I emailed Speer Ammo today to get a fps figure on their .38 Lawman 125 gr. TMJ and they got right back to me with "865 fps from a 4" VB." Is 4" a standard barrel length for fps numbers?

It is "A" standard length but not "THE" standard length. Depending on the caliber and type of handgun the load is designed for -- or even just whatever length barrel their test gun has -- you may see velocity figures quoted for 2,4,6, 7.5", or whatever. The ones that drive me nuts are the ones that don't specify length. At least with a length given, you can make some reasonable guesses up or down depending on your gun's length.

Thanks, Pisgah. Is there a rule of thumb for those reasonable guesses? For instance, a likely percentage you'd raise the fps going from 4 to 6"? And guessing further, is it reasonable to guess that WC numbers would be based on 6" while might be based on 4"? Seems to me most of the fps numbers I've seen are given without a barrel length.

Oops. Sorry, please insert "JHP" between "while" and "might" in my second to last sentence.

FPS differences with barrel length can vary depending on powder and cartridge. One rule of thumb I have seen used for pistols is add 60 to 100 FPS for 6" versus 4" but this is general.

A lot of velocity variation can be seen even between two guns of the same make, model and barrel length. The new Speer #14 reloading manual has a discussion of this.

Real world a "fast" 4" gun might give the same or higher velocity with a given load than a "slow" 6".

Lots of variables involved, everything from chamber dimensions to barrel-cylinder gap to barrel dimensions and interior finish can affect things in addition to variables such as test temperature etc.

I also wonder about chronograph accuracy and stability from one unit to another. I worked in QA for years and in industry measurement tools are calibrated regularly, minimum of once every 6 months to year depending on type of device. I have not heard of amateur level chronographs being lab calibrated regularly.

Speer #14 lists specific firearm used for velocity testing. On a couple of the super magnum revolvers they note that the listed test gun velocities are over 200 FPS lower than the velocities they got from their pressure test barrels.

Hodgdon lists barrel length only and many of their velocity readings are high enough I suspect that they are from the pressure test barrels rather than actual revolvers.

Very enlightening post, Rich, thanks. The variables are interesting to know about, and I will use your 60 to 100 fps add formula for considering 6 vs 4" barrel lengths. Thanks again.

quote:

Real world a "fast" 4" gun might give the same or higher velocity with a given load than a "slow" 6".

From using my own chronograph, I can confirm that the "rules of thumb" for velocity are unreliable in comparing different guns.

Example: I chronographed the same .40 load with the same chronograph and found my Browning .40 HP Practical is 40fps FASTER than a G35 owned by another family member, even though the G35 barrel is 3/4 inch LONGER and "everybody knows" the Glock rifling is faster.

I have a 4" 686 that is consistently 30 fps faster than my 4" 66.

Until you chronograph a specific load in a specific gun, you don't know what you are getting.

This message has been edited. Last edited by: OKFC05,

I guesstimate by adding 75 fps. per inch, but that is all it is, a guess.

Thanks, guys. Interesting and humbling. Every time I think I've figured something out about bullets in particular more complexity lies beyond. I was collecting fps numbers to calculate felt recoil. It's not going to be as easy as I thought given especially the meager info manufacturers report. Plus these issues about barrel quality. Thanks, nevertheless.

It seems to me that in years past the gun mags would do tests. They would cut off a barrel one inch at a time and chronograph the velocity. IIRC the loss was 35fps per inch in a revolver.I sure do wish I had saved those articles to confirm that figure. I'm sure someone out there has copies and may post that info. Guns do vary as stated. My 686 gives higher velocites than my model 19 with the same loads. Bruce

quote:

Originally posted by Bruce Lee M:
It seems to me that in years past the gun mags would do tests. They would cut off a barrel one inch at a time and chronograph the velocity. IIRC the loss was 35fps per inch in a revolver.I sure do wish I had saved those articles to confirm that figure. I'm sure someone out there has copies and may post that info.

I had the said aarticle that was published in the Rifleman many years ago, along with the retraction and apology.(lost it in a move) The numbers given were wrong, and even showed HIGHER velocity with a 1" barrel than with a 3"!!!!

Why? The got too close to the chronograph (nothing to sight with) and were measuring the pressure wave, not the bullets. A basic rule of chronographs is to stay far enough away (like 10') to NOT measure the pressure waves.

The rules of "so much per inch" aren't even logical, since 1" off 8" is 12% reduction, while 1" off 3" is 33% reduction.

Everyone's right. The only "rule of thumb" is that "Shorter=slower and longer=faster" -- and even that one is not always correct.

I once owned 2 Ruger Super Blackhawk .44s, one with a 7.5" barrel, the other 4 5/8" .The shorter one consistently equalled or exceeded the velocity of the longer. Go figure...

quote:

Originally posted by keppelj:
Is 4" a standard barrel length for fps numbers?

Wouldn't that be nice?

Remington publishes using 4" as the barrel for most of their .357 Magnum ammunition, and 8⅜" for their heavy hunting bullets. Smart move on their part.

Winchester describes using an 8" barrel for their 180-gr Partition bullet but makes no mention of barrel length on all their other .357 ammo.

I can only mutter to myself when I see some of the crazy barrel lengths used for load data in reloading manuals.

Hodgdon used a 7.7" barrel for their .38-Special loads. And an unbelieveable 10" barrel for their .357 Magnum recipes! WTH?????

Thank God for Speer using real guns with normal barrel lengths (most of the time).

Here's the real answer.

http://www.leverguns.com/articles/ballisticians.htm

I once did a test on 9 kinds of .45acp factory ammo and 1 reload, comparing their velocity in a 4" M625 and in a Marlin Camp Carbine. I did this after being told on another forum that there would be no significant increase in velocity with the carbine. I no longe have the Marlin, but think the barrel was 16.5", or something like that.

In some rounds there was a meaningful difference. The respective velocity increases in fps were 60, 80, 80, 122, 139, 166, 191, 194, 222, and 251. The wide variance I believe was determine by whether the powder was "fast" or "slow" burning, or somewhere in between. A 225 grain lead SWC traveling at 1142 fps packs quite a whallop!