Freebore, Throats and Bullet Jump
Wow, you broke the bank and went all out to build that super gun and when you went to the range some old codger shot a smaller group than you with his lever gun! It happens all too often but why does one gun shoot better? It might be the first ¼ inch of the long trip to the target that’s the problem.
Even before shooting progressed to breech loading cartridge guns shooters had discovered how important it was to have the bullet enter the rifling straight. Early muzzle loading target rifles had false muzzles that attached to the barrel for loading. The false muzzle was bored and tapered to gently center the bullet and guide it into the rifling. Even after switching over to breech loaders most accuracy enthusiast would load the bullet from the muzzle and only have powder in the cartridge. Why did they go to so much trouble? Because it made a difference. The ones that didn’t load from the muzzle used special tools to seat the bullet in the rifling (called breech seaters) and then loaded the cartridge with powder and a paper card atop the powder to hold it in place.
After jacketed bullets became commonplace muzzle loading was no longer possible due to it being impossible to push the jacketed bullet down the barrel. Modern guns blow the bullet out of the case and it hits the rifling hopefully straight enough to enter and start spinning on center. If a bullet enters the rifling crooked it can’t straighten up. Since the average rifle bullet is spinning over 200,000 rpms you can imagine how much effect this has. The effect is more pronounced as the range increases.
So how do we get our bullets started straight? We reduce the tolerances as much as possible and make sure everything is straight. In benchrest rifles we make the neck portion of the chamber slightly undersize and turn our brass to just slide in with zero tolerance. This is good as long as experienced handloaders make the ammo. If such a gun has a standard round of ammo loaded in the chamber it acts like a clamp on the bullet and chamber pressure goes into the danger zone.
After doing the best we can with the neck we turn our attention to the throat. The throat is a short section of barrel at the end of the chamber where the rifling has been removed. The throat allows a bullet to be loaded into the bore. Without a throat the bullets would hit against the rifling and keep the round from going into the chamber. The throat doesn’t have to be the length of the entire bullet, only the length of the bearing section on the bullet that protrudes from the cartridge. In most rifles it is around ¼ of an inch. If the throat is longer the bullet will have more problems maintaining alignment coming from cartridge case to rifling. In most guns the bullet engages rifling before the base of the bearing section has exited the case. If the throat is not concentric and some rifling remains on one side the bullet will be pushed off center. This is a frequent problems and hard to detect unless you know how to look.
Figure 25The 155Palma bullet has a short bearing section. The throat can be shorter
My favorite way to check the throat is to make a casting of the throat with cerrosafe metal. It is a low temperature metal that looks like lead but melts at 180 degrees. I get mine at Brownells. Just plug the bore with a bit of paper towel in front of the throat and pour in a little metal through the breech. No need to fill the chamber. You are only interested in the throat. Tap it out from the muzzle and you have a faithful reproduction of the throat. You want to see that the rifling starts at the same distance from the chamber. This is also a good time to measure the neck diameter and bore diameter
Figure 26The throat is clearly visible on this Weatherby 7mm mag. This is what is called freebore.
If your throat looks uneven you might be able to cut the throat deeper and get it concentric. This is done with the throating reamer and can be done by hand however the best work will be placing the barrel in the lathe and using the tailstock to hold the reamer. Remember you are only cutting the rifling so it won’t take much effort to cut away too much. The cure for the deep throat is cutting off the chamber and setting the barrel back. A lot of work.
So what if you have a “deep throat” and just leave it. Depending on your objectives you might be happy with it. The famous Weatherby Magnums were designed with deep throats to get more velocity. It is called freebore. It works sort of like this: burning rate on powder goes up as pressure rises. When the pressure rises enough to push the bullet out of the case and into the rifling it has the same effect as making the case larger and this will drop the pressure. This allows rifles with deep throats to be loaded with more powder and achieve higher velocities. Accuracy is not enhanced but if you are shooting a deer you (or the deer) won’t know the difference. You can do this at any time with a throating reamer or the rifle will do it for you as you continue to shoot. A 243 Winchester will have the bullets fall out of the case before touching rifling around 2000 rounds. That’s a lot of deer to be skinned but only one moderate season for a highpower match shooter.
For the gunsmith you need to understand where the throat is, what effect it has on accuracy and velocity. How to repair or correct it. And how to explain it to your customer.