velocity vs barrel length

[old #7]

I don't have a chrony YET. It seems like a lot of loading data spec's a 24' barrel but most of my rifles have 22' barrels. What would the average drop in velocity be for a 2' shorter barrel? How much of an increase would a guy have in 26" barrel?

 

[CamoWildcat]

I would say somewhere between 25 and 50 fps per inch. Figures seem to vary greatly.

 

[Guy Miner]

Same 155 grain target load from two different .308's of mine:

24" = 2850 fps avg
30" = 2980 fps avg

Both shoot real well, but there's more differences between them than just barrel length.

I saw a surprising difference between my 18.5" Marlin Guide Gun and the 22" Marlin 1895, both with the same .45-70 loads... Will have to look it up, but it was around 180 fps difference between the two guns, with the same load. That one surprised me, I hadn't expected so much.

I think that variations in bore diameter, chamber dimensions, twist rate, and the roughness of the throat and bore in general all play a role in determining velocity, not just barrel length.

Get yourself a chronograph - then you can befuddle yourself plenty! :grin:

Regards, Guy

 

[CamoWildcat]

Amen. They should have called them 'Befuddlers' instead of Chronys. :lol:

 

[Eagleye]

This is something that is very difficult to actually predict, since each barrel seems to be a bit of a law of it's own. If you had a 24" barrel, and cut it to 22", then you would actually realize a drop in velocity. How much would depend on several factors, such as: cartridge capacity, powder burn rate, bullet bearing surface, etc. However, two barrels of different length may really surprise you. I will illustrate from personal experience. I had two Ruger #1 rifles chambered in 30-06. One was the #1AB with a 22" barrel. The other was a #1B with a 26" tube. I shot both rifles with a couple of identical loads. Load number 1 was 180 Nosler Partition and 60 grains of IMR 4831. In the 1AB it chrony'd 2824 average for 5 shots. In the 1B it chrony'd 2766 for 5....4" more barrel length, 58fps LESS velocity!! Load number 2 was 165 Nosler Partition and 58 grains of Nobel 104. 1AB chrony'd 2915fps for 5, the 1B only 2871. Again the longer barrel gave less velocity, this time 44fps. Thinking there must be a simple answer, I checked into these rifles further. I slugged both bores, but could not find any difference in the diameter of the lead slug from either bore. I measured throat length, and found the two throats only .005" different in length, with the 1AB having the longer throat. Go figure! You can never be sure what will happen. But usually, shorter barrels will deliver somewhat less velocity than will their longer counterparts. In the 30-06, about 20-25 fps per inch. In a 220 swift, 50-60fps per inch. Regards, Eagleye.

 

[Guest]

I'd like to second what Eagleeye said about powder burn rate. I helped a buddy of mine load for a T/C Contender Pistol in 30-30WCF many years ago, and he was getting about 1900-2000fps on his chrony through a 14" barrel. My 30-30WCF in a Model 94 Winchester with a 20" barrel was giving, at the time, about 2100-2200fps with the same bullet weight (150gr). The difference? Powder choice. In a shorter barrel, you can't realize the full effect of slower powders, and as such the last few inches of barrel may be some of the best as far as acceleration is concerned. In this case, with 6" of barrel length difference, we were seeing only a couple hundred fps difference. I have since found some other powders perform better in my gun, and I can reliably get 2300fps on a chronograph without being over max load and with no pressure signs. But the lesson is there. Check pistol loads if you really want to see the difference. Loaded with 19gr H110 under a 125gr bullet, my 4" 357Mag is significantly slower than a friend's 7.5" barrel with the same load. He gets the most out of the slow powder.

 

[FOTIS]

I don't have a chrony YET. It seems like a lot of loading data spec's a 24' barrel but most of my rifles have 22' barrels. What would the average drop in velocity be for a 2' shorter barrel? How much of an increase would a guy have in 26" barrel?

A lot of this depends on the caliber too.

IE a 257 wby will lose a great deal more velocity in 2 " than say a 308 or 30-30.

 

[rick smith]

Good article in Rifle Or Handloader on this subject in the last month.Rick.

 

[fredj338]

A good rule of thumb is 25fps for large bores & std. cases. Up to 50fps for overbores. It's only a guess & since you have not chronographed your own rifle, you have no idea how fast your bullets are going. Next time you are at the range & someone has a chrono, ask if you can run a shot over the screens. I have never turned anyone down. :grin:

 

[old #7]

I am mainly concerned with my 300 wm and 270 win.
Thanks for all of the input.