Constriction in the Barrel Near the Frame?

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Mixed Nuts

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The High Road is a great resource :)

Recently I've been looking into revolver accuracy. I ended up on this kick after I got some serious leading shooting hardcast bullets at low power and folks here at The High Road explained about bore size vs. cylinder throats. Very enlightening.

Reading around the web, I've seen some people suggest that some revolvers have a constriction in the barrel near the frame. A constriction caused by stress as the barrel is screwed or fitted tightly into the frame.

The idea seems plausible. And the solution is said to be fire lapping with cast bullets treated with abrasive.

MY QUESTION... is, how can anyone know their revolver has this issue? How can it possibly be measured? Slugging would just leave a slug that measures one size - the size of the constriction.
 
Good question.

Pin gauges is one way, but expensive. I prefer to mark the slug as it enters the bore, recover it on exit, lube it, and restart it in the same orientation in the bore. On the second trip through, since force is much lower, you can feel the constriction if there is one.
 
Mixed Nuts said:
The idea seems plausible. And the solution is said to be fire lapping with cast bullets treated with abrasive.
Click to expand...

This solution is "said" by fools, in my opinion.

The Solution is to send it back to the OEM to have the barrel replaced. It's a manufacturing flaw, their mistake, so in my experience, at least with Ruger and S&W, they'll take care of it.

In lieu of sending back for factory repair, HAND lapping is the appropriate course, not fire lapping. Fire lapping will open up the constriction, but it is non-discriminatory, so the bullets will round the edges of the rifling, and will open the entirety of the bore, not just the trouble spot. Hand lapping can be done in a far more targeted manner - but it's a hell of a lot more labor intensive and most guys aren't set up to do it, so they take the easy road and fire lap instead. It absolutely makes me cringe whenever I see fire lapping recommended online. You see a lot of custom barrel makers hand lapping, you don't see them fire lapping, for good reason. Consider what the lapping bullet is doing to your cylinder throats, forcing cone, and the rest of your bore - NOT just the constriction.

Slugging the bore with a dead soft slug is the method to determine whether you have a constriction or not. Pin gauges, or more appropriately, a tight fitting range rod, will be able to feel the constrictions on really bad examples, but the dead soft lead slug is the easiest means. You'll feel it. If you don't feel it in the first pass (usually means your slug ball is too hard or too big), as @edwardware mentioned, mark the slug and start it back in the same grooves for a second pass - you'll have a pretty regular resistance through the length of the barrel, then the ball will stop at the constriction. If you're well equipped and paying close attention, you can even feel when the barrel stamping presses through into the bore,, or the dimple where the front sight screw hole is drilled, as the slug will offer slight resistance at that point of the barrel, moreso than other sections.

If you're willing to remove your barrel, or use a tethered slug, you can run the ball part way through, say all the way up to the frame, but not cross the boundary, then drive it back out the way it came or PULL it back out the way it went in. Compare against a ball which is pushed all the way through. If a guy is creative and uses multiple dowels as the drive rod (cylinder window length), you can drive it from the rear too, but I've always found tethering is just as easy.
 
Edwardware, thanks. Your slug method makes a lot if sense. I guess, with the pin gauges, you just find the snug fit at the muzzle and then see if it will move beyond the forcing cone at the frame.
 
Varminterror said:
This solution is "said" by fools, in my opinion.

The Solution is to send it back to the OEM to have the barrel replaced. It's a manufacturing flaw, their mistake, so in my experience, at least with Ruger and S&W, they'll take care of it.

In lieu of sending back for factory repair, HAND lapping is the appropriate course, not fire lapping. Fire lapping will open up the constriction, but it is non-discriminatory, so the bullets will round the edges of the rifling, and will open the entirety of the bore, not just the trouble spot. Hand lapping can be done in a far more targeted manner - but it's a hell of a lot more labor intensive and most guys aren't set up to do it, so they take the easy road and fire lap instead. It absolutely makes me cringe whenever I see fire lapping recommended online. You see a lot of custom barrel makers hand lapping, you don't see them fire lapping, for good reason. Consider what the lapping bullet is doing to your cylinder throats, forcing cone, and the rest of your bore - NOT just the constriction.

