Determining the COAL or cartridge-over-all-length of a pistol or rifle round is a very important part of reloading your own ammunition. Insuring that a round is the proper size to feed from your magazine and fit in your firearm’s chamber is extremely important. If a round is too long than it simply won’t function properly or even at all. Different firearms of course have different chamber sizes and the optimal COAL may vary significantly. This is further complicated by the fact that precision ammunition may be loaded with a longer than usual COAL to insure that the actual bullet is seated an optimal length from the barrel’s rifling. This insure that the bullet enters the rifling as stable as possible and can promote better accuracy. Factory ammunition, whether it be for a rifle or a pistol, tends to be loaded with an COAL that is designed to function in a wide variety of firearms. If you shoot much you likely have noticed some ammo works better in some firearms. Almost everybody has heard someone say something like: “My AR really likes that PMC X-TAC M193.” Fundamental to COAL is how deep the bullet is seated in the cartridge case or brass, therefore the other important consideration in regards to COAL is the effect on pressure. We have mentioned seating a bullet with a long COAL and having issues with function, seating a bullet too deep and having a short COAL can cause some serious issues as well. Primarily those issues revolve around changing the pressure of each round. How that change in pressure can effect the firearm may be minor in the case of a deformed cartridge or catastrophic in extreme cases damaging the firearm and/or shooter.
COAL can be a complicated subject and I don’t intend to go into any greater detail in this article. My suggestion is that for each type of round you are reloading, do some research. You should of course start with your reloading manual. Every reloading manual will have a recommended COAL for each type of round and bullet type. This recommendation is based on that manual’s tested data and may not be optimal for each and every firearm. In general though it’s a good and safe place to get started. When in doubt, stick with your reloading manual’s recommendation. This article of course is about the 9mm cartridge. I have done quite a bit of research on the optimal COAL for the 9mm round and it has gotten me few specific answers. It appears that the overall length of the 9mm round can vary quite a bit. In an effort to confirm this I have taken measurements from four common 9mm manufactures. The table below shows the results of those measurements. Measurements were done using Hornady dial calipers and then again using a Hornady bullet comparator. A bullet comparator measure COAL from a bullets ogive. A bullet’s ogive simply defined is the curve of a bullet’s forward section. It inherently shows less variation that the point of a bullet and therefore will measure more precisely from one round to the next. It is possible to have variations within a hundredths of an inch measuring from a bullet’s tip while variations within a thousands of an inch are the norm when using a bullet comparator and measuring from the bullet’s ogive.
As you can see from the chart above there is quite a bit of variation between these 9mm cartridges. The shortest averaging 1.118 and the longest averaging nearly 1.160. That’s a significant difference. Using the bullet comparator we have a similar difference with the shortest coming in at 1.926 and the longest at 1.971. The difference regardless of whether the calipers are used by themselves or with the bullet comparator is roughly .04″. Not likely a difference you can spot with the naked eye but possibly significant enough to matter to your particular firearm. All of these rounds have been used extensively by me in a variety of firearms. All function well. The point? Variation in COAL is okay and to be expected. Optimal COAL depends on a wide variety of factor and ultimately needs to be determined by the reloader through experimentation. I do want to reemphasize that you should always start with your reloading manual when creating new cartridges. Happy reloading!