I have been concerned about cylinder gap and have read bunches of articles on the subject. I have found an article on the internet that is a great Readers Digest version. Some of you might want to check it out. Try going to Revolver vs. auto: the effect of barrel/cylinder gap and barrel length on velocity. To look at the article on the internet search cylinder barrel gap pistol
Guns Magazine, Jan, 2005
Not long ago somebody asked me how much velocity was lost due to the barrel/cylinder gap on revolvers. Of course there is no pat answer but very often I have seen cases where identical cartridges had a higher velocity when fired from a revolver than from a pistol with the same length barrel. This would seem to fly in the face of logic because there is a space between the barrel and cylinder, usually called the b/c gap. In a well-built gun this isn't very big--usually less than .010"--but it is logical to think that some gas is going to escape there. Of course it does, but we have to remember the bullet itself seals the gap for a little while as it passes. What if, before the gap opens up, the powder has all burned and the bullet received all the acceleration it is going to get? Of course we are talking about the tiniest little increment of time ... but what if?
There is another set of variables to consider, though, and are the conditions within the barrel itself. Are the dimensions constant from one end to the other? Is it smooth or rough? Is it slow or fast?
We need to have a cartridge that is fired in both types and we also need a variety of barrel lengths. Well, the ancient .45 ACP fits those nicely and rummaging through the safe produced S&W .45 ACP revolvers with barrels of 4", 5" and 6.5" and a host of pistols with both 4.25" and 5" barrels from several different makers. When the dust settled I had seven 1911 type pistols and three revolvers.
For ammo I chose CCI Blazer Brass 230-grain ball ammo and velocity was measured with a shiny new PACT Professional Chronograph.
Of course, I went into this experiment knowing that b/c gap didn't matter but length did. When I was through those assumptions were left in tatters. Some observations:
Revolvers
1. The size of the b/c gap is irrelevant.
2. There is no difference between 5" and 6.5" barrels.
3. The difference from the 4" is minimal, if it is relevant at all.
Pistols
1. 3/4" does matter
2. There is no meaningful velocity difference in the other 5.
I am not a fan of standard deviation. Even though it does give us some ideas about the consistency of our ammo, it in no way tells us that one load is better or worse than is another. I include it here because there is one valuable application. We always tend to take numbers as absolute values but if you look at a string of chronograph data it would be exceedingly rare to see the same exact number twice. The system is simply too variable. The standard deviation comes in handy if we use it as a plus or minus value when comparing two sets of data. For example if we use the standard deviation of one set as a plus/minus value and compare it with the same numbers from another, if the two results overlap the chances are pretty good that the differences are not meaningful.
Sometimes this is a hard concept to accept because we are so accustomed to thinking of numbers as absolute values. Sometimes--but rarely--they really are, but more often than not our method of measuring has a small variability built in. When we talk about firearms and ammunition there are variables associated with every element of the process in both gun and amino. Using our data, we might be able to conclude based on the lower SDs, that Kart barrels are a little more consistent than are the others.
I began this project with two expectations that proved (at least this time) to be untrue. First, I thought it would be possible to show that the b/c gap had no effect--it certainly seems not to here--and, secondly, I expected to show a wide range of velocities from different makes of barrels. Actually they were remarkably consistent. Is there a lesson here? Sure there is. The only way to confirm theories is to test them in the real world.
VELOCITY COMPARISONS: REVOLVER VS. PISTOL
S&W REVOLVER
LENGTH CYL GAP STND
(INCHES) (INCHES) VEL (FPS) DEVIATION
4 .004 808 14
5 .006 823 14
6.5 .007 822 16
1911 PISTOL
LENGTH STND
MAKE INCHES VEL (FPS) DEVIATION
Brown 4.25 766 18
Wilson 4.25 812 22
BarSto-1 5.0 843 16
BarSto-2 5.0 864 15
Kart-1 5.0 869 9.3
Kart-2 5.0 873 6.6
Colt NM 5.0 859 14
All ammunition is CCI Blazer Brass 230-grain TMJ from the same lot.
