Shortening revolver cylinder, no chamber throats?

We all know that using a short cartridge in a cylinder meant for a longer version of the same cartridge (like .38 Special in .357 Magnum cylinder) is no problem. In an ideal situation where "fit is king" and accuracy gains are vital for a competition gun, we would like to eliminate the freeflight where the bullet is unsupported before it reaches the chamber throat and steers toward the forcing cone.

Another part is that in order to achieve a great trigger job, the lighter the cylinder is the less weight there is to rotate in the double action pull. This is also true for a speed shooting gun where you want a light cylinder so that it can rotate faster, hence the titanium cylinder on new guns like S&W 986 and 929.

I've got a S&W 625 in .45 ACP. I've been using it with .45 GAP with great success. The advantage is a shorter cartridge that is easier to do fast reloads with in speed shooting competitions. The disadvantage is the long free flight of the bullet. With the new revolver rules coming up in IPSC competition I'm thinking about fitting a new barrel and shortening the cylinder. The longer internal barrel length would increase velocity, making me able to reduce the charge somewhat for lower recoil with the same velocity output as with the old barrel.

So to the main question. How would the bullet behave in terms of internal ballistics without a chamber throat?
I know there are methods to sleeve the chambers for a new throat, but I would rather avoid it due to costs and pain in time and labor. Also, a throat would probably mean the cylinder would have to be longer than in my sketch below.


(click on it to view larger version)
Attachment 116397

You would definitely want a throat. That much unsupported freebore will do nothing but cause problems. Also, the barrel coming through the frame window would be VERY thin between the frame and cylinder. How are you going to add support for the thin barrel walls? Better off with a titanium full length cylinder, WITH proper throats..

DougGuy: Thank you for your response.
How is the barrel thinner in the window? It has that exact thickness today inside the window, just not as long.
I have no means of manufacturing a new titanium cylinder, I will be using the old factory cylinder.

With a full length cylinder there is a freebore before the throats align the bullet and send it to the forcing cone for more alignment. How is that different from no freebore and direct-to-forcing cone?

Take these guns for example. How was this achieved?

http://www.americanrifleman.org/wp-c...sofSteel_2.jpg
http://www.pinnacle-guns.com/images/...l/DSC02098.JPG

I doubt these cylinders are sleeved or custom made, but instead just shortened factory cylinders.

It would seem to me that you would want a little throat to transition the bullet from the cylinder to the barrel. If you have freebore going straight into the forcing cone, there's going to be a point where the bullet nose is in the forcing cone but the bullet base is still in the cylinder with excessive clearance around the bullet. That will result in a tremendous amount of lost pressure and a lot of blow-by. I think the bullet base needs to seal the end of the cylinder while the front of the bullet is beginning its journey into the rifling.
Just thinking out loud here, I don't really know....

DougGuy: I noticed now that you wrote "that much unsupported freebore". You did see that the upper picture with the long freebore is "before" (as in today) and that the lower sketch in the picture has no freebore (but also no throat)?

You will need to keep the loads light to keep from.bulging or splitting the long, unsupported barrel extension.

Loads will exclusively be 230gn RN at 760 fps. The longer internal barrel will mean lower powder charge than normal to achieve that velocity, probably around 3.2gn N310 in .45 GAP.

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Originally Posted by Whistler

DougGuy: I noticed now that you wrote "that much unsupported freebore". You did see that the upper picture with the long freebore is "before" (as in today) and that the lower sketch in the picture has no freebore (but also no throat)?

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I was looking at the bottom picture. You will still need a throat no matter how much travel the boolit makes between the case mouth and the forcing cone. You can't leave it the same diameter as the outside of the shell casing all the way through to the forcing cone.

The upper picture appears to have a throat but in the case of shooting .45 GAP in a .45 Colt cylinder there is freebore which would be very detrimental to accuracy as the boolit can bounce freely until it slams into the throat. You need a .4515" throat if you are shooting .451" or a .4525" throat if you are shooting .452" boolits. I am sure anyone who would put that much work into a special cylinder/barrel just for this style shooting will design a proper throat into the cylinder.

For your use, wouldn't you need a .44 cylinder to properly ream a tapered throat at the end of the .45 GAP case mouth? .45 Colt would have no throat at all, .45 ACP would have the sharp shoulder where the case mouth would headspace, this would not work well although if it is in the right position from the breech face you could use a reamer to cut it into a chamfer and maybe a .45 ACP cylinder would work for you.

