[dbooksta]

In general a shorter barrel is capable of producing lower velocities from a given cartridge, holding bullet weight constant.

Also, a heavier bullet is slower than a lighter bullet in the same cartridge.

Does any general relationship hold between all three factors? E.g., are either of the following statements true?

A. A heavier bullet loses less velocity (in percentage terms) in a shorter barrel than a lighter bullet in the same cartridge?

B. A heavier bullet loses more velocity (in percentage terms) in a shorter barrel than a lighter bullet in the same cartridge?

Example: I am used to shooting 5.56mm from 16" barrels. Now I'm getting a 11" barrel. If I work up optimal loads for each, can I expect my 11" muzzle velocity to be closer to the 16" (in percentage terms) with 50gr bullets or with 70gr bullets?

 

[bsn]

I hope that 11" barrell is in a pistol or a SBR registered rifle.

 

[broom_jm]

It has been my experience that A is true: Heavier bullets suffer less velocity loss from a shorter barrel than do heavier bullets. I will throw in the caveat that I have noticed this to be the case with larger calibers than 22, but I would expect that it's true for small bullets, as well.

 

[gene]

I believe A is generally true also, however you should check out the AR 15 and M4 websites for their recomendations. There are some strong arguements for lighter bullets in the shorter barrels.
Hope that helps.

Regards
Gene

 

[pisgah]

I would suspect -- guess, really -- that the lighter bullet load would lose less velocity, percentage-wise, than the heavy bullet load.

Regardless, it is my opinion an 11" Ar15 is a great enrty-type close-range and home defense choice, but beyond about 100 yards you give up a lot more than just the extra 5" of barrel.

 

[unclenick]

What an interesting question. Thanks for posting it.

The answer is A, if you use the same powder and peak pressures for both bullets. However, if you pick the best performing powder for each bullet and run the two rounds at the same peak pressure, the difference will favor A less.

To understand this, first keep in mind that as a bullet goes down a barrel, the propelling gas has to chase it. This means, the faster the bullet accelerates, the more energy is used pushing the propellant's own mass down the tube after it. Since no system is perfectly efficient, it can't keep up 100%. As a result, a pressure gradient develops in the bore with pressure back in the chamber being higher than it is at the base of the bullet by a few percentage points.

The lighter the bullet, the faster a given pressure accelerates it, so the more difficulty the propellant mass, a mix of mostly gas and some burning powder mass, has playing catch up. This means, even if the chamber peak pressures are identical and the powder is identical, the lighter, faster accelerating bullets will be at a disadvantage because they have a bit less pressure at their bases. More powder energy goes into accelerating the propellant gases, so less is translated into kinetic energy in the bullet. The measure of this is called Ballistic Efficiency (B.E.), and is given as a percent of the chemically stored potential energy in the powder that becomes kinetic energy in the bullet by the time it reaches the muzzle.

Having said all that, you come to the question of what happens when the peak pressure is the same, but the powder is different. Usually one uses larger charges of slower powders with heavier bullets precisely because the heavier bullet's slower acceleration gives that slower powder more time to put its greater total energy into the bullet. This is matching the bullet to the powder, and when you do that, the percent losses get closer to one another.

This is the kind of problem QuickLOAD is useful for because, even without perfect information on the guns actually used, you can still make a relative comparison. Below are some results that illustrate the point. For the pair with each powder there is about 0.8% more velocity loss for the lighter bullet. But when you compare the H322 load for the 50 grain bullet, which produced its greatest muzzle velocities, to the IMR3031 load for the 70 grain bullet, which produced its greatest velocities at the same peak pressure, the difference drops to 0.5%. For the IMR4198 loads, the 50 grain bullet actually beats the 70 grain bullet's loss percentage with either of the other powders. Just not with the same powder. If I seat the 50 grain bullet out to 2.260" COL, the loss difference within powders increases to about 1.2% because the larger starting volume relative to the powder doesn't light up quite as quickly.

In any event, within normal velocity extreme spreads, especially for a short barrel which will show ignition inconsistencies more readily than a longer barrel does, the practical differences for 50 and 70 grain bullets is ignorable, IHMO.

For fun I also ran a 33 grain Speer TNT varmint bullet and the 77 grain Sierra MatchKing with their best powders. That opened the velocity loss difference up to 2% (15.7% and 13.7%, respectively, with H4198 and 748, respectively). Still nothing to write home about, but still obeying rule A. It should be born in mind that some bullets in some chamberings will have a more perfect powder available for them than others do, and in this case 748 was an exceptionally good match to the 77 grain SMK.

70 grain bullet seated 2.260" COL (SAAMI max)
50 grain bullet seated 2.165" COL (0.224" into case mouth)

                       20"             11"        % MV     Case
                     FPS, BE         FPS, BE      Loss     Fill

IMR4198, 50 kpsi

70 gr Speer SSP    2705, 31.6%    2324, 23.4%    14.1%     90.5%
50 gr Speer SP     3165, 27.8%    2691, 20.1%    15.0%     97.3%


H322, 50 kpsi

70 gr Speer SSP    2770, 28.7%    2354, 20.7%    15.0%     94.5%
50 gr Speer SP     3206, 24.7%    2701, 17.6%    15.8%    101.5%


IMR3031, 50 kpsi

70 gr Speer SSP    2833, 29.5%    2399, 21.2%    15.3%    102.7%
50 gr Speer SP     3274, 25.4%    2746, 17.8%    16.1%    110.5% (too compressed to load in most instances)

 

[broom_jm]

Nick, that was very insightful and I think it helps me better understand why this is less pronounced with larger caliber bullets.

