Conversion Cylinder Technical Question

[rbertalotto]

I bought and installed a pair of Kirst 45LC conversion cylinders into a set of Pietta 1860 Colt Sheriff models.

One concern I have is the chamber dimensions. I can't figure out why there is a step in the chamber for a 45LC. Looks like the chamber for 40S&W or 45ACP or other cartridges that headspace on the mouth. Not a cartridge that headspaces on the rim.

I've attached a drawing that shows the Kirst chamber in comparison to a standard 45LC chamber.

The pistols shoot fine, but there are flyers.  I have another set of 1858 Piettas with the Howell cylinders and they too have this step, but not as pronounced. The 1858 revolvers do not exhibit the flier issue and are quite accurate. I'll measure those cylinders and post later.

Using Cowboy 45 brass, the 45 SLIM bullet at .452" diameter is jumping .405" and then trying to get through the step in the chamber. This simply doesn't sound like a good thing!

I'm thinking these pistols might shoot better if the step was tapered or simply removed all together.

Comments?

[rbertalotto]

When I pulled the Howell cylinder to measure them, I didn't realize the chambers were tapered at the mouths. This makes a lot more sense and might be the answer why the 1858 revolvers with the Howell chambers are so much more accurate with the short Cowboy 45 brass.............(That "unknown taper" could easily be calculated with some simple math)

[Dick Dastardly]

Howdy rbertalotto,

I also use Kirst Konverter cylinders in my 60s.  I find them very accurate with C45Spl ammo.  I can load and shoot EPP-UG, 45Slim, J/P 45, DD-PUK FLAT and PRS bullets very accurately.  I usually don't shoot the heavy 250 grain PRS bullets in this rig, but they do fly well.

My barrels slug at .454".  The chamber mouths are .454".  A perfect match.  I lube size my bullets .454".  Everything fits.

So, since you are using .452" bullets, do you know if they are the right size for your barrel?  Or, in other words, have you slugged your barrels?

It makes a difference.

DD-MDA

[rbertalotto]

Hi Dick,

Yes, I should have added that I slugged the barrel and they are .452+, I have zero leading in the bore.

I just can't be convinced that a bullet can "waddle" down the cylinder, slam into a ridge in the chamber, and then into a forcing cone and retain accuracy.

I'm having Dave Manson make me a reamer to taper that area. I'll do one cylinder and compare it to the other.

We all know that all of this is foolish for CAS. The revolvers as they are are plenty accurate enough for this game. But I'm a NUT about accuracy. "The only interesting rifle is an accurate rifle!"...............And I love screwing around with this stuff!

[rbertalotto]

I slugged and Cerosafe cast my barrels on the two 1860 revolvers. Both were exactly .448 with both Cerosafe and with a soft lead slug.





I also photographed the forcing cone on the barrel. Looks like Pietta did an excellent job on both barrels. The large diameter is .452





I also forced a lead bullet as far as I could into the 1858 revolvers. Unlike the 1860, the barrels are not removable so I wasn't able to slug the whole bore. But for the first 3/4", they both slugged .448 also. Someday I'll slug them from the muzzle just to see if they are tapered or if they have loose spots. But I doubt it as the 1860 barrels felt perfect, end to end.

And here is a picture of the cylinder....

[Driftwood Johnson]

Howdy

Ever since the mid 1870s, just about all revolvers have a 'step' in the chamber.

It has nothing to do with whether the case headspaces on the case mouth or on the rim. It is because other than 22 rimfire ammo, all bullets are sized to fit inside the cartridge case. In the old days, many of the older calibers used what are called 'heeled bullets'. These were the same diameter on the outside as the cartridge case. There was a narrower section, the heel, that fit inside the case and the case was crimped on the heel.

With any caliber where the cartridge headspaces on the case mouth, the 'step' will be cut at a right angle, so the case has a positive stop to seat against. With calibers that head space on the rim, the step is usually cut at a sharp taper, and it is always set a bit ahead of the end of the cartridge, so there is never interference as the case is seated. When the rim seats against the cylinder face, there is always a tiny space. This is done because manufacturing tolerances make it impossible to reliably seat the cartridge in two places.

Here is the SAAMI drawing for 45 Colt cartridges and chambers.



If you bore out your chambers to remove the 'step' you will be allowing hot combustion gasses to escape around the bullet. That is why there has to be a throat. If gasses escape around the bullet, they can soften the sides, and this can cause lead to be deposited in the bore. A tight fitting chamber throat keeps the hot gasses behind the bullet, where they belong.

[rbertalotto]

Yup, I know all that and them some!....

I never intended to "bore out the entire step", but I will have a chamber reamer made to remove the step and introduce a soft angle for the bullet to be guided by.

In the dozens of 45LC cylinders I've now inspected, not one has an abrupt, 90 degree angle for the bullet to go through. They are ALL chambered like the Howell cylinder drawing that I posted above.

If I was shooting only 45LC brass, I'd have no issue as the mouth of the case would be up against this 90 degree shoulder and the bullet wouldn't be "jumping through a hoop" to get to the barrel.

But with the Cowboy45 brass, the bullet is totally unsupported for nearly 1/2" until it then need to fit through the "hoop".

You can easily see "half moons" of lead being shaved off the bullets as they attempt to clear this 90 degree abrupt shoulder.

[Lefty Dude]

If you are the expert on the subject, why did you post the Question ?

[Cuts Crooked]

If you are the expert on the subject, why did you post the Question ?

I reckon that even the greatest "expert" can sometimes learn new things by askin' fer others opinions.

[wildman1]

Never met anyone I couldn't learn somethin from, even if it might be what NOT ta do.  WM

[Hoof Hearted]

I am a bit confused by the data posted above.

I'm sure rbertalotto has done his homework but when he say's that he could not slug the remington barrel fully.....why not just push the slug all the way through from the muzzle end (with cylinder removed) this is how most do it. Also pure lead would be better than wheel weight alloy.

I think .448 would be the bore dimension and not the land (with the greatest dimension being correct).

Think of this as increasing dimensions:

If the bullet is .454 we want a throat of .454 or slightly tighter and a barrel of .454 or slightly tighter. (.454-.453-.452 would be ideal)
I would not think that .452-.454- .448? would be ideal.

Twist rate and bullet weight can do some funny things that we shooters relate to flyers and a prudent reloader (after getting the sizing thing down) should try different lengths (weight) to see if the twist stabilizes them better as well as softer material in reduced pressure loads. Wheel weight is not the best choice for reduced loads  as there is very little obturation.

rbertalotto and I have discussed this issue and slightly changing the "shoulder" which is a byproduct of the Kirst machining operation might be a great idea but I would not make the chamber look like his drawing of the Howell chamber.

I also feel that his picture showing the forcing cones of the 1860 barrels shows that a deeper and more generous forcing cone would help his issue greatly!

Regards, HH