Slugging the bore with a dead soft slug is the method to determine whether you have a constriction or not. Pin gauges, or more appropriately, a tight fitting range rod, will be able to feel the constrictions on really bad examples, but the dead soft lead slug is the easiest means. You'll feel it. If you don't feel it in the first pass (usually means your slug ball is too hard or too big), as @edwardware mentioned, mark the slug and start it back in the same grooves for a second pass - you'll have a pretty regular resistance through the length of the barrel, then the ball will stop at the constriction. If you're well equipped and paying close attention, you can even feel when the barrel stamping presses through into the bore,, or the dimple where the front sight screw hole is drilled, as the slug will offer slight resistance at that point of the barrel, moreso than other sections.

If you're willing to remove your barrel, or use a tethered slug, you can run the ball part way through, say all the way up to the frame, but not cross the boundary, then drive it back out the way it came or PULL it back out the way it went in. Compare against a ball which is pushed all the way through. If a guy is creative and uses multiple dowels as the drive rod (cylinder window length), you can drive it from the rear too, but I've always found tethering is just as easy.
Click to expand...

Thank you for this very generous and informative post. I wouldn't try to remove the barrel but how would one get a tethered slug? A shop built thing? A slug cast around a nut tied to a fishing line or suchlike?

Now off to look up hand lapping. :)
 
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Varminterror said:
Consider what the lapping bullet is doing to your cylinder throats, forcing cone, and the rest of your bore - NOT just the constriction.
Click to expand...

For what it's worth, I've firelapped several revolvers with positive results, and downright miraculous results on a recent production Blackhawk 45 Colt.

Beartooth's online articles are quite good; start there and read up.
 
Mixed Nuts said:
MY QUESTION... is, how can anyone know their revolver has this issue? How can it possibly be measured?
Click to expand...

First, since one rarely hears of this occurrence, it must be just that....rare. Other than by using the methods described above, about the only way I could see to see to discover the problem would be by consistent inaccuracy and excessive leading in the forward part of the barrel, both, when all other solutions have been exhausted. It would seem that unless one encountered the two above problems, that like a bulged barrel, it might not even be an issue.
 
Howdy

First of all, I am no expert on this, but I believe tight bores in S&W revolvers are a result of the crush fit barrel threads they have been using most recently. In other words, an older pinned barrel will not have crush fit threads, the pin keeps it from backing out, so the bore will not have been squeezed down. At least that is what I think. Could be wrong

I have slugged lots of barrels. I have never heard of a tethered slug, but it sounds like a good idea. But I have slugged many old rifle barrels, mostly Winchesters and mostly from the muzzle end. Often times old rifle barrels, particularly from the Black Powder era, will have more erosion in the bore near the chamber. I always know because as I drive the slug down the barrel the going gets easier when I get close to the chamber. Instead of the slug creeping along with every hammer blow, it slides very easily. Slugging a bore the conventional way the slug will of course size itself to the narrowest diameter in the bore. So as my slug reaches the eroded part of the bore, it has already been sized down the most by the rest of the bore. So I don't get an accurate measurement of the eroded section. I don't really care exactly how much erosion there is, in my case as long as the old barrel still puts a good spin on the bullet (they usually do) I'm happy. But the slug sliding through near the chamber tells me there is some erosion there.
 
Almost every barrel has a restriction somewhere in it... unless it's just a plain, free-floated barrel. Dovetails cut into the barrel, for sights and such, are one of the biggest things.
 
Recently I've been looking into revolver accuracy. I ended up on this kick after I got some serious leading shooting hardcast bullets at low power and folks here at The High Road explained about bore size vs. cylinder throats. Very enlightening.
Click to expand...

If you are having accuracy issues with hard cast bullets at low velocity you may want to try softer lead and see what the results are. I shoot hard cast but only at higher speeds. The hard cast may not slug up to fit your bore at lower pressures. I have always gotten better accuracy with softer lead bullets than super hard lead like the Laser Cast slugs. This may save you a lot of work looking for tight spots in your barrel that may not exist. Also have a look at castboolits.com and do some reading there on bullet hardness. Its a good site with lots of help.

I had a S&W model 629 that had the newer EDM rifling or whatever its called and it did great with jacketed bullets but not so hot with lead. Plus the throats were a very tight .429 and I suspect just a little smaller than the bore. I didn't have it too long before I sold it. If I had of kept it I would have had he throats reamed to .430.
 
Mixed Nuts said:
how would one get a tethered slug?
Click to expand...

Steel cable, cast the ball around the cable. The smith from which I learned it drilled a hole in a ball, ran the cable through, and put a stopper on the back, but I prefer casting to eliminate some of the potential issues (ball crushing/deforming down around the cable, stopper pulling through the ball or cable out of stopper, stopper dragging against the barrel, etc.
 
