fps question vs barrel length

223

Distinguished Member
how much fps velocity would you expect to loose, if a load says 2900 fps in a 26 inch barrel but you use that load in a 20 inch barrel instead

i know the only way to be certain is to chrono but anyone know roughly ????

ATB

Dave
 
I have a 20" barrel and most manufacturer load bullet speeds are based on 24" or 26" barrels. Based on the chrono results I got on factory Norma I think you will find it that you will loss significantly less than 100 fps per inch (although I have heard that figure being given before as rule of thumb). My guess (there are a number of unknown variables) is you will lose between 65-80 fps so 390-480 fps overall.
 
It really depends on the calibre.

My 14.5" .243 shoots 75gr v-max at 3200fps. The same load through a mates 22" barrel is 3600fps.

You are looking at 50fps per inch where as .308 you are looking at 25fps per inch.
 
In Shooting Times of 16th August 2007 (page 24) there was an article written (I think) by Pete Moore. He made practical tests with a .308 and a .223 by progressively shortening the barrels and measuring the MV over the chrono. He compared it with the computer generated predictions of 'Quick Load'. For all practical purposes the results were very similar.
Practical test for .223 gave 24'' 3,402 fps mv, whilst 20'' gave 3,253 fps mv and 18'' gave 3,185 fps mv. Using a 52 grain bullet.
The .308 gave 24'' 2,905 fps mv, whilst the 20'' gave 2,851 fps mv and 18'' gave 2,748 fps mv. Using a 150 grain bullet.
Reducing from 24'' to 18'' the computer prediction was a loss of 194 fps whilst the practical test showed a loss of 217 fps for the .223. Difference 23 fps.
The .308 predicted loss was 189 fps whilst the actual was 157 fps. Difference 32 fps.
It was after reading the article that I had my Sako 75 .243 shortened to 20 inches and ordered my Sako 85 .30-06 with a 20'' barrel.
One cautionary note with the .243 is whether it will generate 1,700 ft lbs muzzle energy with the shorter barrel. Some factory loads, particularly 100 grain could be maginal as they do not meet the claimed mv when tested over the chronograph. I home load using a Speer SPBT 85 grains at approx 3,100 fps mv, so no issue for me on that score. I hope you find this useful.
 
Uncle I think you are mistaken with regard to the author of the article, it was Bruce Potts. Pete Moore is the editor of Shooting Sports. The one is well qualified and respected the other is ..... well the editor of a magazine.
 
Uncle I think you are mistaken with regard to the author of the article, it was Bruce Potts. Pete Moore is the editor of Shooting Sports. The one is well qualified and respected the other is ..... well the editor of a magazine.

8x57 I like that. Very asute.

I would certainly take Mr Potts advice over that of Mr Moore's.
 
Thanks for the correction and my apologies to Mr Potts. I cut the results part of the article out and kept is but alas, not the name of the author. It certainly helped me in drumming up the courage to shorten my Sako 75.
 
As a rough guide it's 100fps per inch.

On what planet?

As the lads stated above, it is quite calibre dependent, some rounds are a lot more forgiving than others when it comes to barrel chopping. Basically the bigger the case and smaller the calibre, the more velocity you lose per inch.

The 308 is very forgiving, 20fps per inch seems typical, but once you go below a critical point, maybe 19" or thereabouts, you can get sudden big drops. My 260 lost about 50fps total when I took 2.5" of the barrel, so again 20fps/inch roughly.

Rounds like the 25/06, 7mm Mag and even the 243 are a lot less forgiving, you could easily lose 50fps per inch.

I have experimented a lot of the past few years, 20" barrel length seems about right to me for a moderated rifle shooting a round on the '08 or similar sized case.
 
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how much fps velocity would you expect to loose, if a load says 2900 fps in a 26 inch barrel but you use that load in a 20 inch barrel instead

i know the only way to be certain is to chrono but anyone know roughly ????

ATB

Dave
The gurus vary between 25 and 100 ft per sec and without a chrony or a good software package it is mostly guess work . In lopping 6 inches from a barrel of say an 30-06 ,( a drastic reduction ) and using the same load of say 4381 you would probably lose velocity at the 100 fsec end but you would gain a very big flash .. using a powder with a faster burn rate to keep most of the burning powder in the barrel where it is intended to be then the loss would probably be at the lower end .. This is where good handloading practise would be helpful .. Rob
 
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