 

[Helix]

The trick also may be that the heavier round may start out slower and lose V at a slower rate and may catch the lighter round as it loses V. also another interesting point may be where that particular round becomes transonic as normal tables/ External balistics formulas become far less acurate at this threshold due to the changing and plummeting trajectory from this point on. When you clock some velocities try looking at the different loads in a quality External balistics program to see the range and flight time for when the hit the transsonic threshold. one may show as superior in that sense although I doubt the 11 in barrel will be used out to ranges that this would be too relevant.

 

[unclenick]

If a bullet has a higher BC, then there is a range of muzzle velocities below that of a lower BC bullet over which it will catch up and pass the lower BC bullet. Too slow, and it will stop first, obviously. But BC is all that matters here. Not the weight in and of itself. A heavier bullet that is a full wadcutter, for example, won't likely have a high enough BC to overtake a lighter spitzer even if they are fired at nearly the same velocity.

The external programs are limited by how close a matching drag function to your bullet they have available. Many now have not only the usual G1, but several others to use that come closer to most modern bullet shapes. They will do better.

 

[dbooksta]

Nick, fabulous analysis! Thank you for the answer, and for continuing to offer your expertise to the community!

 

[Jakeway]

Too many variables to generalize.

I read an article last night (Field & Streammagazine, can;t remember what month) where someone was testing various bullets for terminal ballistics. He was shooting with a Ruger Compact with 16.5" barrel, to simulate velocities at farther range than he was actually shooting. For comparison, he also shot with a 24" barrel.

He kept the same bullet weight (150) with the same powder type and charge, in a 308 WIN. The 6 or 7 different bullets had different velocity losses between the 16.5" barrel and the 24" barrel.

 

[unclenick]

Yes, this was an all-else-being equal comparison, which in real life is hard to find, except on averaging over large numbers of rounds. You'll notice that at 0.5% you are under the velocity extreme spread for many loads in many rifles, so it can be hard to see in the noise under the best of conditions. Hence the need for a large averaged group to see the results at all. Nonetheless, if you take the same brand and bullet shape and construction and change the weights, I believe you will find that over a running average it tends to be true. Change both weight and construction (one boattail and one flat base, one jacket and core and one solid, or even just the same shape from two different manufacturers whose jacket thicknesses and jacket alloy and core alloy hardness aren't quite identical, etc) and the tendency most probably will be overwhelmed by barrel friction and start pressure differences.

 

[kraigwy]

For the What its worth dept. I built an across the course gun in 308, using a Rem 700 and a 26 in. douglas priemium barrel. Never did like the way it shot. I desided to use it as a doner rifle for a hunting rilfe for my Grandson. Before that, I let my Grandaughter and her little friend use it for their MiddleSchool Science progect.

There Goal; To determine the shortest barrel lenght for a hunting rifle, yet, have the assumed energy needed to still produce 1800-2000 Ft Lbs of energy @ 300 yards to humainly use on elk size animals.

They started with 26 inches, measuring the velocity after cutting two inches at a time down to 16 inches. We stopped there because I didnt want ATF to throw a hissy fit.

They used Factor Rem 180 Corlocs. I didnt see the need to buy another barrel to see if it made a differnace with lighter bullets. But any ballistic program will give you that info: Ref bullets slowing down via weight.

Anyway here is what she came up with: For the project purposes, you need 22-24 inchs for a 308 witch matches the sales of 308 hunting rifles. It was fun, but I wish I could have recorded all the giggling, thats what made it fun.

The first colum is Barrel lng. the MV, Me, 300 V, 300 E

Barrel Lenght 180 Vel Ke Vel KE
26 2826 fps 3,192 2240 2,005
24 2722 fps 2,961 2149 1,846
22 2713 fps 2,942 2141 1,832
20 2677 fps 2,864 2109 1,778
18 2654 fps 2,815 2089 1,744
16 2552 fps 2,603 2000 1,599

 

[Cavitation]

I dont know about others but it must be a very small difference. My 308 target 168grn A max load drops a hair over 150 fps from 24 inch to 16 inch barrels. Just about the same for my 110 vmax load.

 

[fishnngrinn]

Interestingly-I read that the .22 has maximum velocity from a 16 inch barrel. Velocity drops as barrel length increased. So why do they all produce 20 in barrels for the .22?

 

[dbooksta]

Interestingly-I read that the .22 has maximum velocity from a 16 inch barrel. Velocity drops as barrel length increased. So why do they all produce 20 in barrels for the .22?

Good question. What you read appears to be plausible, given the Ballistics By The Inch results here. And I can't think of a good reason for a longer barrel other than to produce higher muzzle velocity.

 

[F2G1D]

... good reason for a longer barrel ...

25% longer iron sight radius?

 

[dbooksta]

25% longer iron sight radius?

Oh yeah, forgot people still compete with iron sights

So the answer is, "The only (good) reason people use longer barrels on .22LRs is because they're shooting iron sights and the rules don't let them extend the front post past the muzzle?"

 

[laidback]

Wow, I just went to rifle college.

 

[kerrek]

Interesting articles on I believe it is either Shooting Times or Rifleshooter websites about velocity loss by barrel length. Know for sure they have one on 300 win mag, but think there are other calibers as well.