Barrel Length and Bullet Velocity Ratio

[CB11WYO]

Alright I need some experienced input...

The way I've seen it/been taught to see it is that 1" of added barrel length will result in about 50 fps added velocity... But I've recently read a few articles that seem to suggest otherwise.

For example: The way I was told, a given bullet which will reach 3000 fps from a 28" barrel will probably be capable of about 3050 fps from a 29" barrel given that the other elements are roughly the same (powder charge, primer, twist rate, bullet weight, etc...)

Is this a correct method of thinking or is it not even close to correct?? How much does it differ with different cartridges? If you could please give some examples from your own experience it would be much appreciated...

Thanks in advance!

 

[Kevin Thomas]

CB11WYO,

There's very little in this game that's linear, and this is yet another example of that. Velocity gain/loss in re barrel length varies substantially with the barrel lengths we're talking about, the expansion ratio (cartridge capacity), the bullet weight and many other factors which come into play. Bottom line is, it's all over the place, and virtually impossible to say that X increase or decrease in barrel length results in Y velocity change.

Sorry, and I wish there was a simple answer to this one, but the question is a little too broad for a single, concrete answer.

 

[Stanm70]

Yea I agree, everything in shooting is relative. Logically it can and does increase velocity, but exactly how much depends on your rifle.

 

[luke]

I agree. There is no positive way to calculate what velocity you will get. Example i have a savage 308 with a 26" barrel. It shoots a 155gr amax at 2955 average on the last 5 shot string through my chrono. My buddy has a savage 308 with a 24" barrel. Same exact load same chrono on the same day. His 5 shot average was 2951. But I have a 168gr amax load that shoots 2870 out of my 308. The same load in my buddies rifle only shoots 2830. Hope my experience helps you out.

 

[CB11WYO]

CB11WYO,

There's very little in this game that's linear, and this is yet another example of that...

That's what I was afraid of... oh well

Thanks!

 

[CB11WYO]

I agree. There is no positive way to calculate what velocity you will get. Example i have a savage 308 with a 26" barrel. It shoots a 155gr amax at 2955 average on the last 5 shot string through my chrono. My buddy has a savage 308 with a 24" barrel. Same exact load same chrono on the same day. His 5 shot average was 2951. But I have a 168gr amax load that shoots 2870 out of my 308. The same load in my buddies rifle only shoots 2830. Hope my experience helps you out.

Yup! Thanks that's a good example.

 

[CB11WYO]

Just too many variables to work with, too many gun "personalities" in the mix I guess.

 

[Kevin Thomas]

Just too many variables to work with, too many gun "personalities" in the mix I guess.

CB11WYO,

No, that's not what I was saying. You can get some very good ideas about what will happen in specific instances, when you're talking about a given caliber, a particular bullet weight and a certain powder. In your original post, the question you asked was just asked too broadly. You CAN get a pretty good idea of what barrel length variations will do to a load, but you need to be very specific about the components involved in that particular instance. Simply saying that an increase of X will increase or decrease velocity Y fps (regardless of caliber, bullet weight, etc., etc.,) just can't be answered accurately.

Hope that clarifies it a bit, and didn't mean to discourage you from asking.

 

[Franklin]

This is a good read for those that haven't.

Lilja Precision Rifle Barrels - Articles: Barrel Lenghts and Velocities in the 338/378 Weatherby Magnum

 

[CB11WYO]

CB11WYO,

No, that's not what I was saying. You can get some very good ideas about what will happen in specific instances, when you're talking about a given caliber, a particular bullet weight and a certain powder...

That's a little more what I was after...

Like say I'm loading for a 7mm Rem Mag, using 180 gr bergers and Retumbo. Say reloading manual brand "X" says my load will do 2900 fps from a 26" barrel, but I'm shooting a 28" tube... what kind of increase should I be looking for? Etc, etc...

(yes in the end I can just validate my velocity using trajectory anyway...)

 

[Stanm70]

Like we said before that is a totally variable question. Logic says a 80-100 fps increase but there is no way to say for sure. It may be no velocity change at all.

 

[luke]

Another example I have for you is my wifes 7mm rem mag. It is a tikka with a 24" barrel. According to my book the load she shoots should be around 29-2950 with a 162sst. That gun bearly breaks 2800fps. The same load in my old savage shot nearly 3100 with the same length barrel. Not sure how exactly it works but there are fast barrels and slow barrels. Hope this is helpful. But probably not.

 

[TOM H]

That's a little more what I was after...

Like say I'm loading for a 7mm Rem Mag, using 180 gr bergers and Retumbo. Say reloading manual brand "X" says my load will do 2900 fps from a 26" barrel, but I'm shooting a 28" tube... what kind of increase should I be looking for? Etc, etc...

(yes in the end I can just validate my velocity using trajectory anyway...)

I shoot longer barrel than the loading data I'm using. Working up my loads I see an increase in velocity with my start loads and most times I'm at published velocity gr or two under max load.

Nosler start load for the 280AI with 26" long barrel was 57.5gr/IMR-7828 @ 2856fps with 160gr AB that same start load in my 27" long barrel was @ 2916fps which was gain of 60fps with 1" more barrel.

One of my 30-06 was playing around with some of Hodgdon data using H-4350 and 150gr E-Tip their barrel was 24" long mine was 25". their max load was 57.8gr/H-4350 @2927fps and my max load was 58gr @ 2998fps. Another 30-06 with 26" long barrel with 180gr Partition max in Nolser data is 2825fps I can run that easy 100fps more with varies loads.

I build a 300WSM with 26.5" long barrel and it's easy getting that extra velocity. I have pretty good list on gains with some of my barrels but in the end I stay pretty close to max velocity and nice to know don't need max powder. 30yrs ago when I was in my 40 looked at velocity little different. Well good luck

 

[Kevin Thomas]

CB11WYO,

Okay, I an this on QuickLoad, with the component combination you cited; Retumbo, Berger 180s in a 7mm Mag with a 26" and a 28" barrel. It showed a difference of about another 50 fps in the longer barrel, or about 25 fps per inch. This is about what I would have figured, but it could well change a bit with a different powder, different charge weights, etc.. As I said, it isn't linear, and will change with the variables in play. Ballpark figures of 20-40 fps per inch is usually the figure that gets bandied about, and it's roughly correct.

Bottom line is, no matter what an interior program like QL says, it's the chronograph that will give you the final answer. Hope that helps.