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Setting Bullet Jump/Freebore for Accuracy

Discussion in 'Reloading' started by Pink_Vapor, Mar 23, 2020.

  1. Pink_Vapor

    Pink_Vapor Well-Known Member Supporting Member

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    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.

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    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:

    1. 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.
    2. 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.
    What Every Gunsmith Wish You Knew
    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?
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    “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:

    [​IMG]
    “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

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    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.

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    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
     
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  2. Pink_Vapor

    Pink_Vapor Well-Known Member Supporting Member

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    Most published materials on reloading repeat a similar idea. The Hornady Reloading Manual simply says, “In general, the less distance to the bore, the greater the accuracy.

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    Another one of my favorite resources are the books by Nathan Foster, an accomplished gunsmith in New Zealand and author of Terminal Ballistics Research. I’ve read a ton of what he’s written, and Nathan has earned my respect because he has both a practical and data-driven approach. Here is what Nathan says on the subject: “Why seat close to the lands? In our last example Joe read an article where the author stated that for best accuracy, he should be seating close to the lands. Many of you will have heard this statement. Target shooters often go to the extreme and wedge the projectiles right into the lands – but why? The statement ‘because it is more accurate this way’ is not really an explanation. Current research suggests that by pushing the projectile into the lands pressures can be made more consistent from shot to shot.” Nathan later adds that, “The projectile may arrive at the lands slightly off center if it has to travel a long way. In this instance, having the bullet close and concentric to the bore can help minimize potential bullet yaw during ignition. Close seating can also simply be used as a start point for harmonic experimentation. We can test the bullet up close to the lands, then step back if need be, monitoring the effects on accuracy.” So, Nathan has more a pragmatic approach, as expected, where he starts close and then experiments to see what jump results in the best accuracy for that rifle/bullet combination.

    [​IMG]
    Dr. Harold Vaughn, who is literally one of the greatest researchers of the past 100 years (you should read about him here), concurs with the Nathan’s point about how important it is to get the bullet absolutely centered with the bore. In his book, Rifle Accuracy Facts, which covers his extensive research, he shares how Benchrest shooters desire chambers with minimum clearance in the necks and they also turn the necks of their brass to ensure the bullet is perfectly centered with the bore. Dr. Vaughn goes on to explain, “They also seat the bullets into the lands, which helps center the bullet. However, seating the bullet in the case so that it contacts the rifling in the throat also increase the peak chamber pressure, which is not desirable. Evidently the Benchrest shooters have found that having the bullet centered in the bore is important, and I think they are right. … Seating depth of the bullet in the case has an effect on just how close to the center the bullet will line up. Obviously, the bullet will be centered if it is in complete contact with the lands, however Reference 1 showed that peak chamber pressure decreases if the bullet has a free run before it contacts the lands. Since a minimum in peak pressure for a given load implies minimum bullet distortion, the author prefers a seating depth that will provide about 0.010 inches into the lands in the case of a Benchrest gun with light bullets and about 0.020 inches of bullet free travel before the bullet contacts the lands in the case of a sport shooting heavy bullets.”

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    Tony Boyer is the most successful and celebrated Benchrest shooter of all time. He has won several World Championships and was named Shooter of the Year over ten times. In his 2010 book, The Book of Rifle Accuracy, he simply says “When shooting with the bullet into the lands, the lands themselves straighten the bullet without forcing over-travel.” However, Tony goes on to say that bullet shape can affect optimal seating depth, and “There are no hard and fast rules dictating the best seating depth for all bullet/barrel combinations.” But, Tony does say there are some common characteristics of barrels, and when he’s searching for the sweet spot “with a button-rifled barrel such as a Shilen, I would have stopped the search for the optimal seating depth at the 0.012 inches off the jam. … With a four-groove cut-rifled barrel (Bartlein or Krieger), be more patient and keep trying until there is no mark on the bullet at all. Again, I personally do not want to shoot off the rifling so that become my stop point.” So, the guy who has proven several times that he can shoot the smallest groups in the world, says he is always at least touching the lands, and often the bullet is jammed into it.

