Recently, on M4carbine.net, a user by the name of “thecolter” put up a very interesting post relating to various barrel lengths, ammo selection, and zero range. With his permission, I’m reposting some his graphics.
A while ago, I put up a post about implementing a RIBZ (revised improved battlesight zero) on my rifle. It was done in the context of carry handle iron sights, but the principle remains the same for any zero. That post had this chart in it.
While interesting to look at, thecolter had a much better visualization for the effect of zero range on bullet impact (assuming the same point of aim). Before I post it, I also wanted to reference my recent post on AR-15 barrel selection. In that post, I mentioned that the primary function of longer barrel length is to provide higher velocity to the bullet. A higher velocity translates to a “flatter” trajectory and improved terminal effect. I’ll talk about this more in a bit.
In order to keep it relatively simple, I’m just going to post the M855 targets. Thecolter also put up graphics for M193 and Mk262 in the thread.
He is working on some more of these, including a 20″ graphic. But I wanted to go ahead and start with these. Since this graphics uses a standard IPSC target, another user went ahead and sketched in a few measurements just for reference.
Each of these shows the effect that barrel length, and therefore velocity, has on point of impact in relation to zero distance. There are a couple interesting things to note here. Notice that as the barrel length grows, and the velocity increases, the overall vertical cluster shrinks. This is quite noticeable on the 50 yard and 100 yard zero charts by looking at the 300 yard impact. As the barrel length increases, the 300 yard POI steadily marches up the target from being a miss with a 10.5″ barrel to a solid B zone hit with an 18″ (and, presumably, a 20″). Looking at the 300 yard zero graphics, you can see that a 400 yard POI is a total miss with a 10.5″, but becomes a solid pelvic hit with 18″.
I imagine that this latter fact is why the Marine Corps has shown such great affinity for the 300 meter zero. Holding the same center mass POA with a M16A4, it provides solid hit potential in vital areas out to around 350 yards with no required adjustment to sights. If one wanted to get fancy, they could learn how to hold over or hold under at various ranges in order to maximize the probability of a hit. This was the principle behind the Swiss S4G system. The same applies to the 50 yard zero.
In contrast, the 100 yard zero shows a steady march of POI down the target as ranges get closer or further. This actually provides a rather simplified method, since the shooter knows they only have to start holding higher if the target is closer or further than 100 yards.
Something to keep in mind, though, is that whatever option you choose for a BZO, you must learn to practice it. A common problem with the 300 yard zero, for instance, is that unpracticed personnel will tend to always aim at the same point regardless of context. This idea works fine for an averaged sized human target standing in the open, but if the only exposed target area is a head peeking from behind a wall, then the unpracticed shooter is liable to miss high or low if they simply hold their POA on the center of the exposed head.
Of course, none of this is ideal for a precision target shooter. A 4″ drop between one range to the next can be a pretty big score drop on a ringed target. But it does work for an everyday shooting context like hunting or self defense.
To summarize, longer barrels impart more velocity and decrease the spread of your points of impact at various ranges. If you go the BZO route, choose one that best suits your needs and practice it at those various ranges.