Velocity is the instrumental average of 10 shots over a PACT
chronograph.
COPYRIGHT 2005 Publishers' Development Corporation
COPYRIGHT 2004 Gale Group
Guns Magazine, Jan, 2005
Not long ago somebody asked me how much velocity was lost due to the barrel/cylinder gap on revolvers. Of course there is no pat answer but very often I have seen cases where identical cartridges had a higher velocity when fired from a revolver than from a pistol with the same length barrel. This would seem to fly in the face of logic because there is a space between the barrel and cylinder, usually called the b/c gap. In a well-built gun this isn't very big--usually less than .010"--but it is logical to think that some gas is going to escape there. Of course it does, but we have to remember the bullet itself seals the gap for a little while as it passes. What if, before the gap opens up, the powder has all burned and the bullet received all the acceleration it is going to get? Of course we are talking about the tiniest little increment of time ... but what if?
There is another set of variables to consider, though, and are the conditions within the barrel itself. Are the dimensions constant from one end to the other? Is it smooth or rough? Is it slow or fast?
We need to have a cartridge that is fired in both types and we also need a variety of barrel lengths. Well, the ancient .45 ACP fits those nicely and rummaging through the safe produced S&W .45 ACP revolvers with barrels of 4", 5" and 6.5" and a host of pistols with both 4.25" and 5" barrels from several different makers. When the dust settled I had seven 1911 type pistols and three revolvers.
For ammo I chose CCI Blazer Brass 230-grain ball ammo and velocity was measured with a shiny new PACT Professional Chronograph.
Of course, I went into this experiment knowing that b/c gap didn't matter but length did. When I was through those assumptions were left in tatters. Some observations:
Revolvers
1. The size of the b/c gap is irrelevant.
2. There is no difference between 5" and 6.5" barrels.
3. The difference from the 4" is minimal, if it is relevant at all.
Pistols
1. 3/4" does matter
2. There is no meaningful velocity difference in the other 5.
I am not a fan of standard deviation. Even though it does give us some ideas about the consistency of our ammo, it in no way tells us that one load is better or worse than is another. I include it here because there is one valuable application. We always tend to take numbers as absolute values but if you look at a string of chronograph data it would be exceedingly rare to see the same exact number twice. The system is simply too variable. The standard deviation comes in handy if we use it as a plus or minus value when comparing two sets of data. For example if we use the standard deviation of one set as a plus/minus value and compare it with the same numbers from another, if the two results overlap the chances are pretty good that the differences are not meaningful.
Sometimes this is a hard concept to accept because we are so accustomed to thinking of numbers as absolute values. Sometimes--but rarely--they really are, but more often than not our method of measuring has a small variability built in. When we talk about firearms and ammunition there are variables associated with every element of the process in both gun and amino. Using our data, we might be able to conclude based on the lower SDs, that Kart barrels are a little more consistent than are the others.
I began this project with two expectations that proved (at least this time) to be untrue. First, I thought it would be possible to show that the b/c gap had no effect--it certainly seems not to here--and, secondly, I expected to show a wide range of velocities from different makes of barrels. Actually they were remarkably consistent. Is there a lesson here? Sure there is. The only way to confirm theories is to test them in the real world.
VELOCITY COMPARISONS: REVOLVER VS. PISTOL
S&W REVOLVER
LENGTH CYL GAP STND
(INCHES) (INCHES) VEL (FPS) DEVIATION
4 .004 808 14
5 .006 823 14
6.5 .007 822 16
1911 PISTOL
LENGTH STND
MAKE INCHES VEL (FPS) DEVIATION
Brown 4.25 766 18
Wilson 4.25 812 22
BarSto-1 5.0 843 16
BarSto-2 5.0 864 15
Kart-1 5.0 869 9.3
Kart-2 5.0 873 6.6
Colt NM 5.0 859 14
All ammunition is CCI Blazer Brass 230-grain TMJ from the same lot.
Velocity is the instrumental average of 10 shots over a PACT
chronograph.
COPYRIGHT 2005 Publishers' Development Corporation
COPYRIGHT 2004 Gale Group
