First you should decide on what format you want, lets take it one piece at a time:
BARREL:
This is probably the most important decision you have to make, as it is the heart of the rifle.
Length:
The original M16 had a 20” long barrel. As the platform has evolved, all kinds of lengths have developed. The 20” barrel has some advantages- including greater velocity and longer sight radius than shorter barrels. The increased velocity is important if you are going to be shooting long range (say over 300-600 yards) or if you are going to be shooting at something which can shoot back at you. The latter is true because some military ammunition relies on fragmentation, (the bullet yawing and then breaking up inside the body) in order to cause damage and stop a person/animal. The faster the bullet is traveling, the farther out it will have enough velocity for the bullet to fragment. Look at the "Ammo Oracle" to learn more about this, as well as more than you could possibly ever remember about AR ammunition. The increased velocity has another advantage, in that the bullet will have a flatter trajectory, and thus hitting objects at unknown distance will be slightly easier, as errors in estimating the distance, or correcting for bullet drop or wind drift become slightly less important. The increased sight radius is useful, as it improves the shooter’s ability to shoot accurately. Because the front and rear sights are farther apart, any error in misalignment at the time of firing will be smaller as it is extrapolated out to the target. Note that the longer barrel is not more accurate, the sight radius simply makes it easier for the shooter to shoot accurately. This advantage disappears as soon as a telescopic or red dot sight is added to the equation. If you want to shoot at very long distances, for example long range target shooting, varmint hunting or sniping you might consider a 24” barrel. This increases your velocity, and adds to all of the advantages noted above except sight radius. Most 24” barrels use the same sight radius as the 20” rifle, unless they are specifically set up as a “race gun” for long range target shooting.
The next common shorter length is the 18”. I believe this length either evolved, or was made popular due to the SPR “Special Purpose Rifle” concept. The idea here is to have a rifle used by someone who is a better than average shot in a sniper type role, without going to a full blown sniper rifle. The goal is accuracy at long range, and the 18” is seen as a good compromise between the 14.5” M4 and the full 20” rifle.
The next common length is 16” which has evolved because of our laws in the US. The Army (feel free to correct me if I am wrong, I am not in the military, nor do I claim to be an expert on this) has decided that for the most part a shorter barrel is more useful. They have gone to the M4 which has a 14.5” long barrel. In the US it is illegal to own a rifle with a barrel shorter than 16” unless it is registered as a “short barreled rifle” which requires special paperwork and some extra $ to the .gov folks. Thus evolved the 16” barrel. The advantage to this is that you have a barrel which is handier to move around with, swings easier, is lighter, and you don’t have to register it and pay extra dough to the government. Probably the 16” is the most common length these days, most shooters having agreed with the army’s assessment that you don’t gain that much with the extra 4”, and the compromises are worth it to have a quicker handling gun. This is especially true for civilians who are not limited to FMJ ammo, and can select an ammunition which is not as dependent on fragmentation and velocity for self defense. It should be noted that the original 16” barrels used the “carbine length” gas system. This means that the distance from the chamber to the gas block (front sight base in most cases) on the 16” barrel is the same as what the military uses on a 14.5”. This distance is significantly shorter than the distance on a 20” barrel, because the gas pressure forcing the carrier and bolt backward only exists while the bullet is in the barrel. This pressure has to be present for a critical length of time, and thus with a shorter barrel, that time has to start sooner in the cycle. This results in a harsher recoil than one sees with a rifle length gas system, and potentially more problems with extracting the bullet from the chamber, as the rifle is now trying to do this while the pressure in the chamber is higher. The smart folks at Armalite came up with the idea of moving the gas system in the 16” rifle down the barrel a ways from where it would be on the 14.5” design, and they invented the “midlength gas system” (middy for short). This design is slightly easier on the components involved, as there is slightly less pressure in the chamber when the rifle is trying to extract the bullet, and there is less force transmitted to the carrier. This design also gives a slightly longer sight radius, and also allows the attachment of a normal designed ever important bayonet.
Obviously, the 14.5” is next. There is actually a way around the law noted above concerning SBRs that we should note here. If you attach an “extended” flash hider (Smith Vortex, SDI Extended A2, YHM Phantom, VLTOR Compensator, etc.) flash hider to the barrel in a “permanent” manner (for example by threading it on and then drilling a hole off center and driving a pin through it to keep someone from removing it) and the overall length of the combination is 16” or more, you have met the requirements. These barrels are usually referred to as 14.5” with pinned flash hiders. The down side to this is it makes things a little more difficult to remove the barrel and install most free float tubes, as with most the flash hider (FH) and front sight base (FSB) have to come off to install them. There are two piece free float rails which can be used to get around this as well. Again, the gains are maneuverability and portability, with the trade off of shorter sight radius and less muzzle velocity.
Once you get shorter than 14.5, you can go as far as you want. These rifles are designed mostly for shooting things in cramped quarters, and for easy maneuverability. The trade off is again lower velocity, and now significantly increased muzzle blast. These guns tend to be more particular about type of ammunition used, gas port location and size and need to be set up exactly right to run well. This is a fairly specialized area of ARs in my mind, and I have limited knowledge in this area so maybe someone else will come along to supplement my knowledge.
