Unexpected chrono results, .45 ACP

So I currently have 3 different .45 ACP handguns, a Sig P220R, a CZ 97 BD, and a S&W 625-6. Both the CZ and the Smith have 5" barrels, the Sig has a 4.4" barrel. Spent all day at the range, put LOTS of rounds over the chrono, and decided to run my standard .45 ACP range reload while I was at it. This is a pretty mild load, I use a 225 gr. coated lead TC, either MBC or Bayou, over 5.1 gr. of Sport Pistol, mixed small primer range brass and (usually) CCI primers. I'd planned on just running it through the CZ, but since I had all 3 .45's with me, and still needed to burn some time while my rifles were cooling, I went ahead and ran all three. This is all the same load, same box, same loading session, so as close to identical as I ever get :rolleyes:. Drumroll, please...

Sig 783 FPS
CZ 827 FPS
S&W 860 FPS!

The Smith is the one that has me scratching my head. I always thought that, all else being equal, a revolver would have lower velocity than an auto, due to the barrel to cylinder gap. Apparently I thought wrong. Is this typical, or is this specific to the particular combo of guns I have? And while I'm at it, doesn't it seem like the Sig is slower than it should be, considering the barrel is only .6" shorter than the others? Appreciate your thoughts, thanks.

Dave

Apparently the revolver cylinder throat added more than the gap took away.
And my P226 is slower than my Colt 9mm or even a friend's Commander with similar length barrel.

A 5” barrel on a revolver, especially one running an auto cartridge like the .45 ACP, is significantly longer than a 5” semi auto barrel. The semiauto barrel includes the chamber. The revolver chamber being in the cylinder, to get a length dimension to compare with the semiauto, you’re going to have to measure from the back of the chamber in the cylinder to the muzzle. I expected your 5” N frame to run fastest, because it is so much longer in the barrel.

Barrels run at different speeds. The Sig running slower might be a result of the barrel length, or wear, or machining differences, or a combination of things.

Quote:

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Originally Posted by Duelist

Barrels run at different speeds.

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Even in the same platform, I have two S&W 1911 Commanders and both run the same ammo just slightly faster than one of my full size government models.

Pretty much what Duelist said above. Revolver barrel length measurements do not count the length of the cylinder. Even though you lose some pressure at the barrel- cylinder gap you also gain some velocity from that extra length in the cylinder. If you go to http://ballisticsbytheinch.com and look at their real world tests the revolvers generally perform about an inch better than their barrel length when compared to a semi-auto with the "same" barrel length due to the difference in how barrel lengths are measured between the two firearm types.

Within revolvers you'll also see variations in velocity between equal length barrels depending on how tight the barrel- cylinder gap is set. Generally a tighter gap will lose less pressure and have higher velocity. The trade off being the tighter the gap the more rapidly debris will cause rubbing between the cylinder and forcing cone interfering with the revolver operation. Pretty sure Ballistics By The Inch has some test numbers on that as well.

Quote:

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Originally Posted by Duelist

A 5” barrel on a revolver, especially one running an auto cartridge like the .45 ACP, is significantly longer than a 5” semi auto barrel. The semiauto barrel includes the chamber. The revolver chamber being in the cylinder, to get a length dimension to compare with the semiauto, you’re going to have to measure from the back of the chamber in the cylinder to the muzzle. I expected your 5” N frame to run fastest, because it is so much longer in the barrel.

Barrels run at different speeds. The Sig running slower might be a result of the barrel length, or wear, or machining differences, or a combination of things.

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DUH! Obvious answer to the question, but one that I totally missed. So yes, if this revolver was measured the same way they measure pistol barrels, it would be just over 6.5", a good 1.5" longer than the CZ's supposedly 5" barrel, and more than 2" longer than the Sig. No wonder I saw a significant increase in velocity. Add in that this one has a nice, tight cylinder gap at no more than .003", and it ain't givin' up nothin' to the autos :rolleyes:. Makes me even happier with that load, though, it's comparable to most of the range fodder out there, but shoots softer and more accurately. It ain't broke, so I don't think I'll try to fix it. Thanks, guys.

Dave