faster / slower barrels?

[njl]

I was chronographing a few .38spl loads today (125gr TC from Falcon, 148gr BNWC from Falcon, and 148gr DEWC from Friendswood) in my 4" 66 and 6" 14. I'd assumed, the 14 with its 2 more inches of barrel would give me higher velocities. To my surprise, the same loads shot back to back in the 66 and 14, the 14 was consistently around 25fps slower than the 66.

Can anyone explain why this might be?

 

[HKSmith]

There's an excellent article about this phenomenon in the Speer Reloading Manual #14 - it's entitled "Why Ballisticians Get Gray" by David Andrews. It compares the muzzle velocity of 3 different .357 Magnum loads in 26 different revolvers with barrel lengths from 2.5 to 10 inches. There's also data on a T/C contender, 3 rifles, and a velocity test barrel.

Although the trend is for longer barrels to give higher velocities, the data show many examples similar to what you found in your revolvers. The usual explanation is differences in bore, forcing cone, and chamber smoothness and dimensions. I would also think that differences in the gap between cylinder and barrel would be a major factor.

The article is on pages 769-771 of the manual. It also appeared in some earlier editions.

 

[Nemo288]

A couple of things come to mind.
The barrel gaps may be different.
The dimensions and finish of the bores may be different.
The chamber and throat sizes may be different.
Each revolver and pistol is unique as there are so many variables.

I have 3 20" 44 magnum carbines. They vary from 1697 to 1775 fps with the standard 240 gr. load.
My 7.5" Redhawk always produces 50 more fps than the manuals that use it as a test gun.
It has a pretty tight barrel gap and no endshake.

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Nemo

 

[rwsmith]

There's an excellent article about this phenomenon in the Speer Reloading Manual #14 - it's entitled "Why Ballisticians Get Gray" by David Andrews. It compares the muzzle velocity of 3 different .357 Magnum loads in 26 different revolvers with barrel lengths from 2.5 to 10 inches. There's also data on a T/C contender, 3 rifles, and a velocity test barrel.

.

They even have several copies of some guns with the same barrel lengths that vary in velocity. The only way to know for sure is to get a chrono and run the numbers yourself.

 

[gwpercle]

That article should be required reading for all reloaders, especially if they have a chronograph, might help them to not get so upset when published data and thier loads don't exactly match. In this game every firearm and load is a law unto itself. Too many people don't understand this.

Gary

 

[njl]

I've got Speer #14...so I'll go read that section now. As to tightness/gaps, the 66 is mine third hand, and according to the previous owner, it was shot quite a bit by the original owner. Everything about it is looser than the model 14, which is mine second hand, and according to the original owner was shot very little.

It's a short section. It's pretty wild that you can have as much as a 20% difference in velocity for the same ammo in different examples of essentially the same gun. Even odder, some of their guns were faster than others with certain loads, slower with others.

I don't know that I'll shoot enough through it to ever find out, but it'd be interesting to see how a gun (like my 14) changes over time. i.e. as the barrel has more rounds through it, is it possible it'll speed up?

 

[Nemo288]

If you have a really tight, strong, gun that won't get loose with age
it may get faster with some barrel lapping due to just plain shooting.
I don't think it's going to amount to more than a few fps.

Keeping track of a gun's performance through its life with a given load
would be a very interesting and demanding project. I think I read about this
kind of thing but don't remember where.

Rough barrels will almost always respond to some lapping but this is
mostly done by the cast crowd to improve accuracy.
I plan on fire (and hand) lapping a couple of revolvers that have a ridge where the
barrel is screwed into the frame.

I am reading Veral Smith's cute little book and becoming a convert.
Some of his ideas are pretty much the opposite of Elmer Keith's.
LBT bullets have given me more accuracy in a revolver than any other kind.

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Nemo