My thoughts on Chamber Dimensions

Here are some random thoughts on chamber dimensions as it relates to accuracy.

Neck Diameter: The chamber neck needs to fit the cartridge as tight as possible while still allowing for proper function and reliability.  Typical neck clearances on production rifles are between 0.008" and 0.014". When brass is fired in a chamber this loose, the pressure forces it all the way out against the wall, leaving the bullet unsupported. In AR15 barrels, I like to see a minimum of 0.004", and not more than 0.008" of neck clearance. In benchrest guns, I like to see only 0.002" of neck clearance. One benefit of tighter chambers is that the brass does not get worked as much. Remember that when you blow the brass out during firing, you have to squeeze it back down in the reloading dies. the further it moves each time, the faster it work hardens, cracks and grows longer. One way you can measure your chamber's neck diameter is to measure a loaded round's neck diameter and then fire it in your chamber. now measure the fired case. Take fired case diameter and subtract the loaded case diameter. This gives you a good approximation of how much neck clearance you have. One thing to note is that the fired cases is usually a couple thousandths smaller than the chamber due to brass spring back.

Neck Concentricity: This is very important because the neck is responsible for holding the bullet while the bullet releases from the case mouth and into the throat and barrel. if you hold the bullet crooked, the bullet will be out of balance.

Neck Length: Most factory chambers have necks that are cut longer than the SAAMI specs require. This is done for liability reasons. By cutting the neck long, they do not have to worry about a handloader using brass that is longer than spec which would cause it to crimp against the bullet and raise chamber pressures to the point that the gun blew up. I have measured chambers that are from 0.050" to 0.090" longer than they need to be. Think about it this way: the bullet is sticking out the front of the case and into the throat, when the bullet leaves, there is now a ring of open space 1/16" long where the bullet is not supported by the case mouth (neck) or by the throat. This would not be a problem with long bullets, but short stubby ones would be free to float around a little more. This is not good. You can measure your individual chamber's length to determine what the maximum case length is in your gun. There are small neck length measuring tools available from Sinclairs. it is basically a little steel cylinder the diameter of a bullet that has a head on it the diameter of the neck. You take a case and trim it extra short, seat the measuring tool in this case and chamber it in the rifle. When it comes back out, simply measure the length. this is the absolute MAX safe length. you should trim your cases 0.010" or more shorter for safety. If you expand 221 cases, you will see that they end up about 1.350" or so, but most books show a spec of 1.400" in case length. Some reamer makers grind the chambering reamer for 1.450" max length. Quick math gives us 0.100" of clearance, way too much. I recommend a trim length between 1.380" and 1.390" in my 300-221 chambers as my reamer cuts a total length right at 1.400".

Throat Diameter: When the bullet begins down the barrel, the first thing it hits is the throat. If the bullet is not held in the center while it engraves into the throat, it will be slightly off center and out of balance. Typical production rifle throats can be 0.0030" to 0.0050" over the diameter of the bullet. The throat is responsible for aligning the bullet into the bore as it is engraved by the lands and grooves. the tighter the fit, the more accuracy you can expect. I get my reamers ground no more than 0.0010", and usually 0.0005" over bullet diameter. This ensures a nice bullet to barrel fit.

Throat length: The length of the throat probably has the least effect on accuracy, but it is still important to know what's happening here. Throat length is also known as freebore. Factory chambers are cut with long throats so that any weight of bullet, when loaded to SAAMI maximum lengths will fit easily into the throat. Weatherby actually cuts their throats longer, allowing the bullet to accelerate more before it hits the rifling. This is part of what allows them to run such high pressures. Generally, we want the throat to be as short as possible with the ammo we plan to use, but for practical reasons, the reamer makers grind the throats long enough to clear most bullets. The benchrest crowd goes to great lengths to get short throats so that they can seat their bullets into the rifling, or at least very close. Most people that are really into chamber work will tell you to have at least 0.025" to 0.050" of clearance between the bullet sitting in the chamber and the rifling (Throat). Throat erosion is what happens as the barrel has a lot of rounds fired through it. the throat generally gets longer, but it also grows slightly in diameter. Both of these degrade accuracy and are the reason a barrel is considered "Shot Out".

Muzzle Crown: The bullet is now all the way down the barrel and sticking most of the way out the front. As the bullet just leaves the barrel, it is basically uncorking the residual 5,000 to 10,000 PSI of gas pressure. It is very important that the crown is centered on the bore and square to the end of the barrel. if the muzzle were crooked, the bullet would let gas rush out of one side while the bullet was being released. This can cause the bullet to tip slightly sideways and throw it out of balance. Keep in mind that a 1:9 twist 223 barrel sending a 62gr bullet out at 2900 FPS is also spinning the bullet at about 230,000 RPM. If the bullet is out of balance, it is more difficult to fly to the same point of impact each time. Here is where target crowns come into play. Dished or 11degree crowns leave a nice path for the gas to escape from behind the bullet as it leaves the barrel. I like to cut a slightly tapered crown, using a lathe to pull the cutter from the bore outwards. This ensures that there is no burr left in the bore and that the muzzle is true to the bore. I do not like deeply recessed crowns. The gas is moving about 5 times faster than the bullet when it is released behind the bullet. The step in the recessed crown just causes the gas to bounce back towards the bullet. I feel that Muzzle breaks and recessed crowns are not good for ultimate accuracy, but when they are done correctly, there is not really any proof that they are bad for accuracy. I will thread barrels for muzzle brakes and cut recessed crowns upon request.

Chamber Gauging: If you request it at the time that you order your barrel, I can cut a chamber gauge for you with the same reamer. With a pair of dial calipers, you can use the gauge to properly adjust your sizing and seating dies. This will be a 2" long steel gauge with the same body, shoulder, neck and throat. You will find this gauge a life saver when working up loads to get the bullet as close to the lands as possible. This gauge will also serve as a standard to measure throat erosion against in the future, allowing you to keep track of the throat erosion on your barrel. I charge $25 for the custom chamber gauge to match your barrel.

 


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