I touched on this in the previous blog on Bullpups, but the main point I want to get at here is that the advantage of a bullpup is NOT simply to reduce the overall length (OAL) of a rifle, as some people have stated (notably, this opinion has been expressed on InRangeTV by Karl and Ian – two people whose views on guns I greatly respect, but in this case, I disagree). But first, a correction, or maybe supplement, regarding OAL to barrel length ratios:
Bullpups and ballistics
The ballistic purpose of a longer barrel in the bullpup is not necessarily just to give a short rifle the ability to act as a DMR (designated marksman’s rifle).
I stated in a previous blog that this was a big point for bullpups, i.e., that the point of the long barrel was that it gave more accurate shots. But as it turns out, I think I was somewhat off on this. It is probably true in theory, since DMRs generally required somewhat longer barrels (a 10.5 inch barrel is unlikely to be very good as a DMR). However, most military bullpups are 3 MOA guns (their shots spread out over a diameter of three inches at 100 yards), which is acceptable by the standards of a frontline combat rifle, but not necessarily good for a DMR. But why, then, is the smaller OAL/barrel length ratio desirable? Why not just go with a short barreled rifle in a conventional design? The reason IS largely about ballistics, but not necessary ballistics in the sense of accuracy.
We can really give three reasons:
- You COULD configure a bullpup to be a DMR, it’s just a lot aren’t made as DMRs (at least, not the ones that are available to civilians in the US). This shouldn’t be surprising – many AR-15s, for example, are 3 MOA rifles, even with their greater overall length. The difference is that you can go out and buy $3000 AR-15s that are designed for long-range marksmanship, whereas most bullpups are only available in one configuration. If someone wanted to design and sell bullpups configured for long range accuracy, that would probably be quite feasible. But more on long range accuracy later when we discuss the physics of bullpups vs. conventional rifles.
- Some bullpups actually do get really small OALs by using short barrels AND the bullpup design, for example, the mini Tavor (x95) with the 11 inch barrel. This achieves OALs which are simply impossible with a conventional rifle (unless you really sacrificed ergonomics).
- The big advantage of having a longer barrel: better terminal ballistics. Because the bullpup has a standard length barrel, it will fire bullets going faster than the conventional short barrel rifle. This translates into more ‘stopping power,’ which is a measure of how likely it is that, when you hit a target, the target is disabled or killed (morbid, yes, but that is the relevant consideration). This is a BIG consideration, especially in militaries. Now, as aforestated, most military rifles are only required to be 3 MOA guns, whether conventional or bullpup. However, a bullpup gives much better stopping power for the same OAL when compared to a conventional rifle. Note that you can get the same stopping power/terminal ballistic performance out of a conventional rifle, but it will have a substantially longer OAL (probably 4 to 8 inches or so).
Now that we’ve corrected this issue about ballistics, let’s discuss the physics of handling a bullpup vs. a conventional rifle.
Moments of Inertia (Advantage: Bullpups!)
Stone’s study (see previous blog) found that bullpups, with 83% confidence, result in better scoring, when shooters have to quickly engage multiple targets. Why would this be? I think there’s a simple reason: moments of inertia.
When holding a rifle, whether conventional or bullpup, and trying to move it around to aim it at multiple targets, the shooter is essentially moving a rod-shaped object around, with a pivoting axis that goes through the shoulder or possibly the centre of the shooter’s torso. If the rod had the mass distributed equally along its length, the moment of inertia (if the base of the rod were pressed against the axis of rotation) would be one third the mass, times the square of the length. In other words, as you lengthen the rod, you REALLY knock up the moment of inertia.
Now, a bullpup is shorter than a conventional rifle, so if the weights were spread along the lengths evenly, the conventional rifle would have a somewhat higher moment of inertia. However, conventional rifles like AR-15s tend to have almost no weight in the stock, which means their moment of inertia is even greater than what we would get approximating them as an homogenous rod.
Why does this matter? Well, when target switching, it’s easier to use something with a lower moment of inertia, because it requires less torque to get the same angular acceleration, or in other words, it takes less effort and strength to move around the item with the lower moment of inertia.
Jerry Miculek, who is a famous competitive shooter of conventional AR-15s, has a video on YouTube entitled ‘Battle of the Bullpups,’ and at one point, he comments on how the bullpup moves from target to target so much more easily than he is used to, that is throws of his aim. Now, throwing off his aim is bad, but it is easy to see that this is simply because he is used to having to manhandle a gun with a larger moment of inertia.
This is why bullpups might be expected to do better in close quarters combat against multiple opponents, or in situations where the shooter needs to acquire targets from disparate directions very quickly: it is easier to move the bullpup around. This may also mean that the gun is easier to fire from the opposite shoulder (although there is still the issue of brass flying in one’s face).
Note that the bullpup will have the smaller moment of inertia if the two designs have equal masses (weights), but that even if the conventional rifle is lighter, the bullpup is still likely to have the better moment of inertia, because the length term gets squared in the formula for moments (and because the stock has so little of the weight in a conventional rifle).
Another advantage to this smaller moment of inertia is that it makes it easier to hold the rifle at the shoulder for long periods of time without tiring, including holding it one-handed. This might be necessary when performing overwatch on a given area, or when needing to cover an area while manipulating things with the other hand.
The potential downside to a smaller moment of inertia is that this makes the rifle a bit more sensitive to any twitching on the part of the shooter when aiming at long ranges without supporting the rifle. Higher moments of inertia mean that a slight torque from twitching will not move the aim point as much as the same torque with a lower moment of inertia. This brings us to…
Firing From Support (Advantage: Conventional!)