Material:
There are three common materials used in making barrels: 4140, 4150, and stainless steel (I believe 416 is the most common). Each has advantages and disadvantages. 4150 is the spec (as I understand it) for the military. There have also been discussions of 4150vs CMV which are over my head. Some companies use these terms interchangeably, and others do not, some will list their barrels as “4150 CMV“ while others will use one term or the other. In my mind these terms are interchangeable, but if someone wants to straighten me out, have at it Regardless, 4150 is apparently somewhat better than 4140 in standing up to the heat and abuse from full auto firing. I am not a metallurgist, but the folks who seem to know on page two say that the "40" vs. "50" has to do with an amount of carbon in the steel. More carbon gives a stronger steel, which is slightly harder to machine, but also more durable. The consensus seems to be that 4150 slightly better, slightly more expensive, and possibly a little harder to work with, but the differences may not be detectable in a rifle which is not used to shoot full auto.
Either of these materials can be chrome lined. Chrome lining is a process which bonds chrome to the inside of the chamber and or barrel in a very thin layer. This is advantageous in that the chrome lining is harder and has more “lubricity” or “slipperiness” than the underlying chrome moly steel (4140 or 4150). This extends the life of the barrel (possibly 2-3 times as long) as well as making it easier to clean. Some companies offer only chrome lined barrels, some offer both, and some offer only non chrome lined. The general consensus seems to be that chrome lining is easily worth the few extra dollars you pay for it. It does have one down side, however. The process of bonding the chrome to the inside of the barrel is not as exact as the process of cutting the barrel in the first place, thus overall chrome lined barrels are SLIGHTLY less accurate than non chrome lined barrels. This is a generalization (as evidenced by the impressive accuracy of the FN SPR series which usually shows incredible accuracy in spite of having a chrome lined barrel). Most shooters will never notice the decrease in accuracy seen due to chrome lining, but if you are that good, and it is that important to you, then think about getting a rifle without it. Many say that the amount of accuracy lost is less than or equal to ½” at 100 yards, so if you shoot Wolf like me, you will never see it.
The final common material is stainless steel. Again, this seems to be a material which become more popular due to competition as well as the REECE/ SPR/ DMR evolution. Most stainless barrels are designed for accuracy. Obviously the alloy of stainless is designed to be less susceptible to corrosion, something which the chrome lining of a chrome moly barrel offers, but without the downside of decreased accuracy. Stainless is not as corrosion resistant, nor as durable as chrome lining, but is used where one decides the trade off for better accuracy is worth it. As I pointed out above, most shooters will not benefit significantly form using stainless, as either they or their ammunition is not accurate to see the difference. There have been some recent threads about the durability of stainless which have been fairly impressive, and obviously it has been meeting the needs of the military for a while now, so if accuracy is your main concern, consider stainless. Stainless is not usually chrome lined, but there have been a few custom runs of chrome lined stainless.
How long do these materials last? Good question. I have seen numbers tossed around concerning a chrome lined barrel lasting around 20,000 rounds (lets see: $200 for 1000 rounds of wolf... ?$4000 worth of cheap ammo?). There was a thread by one of the industry sponsors outlining a SBR with a stainless Wilson barrel with 16,000 rounds through it which still had decent accuracy. A non chrome lined non stainless barrel is often said to last half as long as a chrome lined one. Remember, that the military definition of "accurate enough" is no where near the same as someone who shoots high power rifle, and regularly goes out to 600 or more yards. Also, you bumpfiring fools, or full auto guys will get less life out of a barrel because as the barrel heats up, it becomes more susceptible to wear by continued firing. Thus, YYMV (Your Yardage May Vary).
Chamber
First you have to know that there is 223 ammunition, and 5.56. The 5.56 is loaded to higher pressures for the military. When a barrel is manufactured the maker cuts the chamber he wants for it. A 223 chamber is slightly shorter in, and provides slightly more accuracy when used with 223 ammunition. This can cause a build up of too much pressure in the barrel if 5.56 ammunition is fired in a barrel with a 223 chamber. If you expect to shoot any kind of surplus ammo, you should make sure you buy a rifle with a barrel chambered to shoot it. This is slightly complicated by the "wylde" chamber. This is a compromise between the 223 and 5.56 chambers. It is intended to allow the user to fire 5.56 ammunition and yet improve on the accuracy of the 5.56 chamber. This is another option if you intend to shoot surplus ammo. Want to see the specs on different chambers?
Twist So, rifle barrels have "rifling" in them. These are lands (high spots) and grooves (low spots) which run the length of the barrel. The lands engage a bullet and give it spin as it travels down the barrel, just like a quarterback puts spin on a football. The rate at which the bullet spins is determined by its speed as it travels down the barrel, and the rate at which the lands twist in the barrel. The twist is listed as "one in x" were x is the number of inches the bullet must travel down the barrel to complete a single revolution. Early ARs had 1:12 twist (1 revolution every 12 inches). However, as bullet design evolved, bullets became heavier and longer. Longer, heavier projectiles require faster twists in order to stabilize them. If they do not spin fast enough, they wobble as they go downrange, and you lose accuracy. If all you want to do is launch 45 grain bullets at prairie dogs with your rifle, you can get away with a 1:12 twist. Actually, the 1:12 will probably work up to 55 grain bullets. Most ARs sold these days are 1:9 or faster twists. The 1:9 will stabilize 55-69 grain bullets, and may sometimes stabilize 75s depending on the rifle. A 1:8 or 1:7 will stabilize 77 grain and up bullets. This is important if you intend to shoot at very long ranges where the ballistic coefficient of these bullets is really needed, or if you want to shoot the heavier Black Hills or Hornady ammunition which performs near or at the top for self defense. You need to decide what you want to do with your rifle from the start so you know what twist you will need.
No comments:
Post a Comment