To what extent does barrel length affect the velocity of your full-bore rifle? In the name of science and with some trepidation takes up the hacksaw
In all my years of shooting and writing, the single most commonly asked question other than "What is the perfect calibre?" is "How much will I lose in velocity terms if I reduce the length of my stalking/fox rifle?" The issue usually arises as part of a dual-purpose enterprise, namely to create a lighter, more toutable rifle and, more significantly, to keep the rifle's overall length manageable after fitting a sound moderator.
When it comes to the velocity loss per inch reduction of barrel length, there are factory-set guidelines at differing velocities (see Table 1), but these seldom reflect the diverse range of calibres most shooters encounter. Each barrel's unique internal dimensions will also affect a bullet's flight, not only in terms of accuracy, but also of bullet acceleration and retardation. Bullet weight, shape and material, as well as the burn rate of the propellant used, have an enormous effect on a bullet's velocity therefore the only truly accurate way to check velocity drop was to cut a few barrels and measure it myself. I chose two calibres from either end of the velocity spectrum to give a clear idea what was actually happening.
Setting up the kit RPA, the rifle manufacturers from Tonbridge, Kent, kindly donated two barrels for test, one in .223 and the other in .308. This would give me a small-calibre fox cartridge that doubles as a roe deer round in Scotland, while the .308 is an industry standard deer calibre. Keeping things simple, I decided to use factory ammunition to reflect an average velocity, but soon realised I needed to use reload data as well. Only velocity loss would be measured, as to quantify accuracy change at the differing barrel lengths would involve the lengthy task of having to recrown the muzzle after each length-reduction, though I did deburr the cut.
To cater for all shooters' opportunities, I wanted to try this test with both factory ammunition and reloads. You haveto remember, however, that most factory ammunition is loaded for the average rifle, so it stands to reason that shot-to-shot velocity consistency is often not as good as that of tailor-made reloads. Initially, therefore, I put the 24in barrel lengths to test by shooting a string of five shots, repeating this twice for both reloads and factory ammunition, then analysing the results. It was instantly evident that both the .223 and .308 factory ammunition showed a wide range of velocity.
The .223 Remington 50-grain Accutips averaged a velocity of 3,425fps, but had a range of 120fps between the lowest and highest velocities, while the Norma 50-grain V-Max averaged 3,515fps and had a velocity range of 80fps, which would make pinpointing the actual loss in velocity-per-inch reduction a bit tricky. Consequently, I made a reload to shrink the velocity range to a minimum and ended up with a 52-grain Speer bullet travelling at 3,402fps from the 24in RPA barrel length, using 24.5 grains of Vit N133 powder. More importantly, the velocity range was a mere 31fps and so very consistent.The same rationale was used for the larger .308 calibre test barrel.
I used factory ammunition, utilising the standard .308 bullet weight of 150 grains and settled on the Federal Power Shok, which generated 2,914fps and 2,830ft/lb of energy, but had a velocity range of 71fps. A reload to reduce the overall velocity range and increase consistency was generated with the aid of the excellent PC-based Quick Load ballistics programme, so I had a Sierra 150-grain Game King travelling at 2,905fps almost the same as the factory fodder but with an overall range of only 27fps, thus reducing the window for inch-loss analysis of the barrel reduction.
The first cut is the deepest
Barrel reduction was actually easy, if initially a little scary: the RPAs shot groups of less than 0.5in from the off taking a hacksaw blade to barrels of that quality and accuracy was a little difficult, believe me. Means must, however, and I had soon measured out inch increments in order to aid cutting, with the barrel supported in a Ken Farrel barrel vice. As I mentioned, it was only velocity reduction, not accuracy, that was being recorded, so a simple deburring of the sensitive muzzle and crown areas was undertaken between each cut. To measure the muzzle velocity of each calibre as the test progressed, a five-shot string was recorded over a chronograph for every inch of barrel reduction.
You will immediately see the correlation between the predicted computer-generated data and the real-world experiment (see Tables 2 and 3, overleaf). Taking into account the vast variances of the differing barrel makes and their internal dimensions, the results were really good and, to my mind, highly valid.
Take the .223 calibre test first: 3,402fps velocity from a 24in barrel is a good starting point a good varmint/fox load and also, for Scotland, a legal roe deer load, generating 1,337ft/lb energy. Reducing the barrel, inch by inch, to 20in only loses 149fps, very little in practical terms, yet it brings real benefits as far as weight reduction and overall length are concerned. Taking an extra 2in off results in an exceedingly "trim" rifle, though you do start to notice an increase in muzzle blast.
At 18in, the 3,185 fps velocity and 1,171ft/lb energy is only 217fps short of the initial velocity so what does that mean to a fox shooter or stalker of Scottish roe? Well, zeroed at 100 yards with the starting velocity, you are -2.1in low at 200 yards and -9.5in low at 300 yards. No problems there, but what about with the shorter 18in barrel? A reduction of 217fps equates to a zero of 100 yards, a drop of -2.5in at 200 yards and -11.3in at 300 yards. At 200 yards, therefore, there is only a -0.4in difference, and at 300 yards only a -1.8 difference. Gains in terms of weight and length reduction far outweigh the minuscule velocity loss. All very encouraging, but how would the venerable .308 deer round fare?
The 150-grain Sierra Game King, starting out at 2,905fps, generates 2,812ft/lb energy and only suffers a 157fps loss for a barrel reduction of 6in. That seems very efficient to me and certainly justifies reducing a .308 barrel to 18in, as there is little loss in velocity. Muzzle blast may increase, but a sound moderator will take care of that, replacing the length and weight of the rifle before it got the "chop" and adding all the benefits of its inclusion. With regard to trajectory, a zero at 100 yards for the 24in velocity yielded a drop at 200 yards of -2.9in and -11.9in at 300 yards. Reduction of the barrel to 18in with a 100-yard zero gives a drop at 200 yards of -3.7in and at 300 yards of only -14.4in. Look at the figures below: does that really make a difference, especially when most deer are shot at 100 yards or less?
There may be a few downsides as the barrel is reduced, such as increased muzzle blast, but smart reloading of faster powders can make up for the velocity loss, though watch out for the pressure increases. The Quick Load programme (available from JMS Arms, tel 07771 962121) is an invaluable tool for this.
I did use more bullet weights and velocities, but there is simply not enough space to print all the data. Besides, I'm sure some of you will have nodded off by now!
What did prove interesting and will be of real significance for stalkers or fox shooters was the total weight of each barrel segment removed. Not allowing for loss in the material cut, I calculated a weight loss of 371.8g for the .223 barrel and 337.9g for the .308 barrel. That's a lot of weight reduction for a little loss of muzzle velocity. More importantly, that weight loss could be substituted for a moderator, to gain sound reduction without the overall length of the rifle becoming excessive. In fact, the excellent PES 32mm muzzle can, which I use regularly and is a real gem, weighs only 350g, so for all the advantages it brings, weight-for-weight, there is no real difference.
In case you are wondering, by the way, I did have each barrel professionally recrowned and threaded for a PES moderator and, apart from impact shift, the accuracy remained the same as for the initial tests with the 24in barrel lengths.
I hope this sheds some light on barrel length without going too deeply into the world of ballistic and mathematical formulae. GMK also recently sent me a Sako Quad in .22LR and .17HMR calibres for me to conduct the same experiments on (see next week's ST), so I'd better go and buy some more hacksaw blades…