Let’s say you put a bipod (or even a monopod or tripod) on the front end of a rifle, and try to fire from prone. Or perhaps, lacking this, simply rest the front of the handguard on something (in the best case scenario, a bench rest; in the real world, probably a log or something similar), and try to hit a target at a long range. We will assume, for the sake of comparison, that both the conventional and bullpup rifles have got the same mechanical accuracy (i.e. if you could hold the rifle perfectly still, they would fire bullets just as consistently to the same place), and both have the point of rest equally distance from the end of the barrel. We will also assume both are affected equally by the support (so either both have free floated barrels (see the appendix), or the pressure on the barrel has the same effect on accuracy).
Here’s the problem with bullpup designs: in this particular context, the bullpup gives two disadvantages.
First, the trigger. A bullpup trigger will never be as nice as a conventional trigger, although it can probably be made almost as nice, to the point of making little effective difference.
Second, and this is the bigger issue: geometry gives an advantage to the conventional design. This is because its greater OAL actually confers an advantage here. As the shooter adjusts the butt of the gun to aim it, the butt is further from the bipod or point of rest. This means a movement in the butt actually gives less of a change in the angle than with the bullpup (imagine two levers, one that is one foot long, and one that is one yard long. If you move the ends of both levers an inch, the one foot lever goes through a much larger angle than the one yard lever). The same goes for movement of the firing hand, since the firing grip is closer to the tip of the barrel in the bullpup, and hence closer to the bipod or other support.
As a consequence, the bullpup design will be somewhat more sensitive to things like the shooter twitching while firing. Plus, the trigger being worse means the shooter is more likely to apply unwanted torque to the grip area, and in any case, the grip is closer to the point of rotation. The upshot is that it will be harder to make supported shots like this using a bullpup than a conventional weapon.
So, for a rifle designed for making long-distance shots from a supported position, the conventional design is better!
Basically, I suspect bullpups are better for close quarters combat, but conventional guns are better for long range shots from supported positions. This seems to tally with Stone’s result on shooters doing better with bullpups when needing to switch between targets at 25 yards, and with other results about the efficacy of bullpups and conventional rifles. It also tallies with the preference that longer-range shooters have for their conventional AR-15s (which is helped by the simple fact that it’s easy and cheap to get a free-float AR-15, and not so much to get a free-float bullpup).
What I think is ultimately also worth noting, though, is that both designs can do both types of shots reasonably effectively. The bullpup may give a slight edge in close quarters (say, within 50-100 yards), but not exactly a game changing edge – the side with better tactics and training will still probably win. And similarly, the conventional design may be better for long range shots, but again, not enough to be game changing, especially since most military rifles are 3 MOA anyway, and thus unlikely to be all that accurate at longer ranges (300-600 yards) anyway.
The upshot is that for a lot of purposes, I think bullpups come off slightly better. This is especially true for things like close and mid-range combat in the military, police work, and home defense. However, as Stone’s study found, many shooters, especially if they are used to a conventional design, do not realize they are doing better with the bullpup design. Furthermore, the difference is marginal enough that for most militaries, other considerations will probably overshadow them.
Appendix: Free float?
A free floated barrel might sound fancy, but it really just means that the barrel is secured at the chamber, and nothing presses on the middle or end of it. As it turns out, having things that touch the barrel is bad for accuracy, so free float barrels are generally preferred by those who want high accuracy at longer ranges. However, many military rifles are not free floated. This is because you can still achieve perfectly reasonable accuracy (3 MOA) without free floating. Free floating is particularly helpful in preventing pressure on the handguard (whether from gripping the gun or from resting it on an object or bipod) from messing with accuracy, however. Obviously, this is mostly an issue when talking about longer ranges.
Appendix: 3 MOA?
Instead of measuring the angle which a shot deviates from the aim point using degrees or radians, it is convenient to use ‘minutes of angle.’ Roughly, one minute of angle corresponds to one inch at one hundred yards, two inches at two hundred, etc. A 3 MOA gun means that if you fire a group of five shots with no operator error and with the gun held perfectly in the same position for all five shots, those shots will all land within 3 inches of one another at 100 yards.
Most militaries expect their battle rifles to fire 3 MOA. This is perfectly adequate because most combat takes place within 300 yards, and even if your bullets are spreading around a disk of nine inches at 300 yards, that’s still enough to hit a human-sized target most of the time if you aim at their centre (one of the big takeaways from WWII was that most combat takes place within 300 yards). While marksmen and snipers obviously want better than 3 MOA, for most people, it’s just not a big practical issue.
Note that measuring what MOA a rifle of achieves is often done in a manner that strikes me as statistically suspect, because it will be done by firing a bunch of five shot groups, and just seeing how each one spreads. However, this has many statistical problems. For example, one gun might have four groups of five rounds that have a 1/2 MOA spread, and a single group with a spread of 36 MOA. How should that be reported? Another rifle might consistently fire 5 MOA groups instead. Does it get labelled a 5 MOA rifle, while the previous one gets called a ‘usually 1/2 MOA’ rifle, despite the one group being truly terrible? Or we might get an example where in each group of 5, we have 4 shots that are within an inch of one another, with a single ‘flier’ that pushes the group to 3 inches. To my mind, that should rank differently than a rifle that consistently fires bullets that sit around the circumference of a 3 inch circle. But the conventional measure of accuracy treats them the same.
So, trying to measure MOA with five shot groups is not exactly a statistically precise affair. To alleviate this, some people prefer to use things like the mean radius of deviation of the shots. This strikes me as a much more useful number.
The advantage of giving an MOA based on the maximum spread, however, is that it gives a worst-case-scenario estimate. For whatever reason, this is the generally-used measure of accuracy among shooters.