Edit: I had a Blackhawk convertible once, .45 caliber, it had .456" throats when I got it. It was a TERRIBLE shooter because of the throat diameters. I sent it back to Ruger and courteously asked them for a pair of cylinders with .4515" throats which they did exactly my request and upon it's return, groups were less than half the size of groups that it shot with the .456" throats. Throat diameter, AND how even the throats are, is critical to accuracy. You would not be happy at all with a shortened .45 Colt cylinder that would have .481"+/- diameter freebore, that much I can promise you.

What do you expect pressures to be in your desired loading? Standard .45 ACP pressure or less? I am just looking at how thin the barrel has to be to go through the frame, most .45 barrels are very thin where they come through the frame, but they have the thickness of the frame supporting the thin threaded area. Once you get through that, what is going to support it?

My .45 GAP loads hold about 1.5" groups at 25 meters in the .45 ACP cylinder. They headspace on the moonclip, not on the case mouth.
My .38 Short Colt loads hold about 2.5" groups at 25 meters in a .357 Magnum cylinder. The .38 Short colt is less than half the length of the cylinder and very much like the proportions shown in the upper sketch.

I've written the gunsmith that made the revolver shown with red background, but have yet to receive an answer about how he did it.

I think you would need a .41 or .357 cylinder shortened, and then ream the charge holes to .45 caliber size, with a .45 Colt style reamer that has the chamfer on it instead of the sharp shoulder like the .45 ACP has. I can only imagine the shorter .45 GAP being fired in a .45 ACP cylinder and the sharp shoulder scraping off the sides of the boolit. You would need a chamber that does not do this. It needs to be a tapered chamfer exactly as the .45 Colt would have, with the correct diameter throat in front of the chamfer. This would be your most accurate and consistent cylinder dimension to use.

Edit: A .44 cylinder will not work for this, as I can fit a .4525" pin gage all the way to the chamfer from the loading gate, the .44 cylinder is already too big to ream a .4525" throat into. It would have to be a smaller caliber to start with.

I know of two ways to make this work reasonably well,just as suggestions. When working with revolvers bored straight through and no chamber throats, I have found that either using heeled boolits in normal length cases or using a case that goes all the way to the end of the cylinder with the boolit seated below flush with the case mouth both worked quite well to get revolvers like that to shoot accurately. I don't know that it applies here, especially if you want to shoot 45 GAP ammo, but it may lead to something useful...

-Nobade

Actually, revolver cylinders aren't bored straight through at all, they are tapered so that you can eject empty brass easier. If they were totally straight, you would have to beat the brass out the full length of the case. Rimmed pistol cartridges are straight walled, but their chambers taper.

SAAMI spec for a .45 Colt calls for the chamber to be .4862" in diameter, .200" from the breech face, and .480" in diameter 1.249" from the breech face, so a shortened .45 Colt cylinder would be close to .485" at the length that would coincide with the mouth of the GAP brass.
The O.D. of the GAP chamber .200" from the breech face is .4793" and at the case mouth is .4744" which leaves a LOT of slop where there is supposed to be a tight chamber supporting the case. The standard pressure for .45 GAP is 23,000psi, so having a chamber that is within spec for the cartridge it was intended for would be highly advised at these pressures.

I don't think this is a conversion well suited to the .45 Colt cylinder, a .45 ACP cylinder, or a .44 Special cylinder or a .44 Magnum cylinder. I think they are already too big. You may be better off sourcing a smaller cylinder as I mentioned earlier, and getting a custom made reamer to cut chambers for it.

What I have trouble understanding is this: I have acceptable accuracy today, where the bullet travels a full chamber diameter free flight of about the full length of the bullet. It then enters the chamber throat, and then enters the forcing cone. How can I get worse accuracy when removing all that freebore and going straight to the forcing cone?

See if you can find accuracy reports on the 1917 Colt in .45 ACP vs. the 1917 S&W. I have read that at least some of the Colts had cylinders bored straight through meaning that you had to use half moon clips; while the S&W had proper chambers so the ACP round would headspace and fire without clips ( at the cost of individually poking the empties out.

I think Dougguy has it right, you need a new cylinder with a .45 GAP chamber reamer, even though you plan on using full moon clips.

You are going to spend enough money on this conversion, why worry about a few hundred more dollars (or your local equivalent). I would hate to build a gun and then not be satisfied with the accuracy, and think "I should have done ...."

