Velocity v Barrel length

Without a shadow of a doubt the .308 Winchester cartridge has to be the best all-round calibre for British shooters. There are others, but for pure performance, ease of reloading and sourcing ammunition, the .308 Win is still one hell of a round. There are factory set guidelines into the velocity loss per inch reduction of barrel length at differing velocities but these seldom reflect the diverse range of calibres that most shooters encounter. Add to this that every barrel has its own unique internal dimensions that will affect the bullets-flight in terms of accuracy, acceleration and retardation. Weight, shape and material, as well as burn rate of propellant used, has an enormous effect on velocity. I wanted to see how far you could cut down a .308 barrel and the effects it will have on performance, so I decided to do it myself!

Stats

It was Winchester’s sporter version in 1952 of the Military trial T-65 round, which was later named the 7.62mm NATO. For all intents and purposes the same cartridge and it proved an instant hit and would spawn so many offspring, such as the 7mm-08, 6.5mm-08 (.260 Rem), .243 Win, as well as .338 and .358 calibre versions, as well as some exotic wildcats too!

With a rim diameter of 0.473”, same as the 30-06 but a case length of 2.015”, 0.479” shorter than its big brother. There was also less body taper, along with a 20° shoulder angle and a 0.304” neck length. Internal powder volume varies from military brass and differing manufacturers due to wall thickness but a typical .308 holds 56-grains (3.363 cm3) of H20 and can achieve 2850 fps with a 150-grain bullet, with a 60,000 psi max working pressure; nice and efficient in my book!

Cut off

To me, the 308 runs best and is most practical for hunting with a short barrel, where shear velocity is second to good handling and manoeuvrability especially with a scope, mounts and sound moderator added. Sure, for long-range shooters, maximum barrel length is mandatory to keep bullets supersonic at 1000 yds.

The .308 Win is a consistent and efficient case design and responds well to barrel shortening. I have used tubes from 32 down to 14.5”; all with amazing results and ballistics are enhanced or perfected for the barrel length and bullet weight and powder burn rate to suit.

The bore cross sectional area of 47.51 mm2, is good in relation to the burn expansion or burn needed to be achieved within the space, so cutting a few inches off the length does not influence the velocity too much. How much and how far can you go is what this article about and so from past experience with many .308 Win rifles; let’s get cutting!

Protocol

This is the hard bit, as you have to make a decision to cut a perfectly good barrel; in this case in 1” increments, as I needed to know the individual velocity results per reduction. I also used reloads to ascertain preferred bullet weights and powder charges for the altering barrel length. Reloads allowed me to mimic factory ammunition but with a better shot to shot consistency of less than 15 fps variation, whereas some factory hovered at over 65 fps, which could cause spurious results. I also chose three differing weight bullets; 125, 150 and 180-grains.

Only velocity reduction not accuracy was being recorded and so a simple deburring of the muzzle and crown was all that was required. At each inch barrel reduction for each calibre a 5-shot string was recorded over a Chrony chronograph to ascertain the results. Although in the same calibre, there can be a difference in velocities due to bore dimension and tolerances at manufacture, wear, fouling or erosion.

Results

These are the reloads I used on the RPA 24” barrel reduction to achieve a good initial velocity.

A.  125-grain Nosler Ballistic Tip; 43.5-grains of RL 10X, 0.274” seating depth and Federal Match primer produced 3131 fps/2721 ft/lbs

B. 150-grain Sierra Game King; 48-grains of RL15, 0.310” seating depth, Federal Match primer produced 2905 fps/2806 ft/lbs

C. 180-grain Hornady SST; 45.5-grains of RL 17, 0.552” seating depth, Federal Match primer produced 2694 fps/2900 ft/lbs

I lost my bottle at 14.5 inch length! At each one inch cut there was this approximate loss of velocity and corresponding energy:-
Average loss per 125-grain bullet 33.3 fps per inch reduction with energy figure from 2722 ft/ lbs to 2174 ft/lbs

Average loss per 150-grain bullet 30.7 fps per inch reduction with energy figure from 2812 ft/ lbs to 2249 ft/lbs

Average loss per 180-grain bullet 29.7 fps per inch reduction with energy figure from 2902 ft/ lbs to 2297 ft/lbs

I was quite impressed that it actually averaged out; an inch cut for each bullet weight was almost the same, only some 3.6 fps difference between the two, marvellous! Also, the total difference between using the light 125-grain as opposed to a heavier 180 is only 36 fps, which in real terms is practically nothing, in fact only the same as a one inch barrel cut difference. Tight and slack areas in the bore increase and reduction friction and rotational spin as the bullet passes and thus effects the pressures and velocity.

I used Quickload ballistics program to predict the loads as a control, as I also use this all the time for reloading and designing wildcat loads. It predicted a loss of 329 fps for the 125-grain bullet, 299 fps for the 150 and 291fps for the big 180, pretty damn perfect!

Reality check

I am not suggesting all .308 Win rifles should have 14.5” barrels; but it makes you think. Some 300 fps loss overall for each bullet weight is really nothing and do you really think at the ranges most deer are shot, they or you will notice?

For legal requirements in England and Wales you need to meet 1700 ft/lbs energy and Scotland it has to be 1750 ft/ lbs and minimum 2450 fps velocity. All bullets, even in a 14.5” barrel, are way above the legal energy minimum and only the 180 fails to achieve the Scottish velocity minimum of 2450 fps when it reaches a 16” length in my RPA.

Also, take the trajectory, most deer are shot at less than 100 yards but sure some are taken at longer range on the hill. Here, putting the data through QuickTarget ballistics program to predict down range trajectories a 150-grain Sierra Game King starting out at 2905 fps generates 2812 ft/lbs energy and only suffers a 157 fps loss from a 6” barrel reduction! To me that is very efficient and certainly justifies reducing a .308 barrel to 18”, as there is little real loss in velocity.

Does it really?

Yes, muzzle blast increases but a moderator takes care of that and just replaces the length and weight of the initial rifle before the ‘chop’ but gaining all the benefits of the inclusion of a moderator. Regarding trajectory, a 100 yard zero for the 24” velocity yield a drops -2.9” at 200 and 11.9” at 30 yards. Reducing the barrel to 18” with the same zero point gives -3.7” @ 200 and -14.4 @ 300 yards. Look at those figures and be truthful and say does that really make a difference?

The RPA barrel at 14.5” is sublime, I use this with a Nosler 125-grain Accubond over 44-grains of Alliant RL10X that achieves an impressive 2825 fps and a stonky 2215 ft/lbs energy, need I say more? Fitted with an MAE Extreme moderator that’s 13” long, but when fitted to the short tube is less than a standard rifle with no mod; sorted!

Conclusion

If you are still unconvinced, just look at the long-range pistol boys with their 14” barrels for shooting out to 600 yards! I love the .308 Win round and even more as a shorty, as you can remove a lot of tube with no real detriment to the ballistics. The benefits being a handier, moderated rifle that weighs near the same as a full length, un-moderated gun that can still do the job just as well! Next time, I chop my .223 and .243 barrels, which should be interesting!

Contacts

JMS Arms 07771 962121 Quickload QuickTarget, MAE mods
Norman Clark 01788 579651 Reloading supplies, Chrony chronographs.
Edgar Brothers 01623 613177 Bullets, powder cases
Henry Krank 0113 2569163 Sierra bullets, powder