Reloading Basics

Precision shooters apply a lot more ‘science’ to their reloading in order to squeeze out every ounce of accuracy from their ammunition, and there are some aspects of their techniques that you can adopt and experiment with, without getting too technical. Bullet jump is one such aspect and you will find that it can significantly improve the performance of your rounds.

What is bullet jump?

Bullet jump is simply the distance that a bullet travels before it touches the rifling/lands in the barrel. It is only a relatively short distance, typically between 0.000” and 0.150”. This tiny distance is travelled in a fraction of a second after the primer is struck, and it might not seem all that important, but it does have a very significant effect on both accuracy and group size.

Why bullet jump matters

There are a lot of very complicated and sometimes contradictory articles written on the subject of bullet jump, but most do agree on some points. Firstly, when a bullet has to ‘jump’ from the case mouth into the rifling, it may arrive off-centre and the further it has to travel, the more it may be out of line with the centre of the bore. This will have a negative effect on the accuracy of your ammunition. Secondly, the closer the bullet is to the rifling, the better it will perform in terms of accuracy. So, when it comes to bullet jump, it is definitely a case of ‘less is more’.

Adjusting bullet jump

One way to reduce bullet jump is to effectively move the rifling closer to the bullet, which is a permanent ‘adjustment’ requiring work by a gunsmith. The distance between the neck of the case and the start of the rifling is known as the Freebore of the gun and a gunsmith can use a chamber reamer to reduce that dimension so the whole round sits further forward, closer to the rifling. As this is a permanent alteration, you need to be confident that the amount of adjustment is exactly what you need and will suit all of your ammunition. Alternatively, you can experiment with and adjust the bullet seating depth when handloading your own ammo. This is a far easier and better way to explore the effects of bullet jump.

Adjusting bullet seating depth

The seating depth of handloaded ammunition can be adjusted by screwing the seating plug in or out by very small amounts on your bullet seating die. Only a tiny fraction of a turn is required to achieve significant change to the seating depth, so you need to take great care not to over-adjust the die. If you have a seating die with micrometre adjustment then it is easy to make changes of half a thousandth of an inch at a time. If not, then it is important to mark your die in some way so that you can record, control and reverse any changes you make. It is important to remember that when you adjust the bullet seating depth you are changing the Cartridge’s Overall Length (COL) and this has implications, including how your rounds fit in the magazine and how well they feed and chamber in your gun. Adjustments will also affect chamber pressure. All of these factors may restrict how much you can change the amount of bullet jump.

How much bullet jump is best?

Instead of experimenting with different amounts of bullet jump, you can simply set it at whatever amount you consider is ‘best’, however, opinions on how much is best vary. For example, you might read “the optimum amount of jump is usually near 0.020” and while quoted figures are absolutely fine to use, there is no guarantee they will be the best for your particular rifle and bullet combination. If you do decide to set your bullet jump to a predetermined amount, you are going to have to measure how much space you have. For this, you will need to either experiment with dummy rounds of different lengths or use a specialised tool. With dummy rounds, you simply keep trying longer and longer rounds until one actually makes contact with the lands (this will be indicated by marks on the bullet where it has hit the lands) and then reduce the COL back down by the amount of bullet jump you want. Because of the very small measurements involved, this is not always easy to achieve. The Hornady Lock-NLoad OAL gauge uses a specially adapted case of the corresponding calibre, one of your bullets and a feeler gauge to find the exact point where the bullet hits the lands. You then measure the COL of the case and bullet on the end of the feeler gauge, deduct the amount of bullet jump you want and the result is the COL you set your seating die to.

No jump

Precision benchrest shooters often seat their bullets so that there is no jump at all, and the bullets are actually wedged into the lands as the round is chambered. The theory behind this is that the bullet is centred best when it is already touching the rifling, before the round is fired. The greater the distance the bullet has to jump, the more it is likely to drift offcentre, so if there is no jump, there should be no drift. The downside of this practice is that chamber pressure is significantly increased, because the rifling holds the bullet back, rather like trying to start a car moving with the handbrake still on, but most agree that chamber pressures are a lot more consistent when the bullets are seated in contact with the rifling when chambered.

Conclusion

Generally, most shooters agree that the minimum amount of bullet jump gives the most accurate and consistent results, and with careful consideration of chamber pressure, zero bullet jump can be even better. It is well worth experimenting with bullet jump and seating (with due care and attention to safety) and the effect on accuracy can be very noticeable.