Penetration .38 Special

[ZVP]

Hope this is the right forum for this question.
Comparitivly, does a 2" Model 36S&W against a 2" (sealed) ba rrel/chambers.38SP Derringer produce more velocity fired from the Derringer since the Chief has a barrel gap please settle this ongoing debate for me OK?

[JohnKSa]

Probably a wash.

The revolver actually has a barrel length advantage since its barrel is measured from the forcing cone forward while the derringer's barrel is measured from the breech forward. In the absence of any other evidence, it's probably reasonable to assume that the length advantage is mostly cancelled by the presence of the gap.

Of course, there are other variables, at work as well. Individual variations between bores can result in significant differences in velocity, even when everything else appears to be the same.

[ZVP]

Thanks John!

[Unclenick]

ZVP,

The Ballistics by the Inch group did some barrel/cylinder gap tests. They fired .38 Special from a .357 Magnum chamber. Their results for 2" of barrel in this revolver, for which they could adjust the gap by screwing the barrel in and out (they started with it 18" long, then cut off 2" and re-fired the same lots of ammo, repeating until they got to 2" of barrel) was the first 0.001" of gap with 13 different makes of .38 Special ammo, cost an average of -1.2% from MV. A 0.006" gap (pretty typical) cost them an average of -4.5% (average of -0..9% for each additional 0.001" additional gap after the first 0.001" of gap).

I looked at the raw data and concluded the added length of a .38 Special chamber in the 2" revolver would increase velocity about 3 times as much as a 0.006" barrel cylinder gap would reduce it, on average. So, if you could make all else equal (bore and chamber dimensions, in particular and chamber spring and etc., between the two guns, the revolver would still be ahead of the derringer, despite its gap. Something on the order of 40 fps with the average commercial loads.

But all else is never equal in two real-world production guns.

[JohnKSa]

So, if you could make all else equal (bore and chamber dimensions, in particular and chamber spring and etc., between the two guns, the revolver would still be ahead of the derringer, despite its gap.

...and assuming the gap is relatively close to 0.006". These days it seems that cylinder gaps closer to 0.01" than 0.006" are pretty common.

Unfortunately the BBTI folks don't have data for anything larger than 0.006". Which is understandable given how much ammo and time it must have taken to do the testing they did.

[Hal]

OP - do you want info on penetration or velocity?

One has little to do with the other - directly.

The sectional density of the bullet is what counts most as far as penetration is concerned.

Stab something with the pointy end of an ice pick, then turn it around and use the handle as the point.

Despite being the same weight and moving the same velocity, the pointy end will penetrate more and more easily.

the Chief has a barrel gap

I had a Dan Wesson and could play with the B/C gap all I wanted.
I don't think the B/C gap makes any significant difference.

[ThomasT]

I have read that the length of the cylinder on a revolver should also be included with the length of the barrel. That would make a revolver with a 2" barrel actually measure around 3.5" for a total barrel length depending on the length of the cylinder. Does it make any difference. I don't know.

But I bet the load from the snubby is faster than the same load from a derringer.

[James K]

Such tests have been run, with no real answers. One test found that the difference in muzzle velocity of carefully prepared handloads due to the barrel/cylinder gap could vary up to 50 fps within factory b/c gap specs. BUT, the velocity of factory loads fired from a barrel with NO gap varied as much as 50 fps in a single factory box.

My advice on velocity variations caused by the b/c gap and barrel length is to (within reason) forget about them. Barrel length is usually more important in the context of concealed carry than it is where muzzle velocity is concerned and is always a tradeoff with convenience.

Jim

[Unclenick]

The BBTI test has several examples that don't follow the pattern. The Fiocchi 125 grain HPXTP load actually shot slower in the 2" barrel with no gap than with one of either size. This was due to random velocity variation of the mean, which is pretty high with their sample size. However, I hacked their Excel file to run my own numbers on their data, and when you average all results for each barrel length or for all of them, the pattern is pretty clearly there. The first 0.001" of gap makes more difference than subsequent additional 0.001" increments. Longer barrels lose more to the gap than short ones because there is more time for gas bleed off.

All the 2" barrel results for .38 Special (I excluded the .357 and .38 LC data) averaged a loss of 1.5% from the first 0.001" of gap, and 4.5% from the 0.006 in gap, meaning about 0.6% velocity loss for each additional 0.001" of gap after the first one. Averaging all the data, which means an average barrel length of 10", the loss was 5% for the first 0.001" of gap and 0.7% for each additional 0.001".

The SAAMI standard test barrels all use 0.008" gaps. I don't know why BBTI didn't think to go with that number. I would have. But the pattern suggests the difference would be about another 1.2% loss with the 2" barrel.

The fact the most difference is made by the first 0.001" of gap explains why Hal would have trouble seeing the difference in his DW revolver using different shims to set the gap. I never noticed in on mine, either. But you'd have to be scratching the cylinder face up or else shoot a very large number of rounds to distinguish it from random variation with any helpful degree of confidence.