https://precisionrifleblog.com/2020/03/21/bullet-jump-and-seating-depth-reloading-best-practices/
Bullet Jump & Seating Depth: Best Practices & Conventional Wisdom
With so many events canceled and stores closed, what a great time to do some reloading! Over the past several weeks, I have been working on a series of posts that I’m very excited to finally share with you guys!
This article provides a comprehensive overview of what most professionally published books and reloading manuals suggest about bullet jump and seating depth when it comes to precision rifles. I have a big stack of books on the subject, and I tried to combine the most relevant info from each of them when it comes to fine-tuning the seating depth of your ammo.
What Is “Bullet Jump”?
Bullet jump is basically the distance a bullet travels before it touches the rifling in the barrel. That gap is relatively short distance that typically ranges from 0 to 0.150 inches, although it could be longer in some factory or magazine-fed rifles.
There is a lot going on in a chamber in the tiny window when a round goes off. Factors like whether the bullet jumps 0.010” or 0.070” before it engages the rifling may seem inconsequential but can have measurable impact on the bullet’s flight and group size. There are multiple reasons for that, which I’ll touch on from a high level below. But suffice to say that these kinds of details are especially relevant in shooting disciplines that are pushing the envelope of precision and/or engaging small targets at long distance.
There are two primary ways to fine-tune bullet jump:
Some shooters obsess over freebore (including me at times), and occasionally insist on specific freebore dimension for their barrel, despite a gunsmith’s recommendations. A gunsmith who is brutally honest might say that is one of the most annoying parts of their job. I feel like I should say that out loud on behalf of all my gunsmith friends. And I admit I’ve been an offender at times, too. While freebore can be an important spec, one of the best pieces of advice I can give is this: Find a gunsmith who specializes in precision rifles and has a proven track record of satisfied customers, then tell them what your goals are and how you plan to use the rifle, and finally: trust their recommendations.
What Are Recommended Best Practices When It Comes to Bullet Jump?
“It’s pretty well accepted that the shorter the gap, the better, and a long-held, and for good reason, accuracy-enhancing tactic is nearly or completely eliminating jump. The longer the bullet is, and the ‘spikier’ its nosecone, the better it will perform moving toward the lands.” – Glen Zediker, Top-Grade AMMO (Published 2016)
I’ve seen countless quotes like the one above in reloading manuals and books. Top-Grade Ammo is one of the best handloading resources I’ve read, but here is how another popular book explains it:
“Handloaders can tinker with the bullet-seating depth so as to exceed the listed maximum [cartridge overall length] significantly. Sometimes they can tinker until the bullet comes very close to touching the rifling. This can dramatically improve accuracy. When COAL places the bullet close to touching the rifling, changes of about 0.005 inch can dramatically alter accuracy. … For most hunting rifles and bullets, the optimum bullet-to-rifling jump is usually near 0.020 inch; for target cartridges used in target-chambered guns, the ideal jump can be near zero.” – Mic McPherson, Metallic Cartridge Handloading: Pursuit of the Perfect Cartridge
Glen Zediker wrote a book entitled Handloading for Competition: Making The Target Bigger, and here are some of the highlights from what it says about bullet jump and seating depth: “Determining the bullet seating depth a rifle likes best is normally ‘fine tuning’ that entices any stray holes to relocate on into the group. Any and every load and bullet responds to seating depth tuning. … How much jump is a good question that has only one answer: whatever shoots best. That may not be the answer anyone was looking for but it’s the right one, and it’s also not always the one we want to accept for various reasons. Most competition rifles will shoot best with bullets seated somewhere between jammed 0.020 and jumping 0.020. … Again, there is no fast rule on the amount of jump a particular combination will prefer, but odds are closer will do better.”
One of the most interesting articles I’ve ever read about precision shooting was called “Secrets of the Houston Warehouse.” It was written by Dave Scott and published by Precision Shooting Magazine in 1993. It’s about an accomplished group of Benchrest shooters who conducted experiments in a large warehouse that redefined extreme rifle accuracy. They used the large, controlled environment where the wind never blew, and mirage never shimmered to test virtually every aspect of rifle building and reloading. They were shooting 5-shot groups “in the zeros,” meaning the extreme spread measured 0.099” or less. In fact, they eventually fired 5-shot groups as small as 0.025”!!! Those experiments contributed to many of the best practices Benchrest shooters still use today, and here is what that article says about bullet jump:
“One thing that IS important is that the bullet be precisely seated against the lands. T.J. Jackson reported this fact in the May 1987 issue of Precision Shooting. In a letter to the Editor, T.J. wrote, “… in all our testing in that Houston warehouse … and the dozens and dozens of groups that Virgil King shot in there ‘in the zeroes’… he NEVER fired a single official screamer group when he was ‘jumping’ bullets. All his best groups were always seated into the lands, or at the very least … touching the lands.” – Secrets of The Houston Warehouse, Precision Shooting Magazine
Bullet Jump & Seating Depth: Best Practices & Conventional Wisdom
With so many events canceled and stores closed, what a great time to do some reloading! Over the past several weeks, I have been working on a series of posts that I’m very excited to finally share with you guys!
This article provides a comprehensive overview of what most professionally published books and reloading manuals suggest about bullet jump and seating depth when it comes to precision rifles. I have a big stack of books on the subject, and I tried to combine the most relevant info from each of them when it comes to fine-tuning the seating depth of your ammo.
