One cylinder is slower than the other

[westczek]

Here is one for the other cyliner switchers out there.

I was testing some 38 Short Colt loadings today to try and make minor reliabley out of a 4 inch S&W 686. I'm already getting good numbers out of my 6 inch 686 (130 pf). The loads were slower out of the 4 inch, but a little slowing than I had expected.

This 4 inch gun is using a replacement blue 6 shot cyliner, the original was a stainless 7 shot. For some reason, my father suggested swapping the cylinders between the 4 and 6 inchers.

With the same ammo, velocity dropped in the 6 incher and rose in the 4 incher when the cyliners were swapped. It is hard to say, but the barrel/cylineder gap looks like it may be a little greater on either gun when the slower cyliner is used. I'll have to measure that.

The increase in the 4 inch gun with the faster cyliner was getting it to averaging a little over 800 fps, almost enough to reliably get minor. With the slow cylinder it was averaging about 25 fps lower.

Westczek

[snertley]

Chech your barrel gap, I was getting almost 120fps difference between my 610's . Checked the gap on the new one and there was almost no gap. Streched the crain to get a proper gap and BANG! Back to the same speed from both guns.

[RAC]

Will that gap also affect accuracy?

And how do you strech the crane?

RC

[snertley]

No change in accuracy. Sorry, I dont remember the measurements.

[Toolguy]

The cylinder gap doesn't affect accuracy, only velocity. The crane stretcher tool is like a tubing cutter with the cutting wheel having a flat edge instead of a sharp cutting edge. You have to put a 5/16 steel mandrel inside the crane so it doesn't collapse from the tubing cutter. A lot of times the crane isn't quite 5/16, so you need a reamer to make the hole round and on size. Then you run the tubing cutter tool around the outside a few times and it displaces the end of the crane. You also need a piloted facing cutter to trim the end of the crane if it's too long or crooked. A lot of them are not square from the factory. Usually there is a ridge raised up on each side of where the wheel goes around the crane. This has to be filed off for the cylinder to fit back on. Don't use a real tubing cutter, you will cut off the end of the crane. Get the proper tools from Brownell's.

[Tom E]

Or better yet, use Power's shims.

[westczek]

Maybe I don't understand what a crane stretcher does... I was under the impression it was to reduce front to back play, pushing the cyliner backward toward the firing pin for proper head space. I thought the only way to reduce the barrel cylinder gap was to setback the barrel.

I switched just the cylinders the yoke/cranes were left on the original guns. Wouldn't this point to the cylinders being of differnt demisions, rather than any issue with the crane?

BTW-- Snertly, you said that the velocity was the same in both guns after stretching. Did the velocity go up in the slower gun to match the faster gun or down in the faster gun to match the velcity of slower gun?

Thanks for the advice, any reason to buy some new tools

Westczek

[snertley]

The FPS came down to match the slower gun with the correct barrel gap.

[Forrest Halley]

Maybe I don't understand what a crane stretcher does... I was under the impression it was to reduce front to back play, pushing the cyliner backward toward the firing pin for proper head space. I thought the only way to reduce the barrel cylinder gap was to setback the barrel.

I switched just the cylinders the yoke/cranes were left on the original guns. Wouldn't this point to the cylinders being of differnt demisions, rather than any issue with the crane?

BTW-- Snertly, you said that the velocity was the same in both guns after stretching. Did the velocity go up in the slower gun to match the faster gun or down in the faster gun to match the velcity of slower gun?

Thanks for the advice, any reason to buy some new tools

Westczek

Mike I recently had some work done to my five inch which was way out of spec for head space among other fun and excitements. The crane stretcher takes and squeezes the crane against a solid steel rod causing the pinched metal to flow(for lack of a better word) to the end of the crane making it longer. The one I saw used looked like a modified pipe cutter with a bearing in place of the blade. A byproduct of this is the increase of the barrel to cylinder gap. The proper way to correct this is to have the barrel set back. The opposite would be true if your headspace was too little and you trimmed the crane which would result in the barrel to cylinder gap closing possibly needing the forcing cone turned down for cylinder clearance. Hope this helps your understanding a little.

Realize that the gun may still be within factory tolerances after stretching so perhaps the answer is to bump the PF of your loads up to see both guns through with the same lot?

Anybody feel I have errors in my statement feel free to correct...Please.

Edited November 16, 2010 by Forrest Halley

[westczek]

Snertly, Forrest,

Great. I thought that was how it works. And Forrest hit the nail on the head, I need to increase my load a little to make power factor in the 4 inch gun. I might just leave the stainless cylinder (the faster, less gap) in the 4 inch gun. Basicaly split the differnce.

Westczek

[cas]

I don't want to sound like the carpenter with only a hammer, but.... differences in throat size will effect velocity too. I've seen it many many times where after the throats were opened up, velocities dropped.* I can only assume it has to do with pressure spikes that are no longer there (or less so). As well as the spike from them bullet swaging down through the throat, I would imagine the now slightly undersized bullet has less resistance in the barrel, speeding things up.

Unless your chambers are EXACTLY the same, you're never going to get the exact same velocities, even if the gap was the same.

* ETA: I'm talking pretty good size changes though, going from say .002" undersized, to not

Edited November 18, 2010 by cas

[snertley]

Do not try to go with too little gap for the increased FPS. After a long COF my revo locked up due to the heat and expanding metal. No free lunch here. I am shooting .40's out of my 610s for what is worth.

[Patrick1981]

I know that a gap is acceptable from 0.006 to 0.011 or 0.013, an ideal middle ground.

0.013 however, even if there is not necessarily lose precision,

but a bit of pressure and speed of the bullet.

If it is less than 0.003 may stop the cylinder rotation.

Personally I have on my SSR a gap of 0.009 and accuracy is very good even if I use 38 special cartridge.

158 grs bullet ( velocity 820 fps ) = 130 P.F.

From 27 yards you can put with a bit of calm and in single action

all the shots in the ICORE zero target, remembering that it is a 4-inch barrel.

Maybe you could do even better using the 357 cartridge, because it decreases the freebore.

I personally leveled by removing from 0.002 to 0.003 inch thick with a 1200 paper

thin ( mounted on a straight piece of metal like a file )

and a little rubbing compound because there was a slight asymmetry of the surface of the forcing cone.

A good job and revolver shoots very well.

You can read my topic (Gap and forcing cone)

Hello from Italy

[westczek]

Ahh, the throats. Yes that could have something to do with it too. I have to admit I forgot all about that issue. I wonder what would happen with .357 diameter bullets vs the .358I'm using now. Could be interesting... Just to see what it would do to the data.

Westczek

[Patrick1981]

you should use bullet diameter as near as possible to that of the throats.

If the throat is narrower than the barrel is a problem because the bullet does not press well against scratches and becomes unstable bullet.

I use 0.358 for lead, galvanically-coated 120-micron copper