Talk to me about Freebore

MarkLeupold

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I've done some Googling around and searching here on the Hide, but freebore is still a tad of a mystery to me. I know that freebore is the distance from the front of the chamber to where the rifling begins (the leade). Currently, I haven't had to worry about freebore too much; factory Remington's have miles of it, so I loaded 175 SMK's to mag length and jumped them as far as I needed. When I rebarrel, how do I know what kind of freebore to put in the barrel?

The barrel is going to be a 6.5 (either .260 or 6.5x47 Lapua) and I want to shoot heavy-for-caliber bullets (either 147 ELD-M, or the new Sierra 150 SMK). I'm probably going to ARC mags with my switch, so COAL is going to be 2.97" max. I'd like to be able to chase the lands a little as the barrel wears. I don't think the x47 will have COAL problems, but the .260 probably will. How do I figure out what I need?

Also, many reamers I've seen have freebore built into them so everything is cut on the same pass. Can most gunsmiths custom ream the freebore longer is it's needed?

Thank you for the help.
 

getoffmylawn

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Sounds like you're already pretty well read up on the subject. I would talk to your gunsmith about what you're looking to shoot and ask him for recommendations. They've likely seen every format you can think of and will know exactly how to satisfy your particular need. Good luck. I'm interested in knowing what fb and twist you end up going with if you're planning on sticking with the new 150 SMK's.
 

buffybuster

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Length of the freebore is going to be dependent on the bullet you're planning to shoot and OAL constraints. Some chamber reamers come spec'ed without any freebore and the smith can cut the freebore separately. Most reamers come with some freebore. Most common cartridges usually have a standardized range of freebore lengths, but what is "optimum" is going to depend on the bullet.
 

918v

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Why don't you make a dummy round based around your bullet and magazine and projected throat wear. Then pick a reamer with the shortest freebore and have your gunsmith cut it to the right length based on your dummy round.
 

MarkLeupold

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Well, I don't have the components or dies to use for a dummy round. This is still planning to figure out my next barrel. I think checking with a smith is my simplest bet right now, but I still would like to figure it out for myself (makes me feel like I know stuff, lol).

Another small question: the point where the bullet engages the riflng, is this at the front of the bearing surface, or further down on the nose of the projectile?

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spife7980

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Another small question: the point where the bullet engages the riflng, is this at the front of the bearing surface, or further down on the nose of the projectile?

Down the front of the bullet just a bit.

https://bartleinbarrels.com/calibers/ So this is a link for all of bartleins barrels land and groove dimensions.

We know that a 30 cal bullet is .308 in diameter and that the bearing length is measured at the full diameter of the bullet.
For a typical 30 cal barrel you will have a bore of .300 and a grove of .308. That means there is .008 of interference fit, or .004 tall lands (one on each side). So because the lands engage the bullet before its full diameter the actual place where the bullet hits the lands is actually a tiny bit in front of the actual bearing surface. If you measure the bearing length on the .308 datum we would measure where the lands hit on the .300 datum.

Now what can further complicate the issue is that the lands arent just a flat square 90 degree cut, they are angled in a bit like a ramp (lead angle). So you cant just necessarily add and subtract, there will be a bit of calculus as youre taking a parabolic bullet shape and mating it to what would be a linear angular introduction of the lands. Knowing exactly what the bullet has and exactly what your smiths reamer is would be required. For this reason the dummy bullet is the preferred method; you let the gunsmith do the math and fitting work and dont risk making an incorrect calculation or transfer of data. [IMG2=JSON]{"data-align":"none","data-size":"full","src":"http:\/\/www.navweaps.com\/Weapons\/Gun_Data_grooves_pic.jpg"}[/IMG2]
[IMG2=JSON]{"data-align":"none","data-size":"full","src":"http:\/\/i286.photobucket.com\/albums\/ll98\/Bullet94_2008\/bulletpartsyp0.gif"}[/IMG2]




http://riflebarrels.com/a-look-at-bu...throat-angles/

 
Last edited:

Dai Bando

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Another way to go about it would be to order a reamer from one of the better companies. Those guys have a good handle on what's what. You won't lose much money on one and would get exactly what you want without the worry of a smith screwing things up.
 

