Can this be more than one way to skin a cat?

[Hang Fire]

On my 1st generation black powder frame Colt 1873 SA I have excessive barrel/cylinder face gap. At first when checked, thought it was 0.012”, but on closer examination it proved to be 0.015”.

From what I have read and been told, the basic fix is to remove the barrel, face off barrel at thread diameter and recut one or more threads, reinstall the barrel, if required face off the barrel to correct clearance and re-cone.

As this Colt was mfg. in 1886, I do not want to do any permanent modification as that would lower the collector value.

As I understand the physics, upon firing, the primer extrudes to the rear and pressure immediately increases in the cartridge case. As pressure builds, the cartridge case moves to the rear, reseating the primer and the cartridge case expands, conforms to, and grips the chamber walls. As the bullet passes into the chamber throat, the cylinder moves forward with the bullet, stopping when the cylinder base pin bushing gas ring contacts the forward frame face. When the bullet leaves the chamber throat and enters the barrel forcing cone, a rapid change of pressure forces transpires. The pressure now forces the cylinder rapidly to the rear and the cylinder ratchet comes to bear forcefully upon the recoil shield.


http://i3.photobucket.com/albums/y92/TANSTAAFL-2/P1010329.jpg

http://i3.photobucket.com/albums/y92/TANSTAAFL-2/P1010327.jpg

The barrel/cylinder face gap at this point is now at its maximum limits.

One can buy or make shims for the cylinder pin bushing to reduce cylinder end shake (AKA destructive battering ram) clearance, this sets the cylinder back, but does nothing to remedy the excessive barrel/cylinder face gap.

I am thinking of making a form fitted recoil shield shim of the correct thickness (0.008”) to reduce the barrel/cylinder face gap to 0.007”. As these are modern solid head cartridge cases, I do not think 0.008” will create a problem with BP pressures. As the shim will be form fitted into recoil shield recess, there will be no movement of it and will be further retained by the base pin. Shim stock can be purchased in precise thickness and of tempered steel. Once the shim fits correctly, a transfer punch of the correct diameter through the front base pin hole can mark for the hole location.

The shim would be form fitted cut as per the masking tape example here.

http://i3.photobucket.com/albums/y92/TANSTAAFL-2/P1010331.jpg

I am not an engineer, nor stayed at a Holiday Inn last night, so any input and critique as to my approach would be appreciated.

[John Taylor]

you might check with this place and see if their replacement cylinder is longer than your original.
http://www.usfirearms.com/shop/category.asp
I think you are wrong about when the cylinder is moving. When the primer is struck it forces the cartridge and cylinder forward, primer fires and tries to back out. As soon as pressure starts to build the case expands gripping the chamber wall. As soon as the bullet starts to move the action reaction starts to move the cartridge and cylinder back until the cylinder or the cartridge is stopped by the frame. If the cylinder stops first then the primer will remain out a bit. If the cartridge reaches the frame first the primmer will be push back in most of the time.. When the pressure drops the brass will shrink back about .003" and release the grip on the chamber wall.

[Hang Fire]

you might check with this place and see if their replacement cylinder is longer than your original.
http://www.usfirearms.com/shop/category.asp
I think you are wrong about when the cylinder is moving. When the primer is struck it forces the cartridge and cylinder forward, primer fires and tries to back out. As soon as pressure starts to build the case expands gripping the chamber wall. As soon as the bullet starts to move the action reaction starts to move the cartridge and cylinder back until the cylinder or the cartridge is stopped by the frame. If the cylinder stops first then the primer will remain out a bit. If the cartridge reaches the frame first the primmer will be push back in most of the time.. When the pressure drops the brass will shrink back about .003" and release the grip on the chamber wall.



Thanks for the info source, will check it out,.

I appreciate your comment as to the sequence of cylinder movements, but beg to differ. Until the bullet enters the forcing cone, all pressure is contained and more or less is exerted equally in all directions soley within the cartridge case and chamber.

When fired and pressure builds, case is thrust to the rear while sealing case to the chamber walls, as increasing pressure drives the bullet forward into the chamber throat resistance, it also carries the cylinder forward with it. Once the bullet enters the forcing cone and bore, it and the pressure are no longer contained within the case/chamber, and the pressure now seeking to equalize, drives the cylinder to the rear and the bullet down the bore.

[Bren R.]

