about checking for tight spots in bore
today i took a 000 buckshot and ran it down a .338 win mag barrel. i had a brass rod with a tip that sat on the lead ball like a shot glass on a baseball, a set up that would keep the rod off the bore. i oiled the bore and the ball lightly. i tapped the 000 in a few inches and then proceeded to feel my way down the barrel. the first third of the barrel starting at the chamber end was extremely tight. the middle third of the barrel it was alot easier to push the buckshot through and the last third of the barrel very little resistance pushing through. my question is, was this a good test to determine a loose muzzle end of the barrel? a 000 buck measures about .350. thank you for any help with this.
Since lead is "dead", that is, soft with no spring back, once you passed the tight spot, any other spot, even .0001 larger, will feel very loose.
Originally Posted by philipbrousseau
Repeat your test, only from the muzzle end, and see if you detect the tightness toward the chamber.
Where you might have went wrong is you did not state if your pusher rod had a free turning handle. The lead slug must be able to spin with the rifling just like a bullet as it advances or it will "strip" and give false readings.
I have slugged barrels in the past, and it takes real concentration to ascertain just what you are detecting. I just used a good Dewey cleaning rod with a good free turning handle, (some do not turn free enough).
The most consistent barrels I have ever "slugged" were cut rifled barrels, the majority being Kriegers. They guarantee that their barrels are within .0001 if straightness in the groove, and never larger at the muzzle end.
I have found this to be true.
Last edited by jackie schmidt; 03-14-2017 at 10:04 PM.
You are not going to get an accurate reading using a ball since the ball will tumble in the rougher places or sometimes in the smoothe places. To do an accurate "slugging" you must start with a cylindrical shaped lead slug that is very slightly larger than the bore.
Grease the bore evenly with a greased patch, then slug. You will be amazed sometimes as to the variations you can feel with just that slug. Since benchrest barrels are hand lapped, not all the barrel gets the same amount of lapping. I have had barrels where the person doing the lapping was trying to lap out an occlusion. If this happens sometimes the bore will have a place lapped out enough the gas will slide past the bullet causing unnecessary coppering.
thank you both for the reply. a great help this being the first time i have done this. i'm going to continue to play with this barrel and see what i can learn.
If you pass a tight spot you can always use another rod from the opposite end and 'bump up' the lead.
A pair of brass rods does the trick.
Or brass blunt ends that fit on Dewey and other rods are good for bumping back up. The issue here is if the oversized spot is not at the end here is no way to measure the ejected lead slug in its bumped up state. But, bumping up in an oversized area does make you aware of that problem. And it is a problem if you have a swelled out spot somewhere between the ends because it will cause premature coppering.
Originally Posted by brickeyee
Slugging barrels with a lead slug is almost a religion in rimfire barrel inspection.
About 15 years ago, I asked Krieger why they did not not lap a choke into their barrels. (They stress to make them dead straight).
Their explanation was upon ignition, the chamber end is subject to pressures high enough to actually expand the barrel a tenth or two, resulting in th barrel having a "choke" so to speak, in it's dynamic state, but not it's static state. As the pressure dropped as the bullet travelled down the barrel, the bullet encountered the actual barrel size.
At first, that sounded like voo-doo, but pressure test with various strain gages do show measurable expansion of the chamber end at the pressures encountered in modern center fire Rifles.
Anyway, I guess the people at Krieger knew what they were talking about.
Very interesting Jackie and thanks for sharing what was shared to you by the barrel maker in regards to choke.
While I could not disagree with the pressure/expansion deal, I suspect you essentially got a techno babble BS reason.
Originally Posted by jackie schmidt
First, how much could possibly expand? An inch....maybe two. What earthly difference could it possbly make that close to the breach ?
Secondly, and more importantly, it takes man hours to taper lap a barrel and unless it is very consistantly tapered through the barrel, as likely to be detrimental.
At present it would appear that anything beneficial from it seems to be attainable with no more that 1/8"-1/4" of gain twist currently gaining some pretty good traction.
Back when that took place, we had a very lengthy discussion on this very Forum about it. The discussion centered around WHY they didn't lap some choke into the bore.
The pressure expansion thing didn't make complete sense because the pressure is behind the bullet and the expansion due to pressure, while real, followed the bullet.
I always figured that since their barrels shoot quite well, they weren't completely out to lunch. I shot Kriegers then, and still do.
Last edited by jackie schmidt; 03-20-2017 at 05:10 PM.
Not too long ago i placed an order to Shilen for 4 6mm 14:1 barrels. At the time taper bore 22 rim fire barrels were the rage. Doug ask me if I would like to try tapered bores for centerfire at the same price. We agreed on them shipping 2 with straight bores and 1 with 0.0005" taper and 1 with 0.001" taper. just before shipment I got a call from him explaining that never again would they offer tapered bores for the standard price. Turned out that it took considerable more time to lap a known taper since to lap a known taper required lapping, cleaning, measuring,etc. till the desired taper was accheived.
How did the tapered bores shoot compared to straight bores? I never could tell the difference!
I have a friend who makes a practice of casting laps to measure the groove diameters of barrels. He runs the lap up and down the lightly oiled bore, tapping on it with another rod when he needs to expand it. IMO a lot of the discussion on forums is between folks who may not have "surveyed" very many barrels, certainly not every one they work with, as my friend now does. I have learned a lot from his experiences. He has run into issues caused by loose spots as well as choke. I have come to the conclusion that slugging barrels is somewhat akin to having a bore scope. Once you get used to the additional information, why would you go back?
Last edited by Boyd Allen; 03-20-2017 at 09:50 PM.
The best "scope" of a barrel is how small the groups are, or how many X's it will collect.
Originally Posted by Boyd Allen
You buy an name barrel. It does not perform to expectations. After that, you slug it (in this case cast a lap) and the probable cause shows up. After an explanation, the barrel is replaced. Do that a couple of times and you will see the economy of slugging before chambering. He builds custom rifles, and sees a fair number of barrels.
Years ago I drilled out an old 2 cavity bullet mold to where it casts 0.245" slugs. These slugs are cast of pure lead not an alloy metal. These slugs are then greased and then inserted in the breech end of a new blank, that is mounted in a barrel vise and then used to slug the barrel, which has its bore greased. It is important that the slug and the barrel bore be evenly lubricated. Don't try to slug a clean, dry, barrel.
The first small amount of the breech end of a barrel is about 0.002" oversize because of the lapping process. Using the blunt end of a cleaning rod I slowly press, walk actually, the slug through the barrel. You would be amazed at the feeling you get with the rod handle pressed against your tummy while pushing the slug..
I start the slug by "hammering" it lightly into the first inch of the bore with a wooden dowel before pushing the slug with a rod that has a swivel handle like a Dewey, of similar, rod.
Don't try to push the slug holding the pushing rod out in your hand. This gives a jerky uneven feel on the rod handle. With the barrel mounted in a bench top height barrel vise, hold the handle against your stomach.
The barrel must be chucked in the vise very lightly. Even tightening the vise clamp nuts over about 50 in/lb will distort the bore enough to stop the slug.
I slug each barrel 2-3 times before it is ever chucked in the lathe for the machining process.