MarkLeupold

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First off, I'll probably build the dummy rounds once I get components and I'm ready to order a barrel, but I'm probably going to grind through the math just for the fun of it.

Down the front of the bullet just a bit.

https://bartleinbarrels.com/calibers/ So this is a link for all of bartleins barrels land and groove dimensions.

We know that a 30 cal bullet is .308 in diameter and that the bearing length is measured at the full diameter of the bullet.
For a typical 30 cal barrel you will have a bore of .300 and a grove of .308. That means there is .008 of interference fit, or .004 tall lands (one on each side). So because the lands engage the bullet before its full diameter the actual place where the bullet hits the lands is actually a tiny bit in front of the actual bearing surface. If you measure the bearing length on the .308 datum we would measure where the lands hit on the .300 datum.

Now what can further complicate the issue is that the lands arent just a flat square 90 degree cut, they are angled in a bit like a ramp (lead angle). So you cant just necessarily add and subtract, there will be a bit of calculus as youre taking a parabolic bullet shape and mating it to what would be a linear angular introduction of the lands. Knowing exactly what the bullet has and exactly what your smiths reamer is would be required. For this reason the dummy bullet is the preferred method; you let the gunsmith do the math and fitting work and dont risk making an incorrect calculation or transfer of data. [IMG2=JSON]{"data-align":"none","data-size":"full","src":"http:\/\/www.navweaps.com\/Weapons\/Gun_Data_grooves_pic.jpg"}[/IMG2]
[IMG2=JSON]{"data-align":"none","data-size":"full","src":"http:\/\/i286.photobucket.com\/albums\/ll98\/Bullet94_2008\/bulletpartsyp0.gif"}[/IMG2]




http://riflebarrels.com/a-look-at-bu...throat-angles/

Spife, I see your point here, and the article you linked is a great piece of information, though from my understanding, Sierra has been manufacturing some neat features into their more recent bullet offerings. The 150 SMK (the bullet I really want to run), has a long, 27-caliber secant ogive. If that was it, then, yes it would take some calculus application to find what I'm looking for, but Sierra has put a 1.5 degree angle bearing surface to ogive junction of the projectile (https://sierrabulletsblog.com/2017/10/13/sierra-introduces-new-6-5mm-150-gr-matchking-bullet/). I believe they did the same with the 6mm 110 SMK and the 7mm 196 SMK, but the theory behind it is that it matches the lead angle of many match chambers, aligning the bullet with the bore. Would this help to simplify the problem some?
 

spife7980

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Maybe. My intuition would say yes like yours does but honestly at this point that math is above my pay grade. It's been so long since I've done math I now struggle to count all ten fingers :p
 

Supersubes

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    Spife layed out the relationships well. I'll only add a few things. 1.5 degree lead angles are pretty much standard for the big three reamer manufacturers, unless you specify something else. Like Spife, im not doing the math to figure out exactly where the ogive on a particular bullet meets the 1.5 degree lead. Fortunately for us, its a very small number. When I spec/order a reamer, I ignore that. Now condisider the relationships between the your two chosen cartridges. One jams the bullet way in past the neck shoulder junction, has a short neck, and mag constraints need to be considered. The other is a polar opposite, and suffers none of those things. They both shoot amazing and reach very similar velocities, go figure. With cartridges like the 47, 6xc, and Creedmoor, you really have a lot of room to play around in with their long necks and short overall lengths. Lastly, unlike the .260, the 47 was a ground up target cartridge. Most all the prints ive seen of it are very similar, and angle towards heavier bullets. The 47 reamer I ordered in 2008 or so makes a beautiful chamber. It has .123 freebore, but honestly it could be .160 and would still be swimming in an AI mag, with plenty of neck gripping the bullet. I view the creedmoor the same way. Grab yourself some brass, bullets, a seater, and start tinkering.
     