When fired and pressure builds, case is thrust to the rear while sealing case to the chamber walls, as increasing pressure drives the bullet forward into the chamber throat resistance, it also carries the cylinder forward with it. Once the bullet enters the forcing cone and bore, it and the pressure are no longer contained within the case/chamber, and the pressure now seeking to equalize, drives the cylinder to the rear and the bullet down the bore.

If that were true, we couldn't have recoil-operated autoloaders.

Physics dictates that as the hammer falls, the spur or firing pin will push forward on the primer, which is set in the case, the rim of which rests against the cylinder (assuming a rimmed cartridge, rimless cartridges headspace on case mouth, etc, etc), pushing them all forward.

Upon ignition, pressure in the case builds, forcing each end of the cartridge in different directions - bullet towards forcing cone, case towards recoil shield and forces the sides of the case to deform, giving a temporary grip against the chamber walls.

If you want to prove that to yourself, heat a .22 uncontained in open air until it goes off... the bullet may travel a few feet forward, but the case will travel backwards at a much higher rate of speed, and land further away. Sticking to the .22 example for a second, the bullet may weigh about 40 grains, the case only 10.6gr, all things being equal, the case will travel around 4x as fast and go 4x as far in reverse as the bullet will forward.

Back to your S/A - even as the cartridge expands and grips the chamber, its pulling the cylinder backward, not forward... and now the force is acting against the combined mass of the cylinder and case together (well, depending on coefficient of friction, etc)...

Cylinder/case stop when one or the other hits something, there will be a slight "rebound", pressure inside the case drops once the bullet enters the barrel (thanks to the cylinder gap venting some pressure), and fully after the bullet leaves the barrel. The brass elasticity allows it to "snap back" a few thou so you can extract it.

It's the very premise by which an recoil-operated semi-auto works.

Bren R.

[Hang Fire]

A true recoil operated semi auto works off a delayed locked bolt, whereas blowback operated semi autos are in effect gas operated.

Which system do you mean?

[Bren R.]

Either way...

A delayed locked bolt (where the bolt is held for a fraction of a second by a physical lock - roller, lugs, etc) or a direct impingement system (like a Savage 1907, where the mass of the bolt and the recoil spring provides the resistance) - the direction of force remains the same, it's the chamber pressure pushing the case backwards that extracts and ejects the spent cartridge... then the recoil spring pushes the bolt back into battery, stripping and chambering the new round.

Easiest to illustrate with a semi-auto pistol - if the pressure upon firing forced the case forward with the bullet, the slide on a semi-auto would want to move forward, not backward.

Bren R.

[Hang Fire]

Apples and oranges.

[Hang Fire]

I will preface this post by stating this is only what I did. And in no way do I imply or recommend the procedure for others to copy or do likewise.

Got a 0.009” recoil shield shim made up and installed, works like a charm, fired 25 rounds and the steel shim is not even marked by the cylinder ratchet. After firing and cleaning, the barrel/cylinder gap is now 0.005”. I had to make up three shims before getting one of correct thickness and fitted where it was held captive in the recoil shield recess with no movement and base pin hole lined up. But was no big deal as they were easy to make.

As the thin shim stock cut so easy with a pair of heavy scissors. Found it best to just sandwich strip of shim stock between two pieces of clamped hardwood and drill for a clean hole. (could have used hand power punch for same) Rough cut the shim stock to size, put it on inserted base pin, then fitted and trimmed with scissors for precise fit in recoil shield recess.

Had to remove three 0.002” cylinder base pin bushing bearings (read as shims, Brownells calls them Endshake Bearings) to obtain 0.002” cylinder end play. The bearing/shims are nice for correcting excessive endshake, (or in my case, removing to obtain cylinder end play) saves having to buy and fit a new cylinder base pin bushing on 1st and 2nd gen Colt 1873 SA or clones that have the removable bushing.

[Bren R.]

I'll agree to disagree.

Glad you got the problem fixed, though!

Bren R.

[Cap'n Morgan]

Just a few thoughts on the physics involved here...

The cylinder is not being pushed backwards by pressure, but the case, being much lighter, is. During recoil the cylinder is forced/pulled backwards along with the gun. When the gun's rearward movement is stopped (by our firm and rock solid grip) the cylinders momentum carries it back against the recoil shield, pressing any protruding primer or case back in place. This is why tapered cartridges like the 22 Jet tends to bind the cylinder; not enough momentum to bang the case back in place.

This is MY theory - Mine alone! And I'm sticking to it: [smilie=1:

http://www.youtube.com/watch?v=cAYDiPizDIs