The Significance of Barrel/Cylinder Gap????

[woodspirits]

After reading Jim March’s excellent post in regard to selecting a “revolver” I found that some of my revolvers had excessively large barrel/cylinder gaps. Does anyone here know of any studies that could help estimate how much fps and fpe that I am losing because of the resulting “blow-by” of escaping gas pressure? I didn’t want to post in Jim’s fine thread and take away from his contribution.

Because of the many variables that are present with chronographs, I do not see how to get a reliable estimate of how the barrel/gap measurements would affect performance of a revolver by using different guns. By utilizing different revolvers even matched up in regard to barrel length, models, etc. there will be many other factors (barrel diameter for example) affecting the readings.

IMHO, wouldn’t using the same revolver and changing the barrel/cylinder gap give test results that would be significant? For example, would there be a significant change in fps and fpe if the same revolver is tested with a gap of say .002” and then changed to a gap of .016” (the range of my own revolvers)?

The only revolver (that I know of) that has an owner changeable barrel/cylinder gap is the Dan Wesson. In my opinion, this would give a fairly reliable method of determining how the gap would affect the performance with meaningful numbers from the same revolver with varying b/c gaps. Does anyone know of any similar studies that could possibly offer something more than a guess? Many thanks for your comments.

[Mal H]

... would there be a significant change in fps and fpe if the same revolver is tested with a gap of say .002” and then changed to a gap of .016” (the range of my own revolvers)?

I can't say definitely for each and every revolver, but in my experience, I would have to venture an absolutely! After all what is it that pushes the bullet along the barrel - pressure. If you increase the pressure leak at the base of the barrel 8 fold, there certainly won't be as high a pressure behind the bullet after it leaves the cylinder.

How much it changes is a factor of many variables; bullet size, weight, and design; barrel length and design; powder load; etc.

I am not aware of any scientific study that has done what you are seeking. You would have to have a variable gap revolver with a wide range. Just using different revolvers with different gaps doesn't work, obviously.

What revolver do you have that has a gap of .016"? That is beyond excessive in my book. (Also .002 might be too tight depending on the ammo and how much you shoot out of the revolver. It could tighten up on you during a session and cause a temporary jam.)

[Magnum Wheel Man]

agree, that the posted gap should be considered excessive...

I have a pair of Dan Wessons, but have never taken the time to shoot various gaps over a chrony... I keep both very tight, which can result in a binding cylinder as carbon builds up on the end of the cylinder & forcing cone... so short of an excessive gap,a looser gap will shoot more reliably for longer periods without cleaning...

some observations though... the cartridge pressure greatly effects the "blow by" of the gap... a higher pressure cartridge will spit out more gasses, & posibly even burning powder than lower pressure cartridges of the same gap...

... not sure of the pressures off the top of my head, but I think my 30 Carbine blackhawk has a typical barrel / cylinder gap, but that gun has burned my off hand so badly numerous times, that I thought the timing was off, & it was spitting lead

[woodspirits]

What revolver do you have that has a gap of .016"? That is beyond excessive in my book. (Also .002 might be too tight depending on the ammo and how much you shoot out of the revolver. It could tighten up on you during a session and cause a temporary jam.)

It is not my intention to bash a manufacturer that I respect but the revolvers with the high b/c gaps are NAA mini-revolvers. I have four or them (all purchased new) and all have large b/c gaps (up to 020") straight from the company. My hand is "filthy black" after 20 or so rounds through them. I realize that they were not intended to be of target quality but I wonder how much energy is available if one was needed in close-up self-defense. Many thanks for your input.

I have a pair of Dan Wessons, but have never taken the time to shoot various gaps over a chrony... I keep both very tight, which can result in a binding cylinder as carbon builds up on the end of the cylinder & forcing cone... so short of an excessive gap,a looser gap will shoot more reliably for longer periods without cleaning...

I have a Ruger 357 that arrived new with a b/c gap of about .003" and it will bind at approximately 50 rounds. But because it is a CCW, that is fine with me and I feel that it would impart good energy levels in an emergency situation. Many thanks for the info.....PS: I have always wanted a good Dan Wesson.

[Chesster]

I'm no expert, but I would speculate the NAA mini revolver large gap has something to do with the method used for loading and unloading. Tight tolerances here would certainly make removing and replacing that cylinder a pain. Also, that short barrel is sure to leave a lot of powder residue which could cause the cylinder to bind if the gap was very tight.

[sw_florida]

A barrel gap of 0.01 inch is not a problem. 0.02 would be my upper limit, but still ok. Put an empty cartridge, a cartridge that has been shot, in the cylinder behind the barrel. With your mouth, blow down the barrel. There you have your escape of gas. Not much, is it?

[Mal H]

Originally Posted by woodspirits

... the revolvers with the high b/c gaps are NAA mini-revolvers.

I never checked the gap in my own NAA mini, but I did tonight. It's about the same as yours - huge.

I think Chesster's speculation about the reason for the large gap makes a lot of sense. If it was tight, the cylinder would be very difficult to remove easily.

Originally Posted by sw_florida

With your mouth, blow down the barrel. There you have your escape of gas. Not much, is it?

That's not a very convincing test since you're comparing a pressure of around 20 PSI or so to that of anywhere from 10,000 to 40,000 PSI. If the additional pressure loss caused by an excessive gap is, say, 10%, you've lost 2 PSI in the first case, and 1,000 to 4,000 PSI in the second.

[woodspirits]

I never checked the gap in my own NAA mini, but I did tonight. It's about the same as yours - huge.

I think Chesster's speculation about the reason for the large gap makes a lot of sense. If it was tight, the cylinder would be very difficult to remove easily.

I was just looking at one of my mini's; I found that when I remove the cylinder and re-insert it, the breech of the barrel and the front cylinder face is not a factor in preventing the cylinder from seating. It is the bushing in the cylinder axis (that rides on the cylinder pin) and this dimension would be responsible for cylinder end-shake. If you hold yours up to the light as you are seating the cylinder into position you may see what I mean.

With my mini, there is a lot of excess space in the barrel/cylinder gap area and not within the axis/pin area. While holding the cylinder in position, it looks like the barrel could be turned in considerably before causing a problem with reloading.

I have a Ruger SingleSix with a b/c gap of .004" and the cylinder is no more difficult to remove and replace than the mini. In fact, I think that it is easier because it is a larger cylinder. IMHO the mini gap could be closed considerably before you could see a problem with binding and reloading. I have heard from some mini owners that their gap is in the .008" range and the mini functions fine. Who knows (???) and thanks for your input