Robert

He needs a custom reamer made with .45 GAP specs except for the shoulder that the case would headspace on. He is headspacing off moon clips so his chamber needs a chamfer at the case mouth like a .45 Colt would have. I suppose he could use the standard .45 GAP chamber, just there will be buildup in this area if there isn't a case mouth snug up to the shoulder, which soon enough would prevent chambering of subsequent rounds until it is cleaned out.

As primitive as it is shooting a short round in a longer chamber, accuracy would still deteriorate if you took out the throat. A forcing cone is not a funnel that directs the boolit into the bore. A forcing cone's job is to help turn the cylinder into alignment with the barrel when the boolit comes into the barrel. If it hits the side of the forcing cone it will push the cylinder away from the side it hits and more into alignment.

I guess if you have acceptable accuracy like it is, it really don't matter what you do with it. 2 1/2" at 25m could easily be improved upon by simply fitting the cylinder and it's throats to the gun and the boolit. I do this stuff day in and day out with Ruger and S&W cylinders, and it works really well, there are dozens of guys on this forum, the Ruger.net forum, my Facebook page, that can attest to the improvement that is made. And before I started doing it, Cylindersmith was doing it with much the same results.

I will say here as I have said before, in a perfect world, the gun acts like a funnel. The larger diameters at the rear where the cartridge loads, tapers as it goes toward the muzzle, every step of the way the boolit is subjected to a slightly smaller diameter that swages it down in size as it goes, ensuring good fit and a tight seal in the bore as it travels to the muzzle. There is no restricted area like throat or thread choke, to upset this smooth transition.

The boolit needs to be .001" to .002" larger than groove diameter, the cylinder throats, and yes it does need throats, need to be .0005" to .001" over boolit diameter. This is what makes a good shooting revolver into a tack driver, or turns a piece of junk into a fine shooter. It's in the math, and it's a linear procession from one end to the other that makes it all work.

I dunno. This is my way of thinking, but this is how revolvers have been built for generations because this is what makes them work the best but it's your gun and I understand the constraints of time and money and also living in a country where tooling and other things may not be nearly as accessible as they are here so you do what you have to do to make it work.

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See if you can find accuracy reports on the 1917 Colt in .45 ACP vs. the 1917 S&W. I have read that at least some of the Colts had cylinders bored straight through meaning that you had to use half moon clips; while the S&W had proper chambers so the ACP round would headspace and fire without clips ( at the cost of individually poking the empties out.

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A similar situation would be firing a .38 Special in a .38 Long Colt chamber, I've heard of this being done. I've also heard of the .357 being fired in a Long Colt chamber, the escape of gas around the billet preventing the cartridge from building up to full chamber pressure which would have unhinged the poor old gun.
They claimed that the loose fit of bullet to throat would allow firing of the 9mm rimmed revolver cartridge in a .38 S&W chambered revolver without destroying it, but I wouldn't try it.

Without the chamber throat too much gas would wash around the bullet base. That would ruin accuracy by deforming the bullet base , though jacketed bullets might be less affected, and would greatly reduce velocity unless a hollow base bullet were used.

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I will say here as I have said before, in a perfect world, the gun acts like a funnel. The larger diameters at the rear where the cartridge loads, tapers as it goes toward the muzzle, every step of the way the boolit is subjected to a slightly smaller diameter that swages it down in size as it goes, ensuring good fit and a tight seal in the bore as it travels to the muzzle. There is no restricted area like throat or thread choke, to upset this smooth transition.

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Tapered chambers have been tried with a revolver intended to fire just about any .38 and 9mm cartridge. Don't remimber the details but the main purpose was to let 9X19 and .380 cases be held against the breech face by a friction fit at different stages of the taper and no shoulder at the throat to scrape the jackets.
The pistol fired a wide variety of 9mm, .38, and .357 cartridges safely though none very well.

When I shot PPC, there were some smiths making short cylinder conversions of the "K" framed S&W revolver. There was never enough difference in accuracy to create sufficient demand so the practice stopped. You want the shorter case for reloading speed. For that reason, it might be a good experiment.

As for the diameter of the barrel, how does it compare to a 1911 barrel? Those are not terribly thick.

Finding a spare barrel and cylinder might be costly but not that hard to do. Any revolversmith should be able to do the work. I would have trouble justifying the conversion for myself.