.308 Muzzle Length vs. Velocity

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PlatinumCore16

New member
In researching what rifle I'd like next(really already have made the decision), I came to a question: what really is the difference between muzzle lengths? There are a plethora of threads here on the subject of muzzle length vs. velocity in general, slightly less so on the .308 specifically. A few link to some awesome articles/experiments including, but not limited to:=
Ballistics By The Inch
Chuck Hawks
Long Range Hunting

If you've never read BBTI, go check it out. Their handgun caliber experiments interested me the most, but they have more than just that.

However I found another experiment with direct affect on my rifle choice (looking at .308):
Rifle Shooter

Now personally for my .308 I am looking for a general purpose rifle/hunting rifle. If I was to get a long range target rifle, I would get 300WM. The Savage Scout 11 has an 18" barrel, which I kind of wondered about accuracy from, specifically 500yds plus, though I understand accuracy is made up of a lot of things. Their standard line of 16 is a 22" in the .308 and I wanted to know if I was losing out on anything except handiness. Also many AR10/semi auto .308 rifles have sub 20" barrels and I never really read anything about them being inaccurate(bolt action vs. semi auto is not within the scope of this post).

So it turns out that a shorter barrel just loses you some velocity (which we already knew), but just how much is extremely dependent on many variables and doesn't really affect accuracy as much as people like to quote. Personally I'll be fine with an 18" barrel and if I need more velocity, I'll be handloading.
 
Even from the same barrel length there can be 100fps or more difference with the same loads fired in different rifles. My 18" 308 shoots the same loads 50-60 fps slower than my 22" 308's. The same applies to other 308 class cartridges as well. You'd do just fine with a shorter barrel in 7-08, 260 or 6.5 Creedmoor.

It wouldn't be unusual to see an individual 18" or 20" rifle of any caliber shoot faster than an individual 22"-24" rifle shooting the same ammo. IF the shorter barrel happens to be the faster barrel. If the longer barrel happens to be the one that shoots faster it wouldn't be unusual to see a 2" longer barrel shoot 150 fps faster than the shorter one.

This is why you see guys swear that you lose a lot of speed from a shorter barrel. If they ever compare a fast long barrel to a slow short barrel they can come to the wrong conclusion. The only accurate way to measure this is to run the same loads through the same barrel and measure velocity as it is cut shorter. This has been done many times and results are available.

Other cartridges will see a greater difference. With 308 class cartridges 15-20 fps/inch is normal. Magnum cartridges may see 25-35 fps/inch with 30-06 class cartridges somewhere in between.

None of this effects accuracy. In fact all things being equal shorter barrels are stiffer and more accurate. You can get to 1000 yards in a 308 with an 18-20" barrel.

If I was to get a long range target rifle, I would get 300WM.
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I wouldn't recommend that anymore. A 6.5 Creedmoor will have the same trajectory and wind drift, but with 1/3 the recoil and equal or better accuracy.

Todays better bullets are game changers. Muzzle velocity isn't nearly as important as down range velocity. A 308 shooting todays high BC bullets is moving faster at about 600-700 yards than a 300 WM shooting old school bullets. This has largely negated the need for fast muzzle velocity and heavy recoiling rifles. This is exactly why the 6.5 Creed has become so popular. It starts very aerodynamic 140-147 gr bullets at a modest 2700-2800 fps with virtually no recoil, but they are still supersonic at well over a mile. Almost 2000 yards. That beats 300 WM by over 300 yards.
 
"With 308 class cartridges 15-20 fps/inch is normal. Magnum cartridges may see 25-35 fps/inch with 30-06 class cartridges somewhere in between."

I think that's pretty conservative. My personal chrono checks indicate that the .223 loses much more-closer to 50 fps per inch. My 257 Wby loses 200 fps when it's 24" barrel is compared with a factory 26".

I'll go out on a limb and suggest that a 16" .308 is much closer to a 30/30 than I'd be comfortable with.
 
It varies with the powder used.

The "Quick Load" program is pretty accurate when comparing rounds using the same components. I found it pretty accurate.

Several years ago my grand daughter did a science project where she cut a 308 barrel from 26 to 16, two inches at a time, and running the rifle through a chronograph. I don't know what I did with the results she came up with, but I do remember checking her results with Quick Load. It was pretty darn close.

I use the same load for competition in my 308s. 42 gr 4895 pushing the 175 SMK, in my Model 70 Target rifle (24 inch barrel) and my M1A Super Match (22 inch barrel).

QL says 2586 fps for the Winchester and 2540 for the M1A. That's right on with my coronagraph.

But a note about the 308. When the Army was shooting international 300 meter shooting, they determined the most accurate velocity for the 308 was 2200 fps.

Years ago when I was heavy into HP competition I tried for the most accurate and tried to get to 2200 fps. It was accurate, but since I was shooting for the Guard, I had to use the M14/M1A rifle. To work the action consistently I ended up going back to the Army's Standard of 2550 for the Match 308 loads. Since I shoot the same load in both my Winchester and M1A, I stuck to my standard target load.

I don't know why people try to get hyper velocity out of a given round.

I have a couple 300 WM Model 70s and lots of 30-06s if I need more velocity out of a 30cal round.
 
At least for me, the .308 has been about 20-25 fps/inch whereas the .223 has been more in the range of 40-45 fps/inch.