Mixed Nuts said:
The High Road is a great resource :)

Recently I've been looking into revolver accuracy. I ended up on this kick after I got some serious leading shooting hardcast bullets at low power and folks here at The High Road explained about bore size vs. cylinder throats. Very enlightening.

Reading around the web, I've seen some people suggest that some revolvers have a constriction in the barrel near the frame. A constriction caused by stress as the barrel is screwed or fitted tightly into the frame.

The idea seems plausible. And the solution is said to be fire lapping with cast bullets treated with abrasive.

MY QUESTION... is, how can anyone know their revolver has this issue? How can it possibly be measured? Slugging would just leave a slug that measures one size - the size of the constriction.
Click to expand...


Veral Smith at Lead Bullet Technologies (www.lbtmolds.com/) sells caliber-specific lead Push-Thru Slugs. They make it very easy to detect a barrel constriction and I've found them very useful in determining when to stop firelapping.
 
I understand how lapping works but I'm not sure I understand how firelapping works.

From the instant the bullet gets underway til the time it exits the barrel, it's lapping...everything it touches.

So how does it remove just the tight part of the bore?
 
If you think about it... using an abrasive lapping covered bullet... the greatest point of abrasion would be at the point of constriction, once the bullet made it past the constriction and was 'squashed' down, the lapping effect would be less (but not completely eliminated!) as it continued down the barrel. That's why hand lapping is a better choice... because you can isolate where the lapping occurs.
 
Buck460XVR, I have no reason to believe my revolver has this problem. Still, it's good to know how to tune things a little or even just to know what might be wrong.

DriftwoodJohnson, thanks for the input. Posts like your make me wonder if I should be taking notes. So many insights on these boards.

Charlie98, this is an interesting remark. Funny to think of steel as being so malleable but I guess it is.

Ratshooter, will try BNH12s from missouri. I don't see any other cast bullet company offering soft bullets. A few sell BNH 15, but 18 is pretty common. I'd love to find somebody casting the lyman mold bullets at 12 or even 9.

Varminterror, yes, of course steel cable. :) My fishing line idea would last until the line met the liquid lead. Ha!

K2rzo, I'm going to make a note of this. Locating the right fishing weights was a pain.

cp1969, I'm still researching this myself.

Charlie98, I think I get this. Gonna do some more reading tonight.
 
Charlie98 said:
once the bullet made it past the constriction and was 'squashed' down
Click to expand...

The bullets typically used are soft lead, so they'll obturate to fit the bore. If they DON'T, then your gas seal is compromised, and gas jetting and scorching of the bore would be an issue.

After borescoping bores before and after, the non-selective method of fire lapping lost my favor.
 
M N;

First, it's very easy to determin if your weapon has the issue you describe. Using a ram rod with proper calibre "spear tip jag" and a tight patch, starting at the muzzle, slowly move toward the forcing cone. You will be able to feel the increased resistance if there is a tight spot anywhere in the barrel. (barrel MUST be clean to do this)

Second, I have run into a tight spot where the barrel threads into the frame on several single action Rugers. In each case the fix was too simple. I simple fired between 25 and 50 rounds of factory jacketed .357 Mag. ammo through the weapon. Presto, no more tight spot. Will this work on your weapon, I don't know. Maybe I just got lucky, several times. What I can tell you is that after 30 years as a LEO armorer I've learned that sometimes the best solution is the simplest.

Good luck
 
When I got my 6" GP-100 way back about 1990 it would lead the barrel. There was no internet to ask questions on. So like the above poster said I just shot it and cleaned it. After a while it stopped leading. And now I can shoot lead all day with just a frosting in the barrel. That is supposed to be there per Ross Seyfried. And that was with my home cast bullets from wheel weights droppen in water. I have no idea of the hardness of them. But they shoot well enough I won a shelf full of trophies with them.
 
rdstrain49 said:
Second, I have run into a tight spot where the barrel threads into the frame on several single action Rugers. In each case the fix was too simple. I simple fired between 25 and 50 rounds of factory jacketed .357 Mag. ammo through the weapon. Presto, no more tight spot.
Click to expand...

Been about 10,000 rounds of 44mag running 300grn XTP's and over book loads of H110 through one of my SBH's. It still has a crush constriction in the bore.
 
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