Velocity Variance ??


Denny Crane
September 17, 2011, 04:31 PM
MBC PPC#2 148 gr DEWC in .38 Sp cases and 2.8 grs Bullseye.

8" Python avg 740 FPS

8 3/8" S&W Mod 27-2 avg 725 FPS

4" Diamondback avg 730 FPS

6" Officers Model Special avg 690 FPS

Makes no sense to me.

What actually causes this? Throat/bore size?

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September 17, 2011, 04:55 PM
Assuming all the loads were put up in the same brand cases with the same case capacity.

One or more of a combination of things.
* Bore size - .3565", or .357", or .3575", or .358".
* Bore smoothness, or drag on the bullet.
* Traces of bore leading or copper fouling.
* Chamber tightness, or looseness? (More case expansion increases case capacity)
* Cylinder length compared to the other guns. (longer cylinder increases the working bore length)
* Barrel / Cylinder Gap different = different pressure loss.

If you can figure out why two identical guns nearly always give different velocity, you can write a book and get rich quick!!


Denny Crane
September 17, 2011, 05:28 PM
All the bores were clean with no leading before and no leading after. Leading had been my problem so I pay close attention to it.

The 4" DB being almost equal to the 2 8" guns surprises me.

The 6" OMS being the slowest? I have always wandered about the bore in my OMS. I can tell this gun had some work done to it at some point. In single action it breaks at about 2.5 lbs. The rear sight has been changed out to a Micro brand and it had a set of thumb rest stocks on it when I bought it. I have to guess it has seen a lot of rounds but really don't know. The rifleing is not near as deep as most bores. No idea if it was made this way or is worn from use. Not sure how many rounds it would take to wear the bore out but I know it's got to be a lot?? Other than shallow rifleing it looks great and shoots great. It performs as well as any I have in the Ransom Rest.

Worn bore cause lower velocity?

September 17, 2011, 05:38 PM
Well, I think the two 8" .357s shooting shorter .38 Spl target loads could explain the 4" Diamondback .38 nearly matching them.

Again, the longer .357 chambers with shorter .38 cases increases the effective case capacity, which lowers pressure to some degree.

I don't know how many rounds of lead bullet .38 ammo it would take to wear a bore bigger either.
But it would be a heck of a lot.

On the other hand, if the original owner went to the trouble of doing the SA, Micro sight, and thumb rest grip conversions, he was a target shooter, and probably shot it a heck of a lot.


September 18, 2011, 08:09 AM
I would bet with confidence that if you changed your powder to something a bit slower burning, say Longshot or HS6 your variance would become more consistent. I load 38 spcl with both of those powders and all though I'm up there in velocity all of my S&W's and even my Taurus keep pretty close company in terms of velocity.

September 18, 2011, 10:12 AM
To me the answer is simple. 2.8 grains of bullseye is burned in the first 2 inches of barrel. The rest of the barrel is simply sight radius for accurate alignment.

The OMS probably has a bit more cylinder gap, especially IF it has been shot a lot. The barrel/rifling is not worn, but check the back of the barrel for gas cutting. Excessive cylinder gap causes a lot of problems.

Like RC said, if you figure out what causes the variance, you could get rich. The fact is you can't. If somebody had it figured out, we'd all know about it by now.

That's what makes the hobby of shooting so much fun. For me at least, the hunt for the most accurate load revolves around eliminating the variables to the point of an accurate load that is repeatable, the ammo performs the same each time.

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