    [​IMG]
    Now and then, 2-3 thousandths change in seating depth can make the difference between average and peak accuracy,” explains Mike Ratigan, Benchrest Hall of Famer and World Champion, in Extreme Rifle Accuracy. Wow, 0.002-0.003 inches in bullet jump can sometimes make the difference! Of the dozen or so published resources I’ve cited so far (plus others), they virtually all suggest testing seating depths in 0.005” increments, meaning most expect that changing the seating depth by just 0.005 inches or less can have a measurable impact on group size on the target.

    I could cite even more books, but it seems like there is overwhelming support that if the absolute smallest groups are your highest priority, then minimizing jump seems to be the way to go. Everyone from accomplished scientific researchers to the absolute best-of-the-best, world-champion shooters are convinced that you’ll likely find the best accuracy with 0.020 inches of bullet jump or less.

    The One Resource That Suggests Testing Longer
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    The only professionally published resource I’ve found that clearly recommends testing beyond 0.020” of bullet jump in a precision rifle is from Berger Bullets. In the Berger Bullets Reloading Manual they say:

    “What has been discovered is that VLD bullets shoot best when loaded to a COAL that puts the bullet in a ‘sweet spot.’ This sweet spot is a band 0.030 to 0.040 wide, and is located anywhere between jamming the bullets into the lands and 0.150 jump off the lands.

    Berger’s VLD bullets are Very Low Drag bullets that have extremely high ballistic coefficients, meaning they are very aerodynamic and ideal for long range shooting, but have been known to be sensitive to seating depth (and probably more precisely bullet jump). Because VLD bullets were known to be very sensitive to bullet jump, Berger apparently performed a lot of research and found that the sweet spot for optimal precision could be as far as 0.150” of bullet jump on some rifles.

    Eric Stecker, previously Master Bulletsmith for Berger and now President, provides detailed instructions for how to find the optimal jump in as few of shots as possible in their manual, but they also republished similar information in this article online: Getting the Best Precision and Accuracy from VLD bullets in Your Rifle.
     
  3. Tim

    Tim I am....an enchanter. Staff Member Benefactor Charter Life Member

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    I was skeptical about this until seeing it myself first hand. Messing with freebore on a Savage that another forum member now owns, made a .75 MOA gun a legit sub .5 MOA.
     
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  4. Pink_Vapor

    Pink_Vapor Well-Known Member Supporting Member

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    My Sako 7-08 has a l-o-n-g freebore. The best 100yrd groups using top shelf Winchester, Federal and Hornady ammo was 2.75"!
    The first handloads I made on my new 550 (in 2000) and setting the freebore to 0.005"-0.008" produced a 5 round 0.355"group using a traffic cone as a rifle rest. Thankfully they still fit in the magazine, the rounds were 1/4" longer than the factory ammo.
     
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  5. jimmyjames8

    jimmyjames8 Well-Known Member Supporting Member

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    Great references in the OP. I just did a test load with 200.20X bergers in 308. Got load data from a thread on Accurate Shooter. Cant remember this second whether it was 0.010" jam or jump but what made the difference for me was getting an accurate measurement of where the bullet touches the lands. Ive tried lots of methods and tools. THis is the only thing that works for me:
     
  6. Gun Lover

    Gun Lover Happy to be here

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    So the video has me intrigued about trying to hone in my distance to the lands using the described method.
    Here's my conundrum...I do not currently own or have access to an ejector removal tool.
    Any recommendations on one over another? Most of the ones I have seen are the Sinclair ones - $35-ish.
    There's also a "complete" bolt tool kit for another $100. Is it worth it to have all of those tools?
    I appreciate any words of wisdom or direction based on your collective experiences.
    Thanks!
     
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  7. Pink_Vapor

    Pink_Vapor Well-Known Member Supporting Member

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    What type of bolt/gun is this tool needed for?
     
  8. Goofyfoot2001

    Goofyfoot2001 Member Supporting Member

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    Coincidently loaded up a few 300 win mag extra long that of course won't fit in the mag. 230gr sitting next to the plain bullet and a 300blk in 220gr
    20200326_205523.jpg
     
  9. Gun Lover

    Gun Lover Happy to be here

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    Bergara BMP, so Rem 700 stuff should work