I'm not overly familiar with the 38 Long Colt round but I know a little something about the 38S&W. I know that the 38S&W is a few thousands of an inch wider than 38 special. I have a S&W Victory Model revolver that was originally chambered for 38S&W but when it was reimported from the UK they rebored the cylinder to allow 38 Specials to chamber and fire. It'll shoot them no problem but not very accurately when compared to 38S&W. I hope that maybe this hepled a little bit.
FWIW, here is some info I picked up at another site.
Try slipping an unfired .38 Special case in a chamber. The .38 LC, .38 SC & .38 Special all have the same base dimension, .378-.379".
Then, try to slip an unfired 9mm Luger case in the chamber. The 9 mm base dia. is .392" which is slightly larger than a .38 S&W (.386") If the 9mm goes almost all the way in, you have a .38 S&W/.38 Colt New Police (same cartridge).
If the .38 Spl is a tight fit with no play, your pistol is probably a .38 LC.
Also, measure your cylinder length; the LOA of a .38 Spl is 1.55"; a .38 LC is 1.32" and a .38 S&W/.38 Colt NP LOA is 1.20".
Naturally, don't fire the gun until you or a gunsmith are satisfied you have the correct ammunition & the pistol is safe to fire
Teakwood is quite right. But let me add, If your gun is chamberd for .38colt .38 spls may fit the cyl. DONT shoot them. the weapon will probly not blow up. But don't take the chance.
If it's .38S&W or .38 police [same thing] no problem the rd.s will not chamber.[too fat]
"I have a Colt Double Action New Army revolver. The barrel is stamped Colt D.A. 38. My ammo supplier list .38 Long Colt and a .38 S&W. Will either of these or both work ok in my revolver."
The .38 S&W ammunition will NOT chamber in your revolver. Colt chambered the New Army and Navy Model revolvers for the .38 Long Colt cartridge. The cases of the two cartridges are different diameters.
I would not recommend using .38 Special ammunition in one of the early Colt New Army and Navy Model revolvers either. While Colt reduced the barrel diameter to handle Smith & Wesson's new .38 S&W Special cartridge in 1904, these early cartridges did not generate the pressure that today's hi velocity .38 Special cartridges do...Using today's .38 Special loads could be quite dangerous!
Since the .38 S&W Special was a proprietary cartridge of Smith & Wesson, Colt refused to mark their barrels with the S&W name, so Colt left the "COLT DA 38" caliber marking on the barrel after 1904 and just said in their advertising that their revolvers would handle all of the .38 caliber cartridges on the market of that time period.
As for the .38 S&W cartridge, Colt never chambered the New Army and Navy model revolvers in this caliber. However some of these early Colt's were sent to England during the Battle of Britain in 1940. The English did ream out the chambers for the .38-200 which is what the English call the .38 S&W case size. These guns will all have evidence of English use and will have "/380" somewhere on the frame near the barrel. Hope that helps..... Bob Best