What Is “Bullet Jump”?
Bullet jump is basically the distance a bullet travels before it touches the rifling in the barrel. That gap is relatively short distance that typically ranges from 0 to 0.150 inches, although it could be longer in some factory or magazine-fed rifles.
There is a lot going on in a chamber in the tiny window when a round goes off. Factors like whether the bullet jumps 0.010” or 0.070” before it engages the rifling may seem inconsequential but can have measurable impact on the bullet’s flight and group size. There are multiple reasons for that, which I’ll touch on from a high level below. But suffice to say that these kinds of details are especially relevant in shooting disciplines that are pushing the envelope of precision and/or engaging small targets at long distance.
There are two primary ways to fine-tune bullet jump:
- Adjust Bullet Seating Depth: When handloading ammo, you can control how far the bullet must jump by adjusting bullet seating depth. However, when a reloader adjusts their seating depth to extend the bullet further out it increases the Cartridge Overall Length (COAL), which may prevent a loaded round from fitting inside your magazine. So, if feeding from a magazine is a priority, you could be limited on how close to the lands you can get.
- Adjust Chamber Freebore: Another way to get a bullet closer to the lands and stay within magazine length is to shorten the throat of a chamber. Instead of extending the bullet out of the case towards the rifling by adjusting seating depth, you effectively move the rifling closer to the bullet. The diagram above shows an area labeled as “Throat,” but that distance is also commonly referred to as freebore. Freebore is simply the distance between the neck and where the rifling starts. Let’s say for example that the SAAMI chamber for a particular cartridge calls for 0.180” of freebore. If we use a chamber reamer that only had 0.100” of freebore, that would effectively move the rifling back towards the bullet 0.080”. Okay, technically it isn’t “moving” the rifling – it just means the chamber reamer would leave more of the rifling close to the chamber.
Some shooters obsess over freebore (including me at times), and occasionally insist on specific freebore dimension for their barrel, despite a gunsmith’s recommendations. A gunsmith who is brutally honest might say that is one of the most annoying parts of their job. I feel like I should say that out loud on behalf of all my gunsmith friends. And I admit I’ve been an offender at times, too. While freebore can be an important spec, one of the best pieces of advice I can give is this: Find a gunsmith who specializes in precision rifles and has a proven track record of satisfied customers, then tell them what your goals are and how you plan to use the rifle, and finally: trust their recommendations.
What Are Recommended Best Practices When It Comes to Bullet Jump?
“It’s pretty well accepted that the shorter the gap, the better, and a long-held, and for good reason, accuracy-enhancing tactic is nearly or completely eliminating jump. The longer the bullet is, and the ‘spikier’ its nosecone, the better it will perform moving toward the lands.” – Glen Zediker, Top-Grade AMMO (Published 2016)
I’ve seen countless quotes like the one above in reloading manuals and books. Top-Grade Ammo is one of the best handloading resources I’ve read, but here is how another popular book explains it:
“Handloaders can tinker with the bullet-seating depth so as to exceed the listed maximum [cartridge overall length] significantly. Sometimes they can tinker until the bullet comes very close to touching the rifling. This can dramatically improve accuracy. When COAL places the bullet close to touching the rifling, changes of about 0.005 inch can dramatically alter accuracy. … For most hunting rifles and bullets, the optimum bullet-to-rifling jump is usually near 0.020 inch; for target cartridges used in target-chambered guns, the ideal jump can be near zero.” – Mic McPherson, Metallic Cartridge Handloading: Pursuit of the Perfect Cartridge
Glen Zediker wrote a book entitled Handloading for Competition: Making The Target Bigger, and here are some of the highlights from what it says about bullet jump and seating depth: “Determining the bullet seating depth a rifle likes best is normally ‘fine tuning’ that entices any stray holes to relocate on into the group. Any and every load and bullet responds to seating depth tuning. … How much jump is a good question that has only one answer: whatever shoots best. That may not be the answer anyone was looking for but it’s the right one, and it’s also not always the one we want to accept for various reasons. Most competition rifles will shoot best with bullets seated somewhere between jammed 0.020 and jumping 0.020. … Again, there is no fast rule on the amount of jump a particular combination will prefer, but odds are closer will do better.”
One of the most interesting articles I’ve ever read about precision shooting was called “Secrets of the Houston Warehouse.” It was written by Dave Scott and published by Precision Shooting Magazine in 1993. It’s about an accomplished group of Benchrest shooters who conducted experiments in a large warehouse that redefined extreme rifle accuracy. They used the large, controlled environment where the wind never blew, and mirage never shimmered to test virtually every aspect of rifle building and reloading. They were shooting 5-shot groups “in the zeros,” meaning the extreme spread measured 0.099” or less. In fact, they eventually fired 5-shot groups as small as 0.025”!!! Those experiments contributed to many of the best practices Benchrest shooters still use today, and here is what that article says about bullet jump:
“One thing that IS important is that the bullet be precisely seated against the lands. T.J. Jackson reported this fact in the May 1987 issue of Precision Shooting. In a letter to the Editor, T.J. wrote, “… in all our testing in that Houston warehouse … and the dozens and dozens of groups that Virgil King shot in there ‘in the zeroes’… he NEVER fired a single official screamer group when he was ‘jumping’ bullets. All his best groups were always seated into the lands, or at the very least … touching the lands.” – Secrets of The Houston Warehouse, Precision Shooting Magazine