My favorite load is the Nosler CC 140 grain at 2800 fps out of the DD5V1 which is a 16" barrel. Out to about 600 it is a great load that is ballistically very close to my .223 match load. That helps me keep the same holds the same under the match conditions of 3Gun where shooting from 1 yard to 600 yards with varying in between distances on the clock is needed. I have found that when I get past the 165s, the SD starts to go up a bit and the losses move closer to 30-40 fps/inch. Accuracy falls off before I get to 600 as well with the heavier bullets. For those reasons, my 16" .308 is now held to shooting 110s to 155s.
 
Interesting point on the 6.5CM. Even with a 200gr+ bullet with high BC in a 300WM, you're telling me a 147gr from a 6.5CM would do equal/better? If that was so, why do long range shooters prefer the .338 lapua? I'm not looking to buy a .338 because I like my shoulder just the way it is and I'm not about to go shooting $5 per pull of the trigger. Maybe when I'm a millionaire, but that's a long ways away.

Not saying you're wrong, but when it comes to wind drift, a heavy, high BC bullet was what I thought did the trick.
 
Not saying you're wrong, but when it comes to wind drift, a heavy, high BC bullet was what I thought did the trick.
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No right or wrong here, been discussed many times. Use a smaller/faster bullet that has shorter flight time, or heavier/slower with a higher BC. Some of the smaller 6 and 6.5 VLD's are posting impressive BC's on paper, whether they verify in the field is another matter.

Velocity isn't a real consideration in your case, and keep in mind that a shorter barrel of the same contour will be stiffer than one that's longer which is an advantage (all other factors being equal).

but they are still supersonic at well over a mile. Almost 2000 yards. That beats 300 WM by over 300 yards.
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Hornady's 4DOF shows the 140 ELD Match at 2900 (I don't think one could even get there) goes subsonic at less than 1400 yards...1099 fps at 1400.

What bullet/MV from the 6.5 Creedmoor will still have a velocity of 1125 fps at 2k yards?
 
308 by the inch

I found an old article by the late Finn Aagaard on just such a thing.The rifle he used was an old Israeli M98.Now this article is almost 30 years old so take it for what it's worth.Five different factory loads and several handloads all grouped in 3.5" at 100 yards without changes to the scope he says.The barrel was chopped from 22" to 20" and again to 18.5" and tests were done to check velocity.Most loads lost less than 100 fps.This is from American Rifleman April 1989.
 
Muzzle length. I think you came up with a new term there, partner. The muzzle I know is the business end of the rifle - length is zero inches.
 
The only accurate way to measure this is to run the same loads through the same barrel and measure velocity as it is cut shorter. This has been done many times and results are available.
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I agree that's the accurate way to do it. There's only two small problems doing it that way.

#1 your results apply accurately, ONLY to that one barrel...
#2 when you get done, you're stuck with the short, lowest velocity barrel, there's no going back without getting another barrel, which may give you significantly different speed from the get go.

Chronographs are wonderful things, they prove and disprove all kinds of notions. Sometimes, they show you things you wish they didn't. ;)

Friend of mine thought his .270WBY was the bees knees, took the published speed as gospel, until he got a chronograph, and discovered (to his dismay) that he had a "slow magnum", and his 22" .270 Win was actually about 12fps faster than his 26" .270WBY.

EVERY gun and ammo combination can be different. Most fall within a close range of each other, but there are some at both ends of the bell curve.

Compare two close together on the curve and you get one kind of result. Compare those at opposite ends, and you get something quite different.

And there's no way of knowing which is which, except by shooting them.
 
I didn't read the links posted by the OP, but another thing that I don't believe has been mentioned yet is barrel length (not muzzle length) vs velocity is not linear. A 26" barrel in .308 will lose less velocity than a 18" barrel if you trim 1" off of both. The bright line "about 20 or 25 fps" is fictional. That may be the case going from 22 to 21, but it will be less if you go from 26 to 25. More going from 17 to 18.

FWIW, .308 is a fairly efficient caliber to run out of a 18" barrel. You will lose velocity and energy, but not as much percentage-wise as heavier cartridges. It will not turn into a 30-30, however a 30-06 likely would effectively turn into a 308 if fitted with an 18" barrel.
 
"I'll go out on a limb and suggest that a 16" .308 is much closer to a 30/30 than I'd be comfortable with."

I'm not so sure I'd go along with that. The .308 is loaded to significantly higher pressure than the 30-30 for one thing and my guess would be that it's closer to a .300 Savage.

I have several rifles in .308 Win. but I'll compare two. One, the Ruger M77 RSI with 18.5" barrel at a velocity of 2550 FPS. The other is a 22" barreled Winchester M70 that clocks at 2610 FPS. The load is a stiff charge of W760, Winchester brass and primer and the 165 gr. Speer Hot Core. That's a 60 FPS difference which is close to .300 Savage in the RSI. I think that with a more appropriate powder probably a good 100 FPS gain might be achieved from the RSI but as history with those three have proved, lousy accuracy.

The powder is a tad too slow for the .308 in my opinion but the load was specifically worked up for the RSI. I have three of those rifles in .308, all purchased for very low prices because their previous owners said they were totally inaccurate. To make a long story short, it took me two years to get the first one to even do 1.5" at 100 yards. On a good day it might do 1.25". The load works for all three rifles, yet nothing else tried has come close. That load also runs at .75" average in the M70. FWIW, the barrel on the RSI is similar to a soda straw, very thin.
Paul B.
 
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