    krw

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    I have a pr of custom 260 rifles with chambers cut with same reamer. Both rifles have 1-8.5T Brux bbls. I dummy lded SMK 123’s in Lapua brass at 2.800”oal. I had PTG make a reamer so neck was .298” and the bullet had .010” (Jump). By PTG figuring this came out to .047” Freebore. I jave had pressure issues and both rifles are a lil on the finicky side. I have contemplated having another .020” freebore added. There is no sense in building a custom rifle and having freebore like a factory Rem, but I think I had it shortened lil too much. Its easy to go longer. Good luck!
     

    Supersubes

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    I have a pr of custom 260 rifles with chambers cut with same reamer. Both rifles have 1-8.5T Brux bbls. I dummy lded SMK 123’s in Lapua brass at 2.800”oal. I had PTG make a reamer so neck was .298” and the bullet had .010” (Jump). By PTG figuring this came out to .047” Freebore. I jave had pressure issues and both rifles are a lil on the finicky side. I have contemplated having another .020” freebore added. There is no sense in building a custom rifle and having freebore like a factory Rem, but I think I had it shortened lil too much. Its easy to go longer. Good luck!


    Whats the OAL of the cartridge when loaded to .010 jump? Using AICS mags? How much room
    left in the magazine?

    My personal .260 reamer is .080 freebore, and was designed for VLD’s in AW mags. It works, but only just. Bullets like yours on the other hand work perfectly in it. If you believe that there is too much bullet down in the case, you can test the theory by loading a similar weight VLD. I dont think you’ll see a difference personally. I just wouldnt expect .033 freebore difference between our reamers to cause any noticable issues. I’ve kinda moved on from the .260, but when I was drawing up a lapua brass specific reamer, the freebore was going to be set at .050 or so.
     

    krw

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    My ltwt rifle is strictly a hunting rifle. I do shoot Nosler 130 Accubonds in it also. I only use BDL bottom metal on it. My 26” hvy rifle uses AI mags. I ld the SMK’s to 2.800” oal and have .010” jump
     

    MarkLeupold

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    Spife layed out the relationships well. I'll only add a few things. 1.5 degree lead angles are pretty much standard for the big three reamer manufacturers, unless you specify something else. Like Spife, im not doing the math to figure out exactly where the ogive on a particular bullet meets the 1.5 degree lead. Fortunately for us, its a very small number. When I spec/order a reamer, I ignore that. Now condisider the relationships between the your two chosen cartridges. One jams the bullet way in past the neck shoulder junction, has a short neck, and mag constraints need to be considered. The other is a polar opposite, and suffers none of those things. They both shoot amazing and reach very similar velocities, go figure. With cartridges like the 47, 6xc, and Creedmoor, you really have a lot of room to play around in with their long necks and short overall lengths. Lastly, unlike the .260, the 47 was a ground up target cartridge. Most all the prints ive seen of it are very similar, and angle towards heavier bullets. The 47 reamer I ordered in 2008 or so makes a beautiful chamber. It has .123 freebore, but honestly it could be .160 and would still be swimming in an AI mag, with plenty of neck gripping the bullet. I view the creedmoor the same way. Grab yourself some brass, bullets, a seater, and start tinkering.
    Honestly, this is why I am leaning towards the x47 over the 260. I have 308 brass that I could make into 260, and I'd have to have bolt work done (bushing the firing pin hole) to fire the x47 without popping primers and whatnot, but I think the extra work is worth it. I'll have what I really want, plus I won't have to worry about the mag-length problems. From the math I've been doing, a CIP standard reamer with a 4.5 mm freebore (.177 inch) would work well with the 150 SMK. I haven't seen much load development for this combination yet, so I'm hoping they'll get to at least 2650 or 2700 fps.

    Sent from my SAMSUNG-SM-G920A using Tapatalk