freebore explantion
- Thread starter jaredc
- Start date
Hi Jared:
Freebore is the distance on a chambered case from the edge of the case mouth to the beginning of the rifling in the barrel. Generally, the distance is shorter for lighter bullets and longer for heavier bullets. It is valuable as to how far out you can seat a bullet before it touches the rifling. So long bullets won't be seated with the tail intruding into the powder capacity of the case. It can get more complicated when ogive and leade taper come into consideration, but this is the general idea.
Freebore is the distance on a chambered case from the edge of the case mouth to the beginning of the rifling in the barrel. Generally, the distance is shorter for lighter bullets and longer for heavier bullets. It is valuable as to how far out you can seat a bullet before it touches the rifling. So long bullets won't be seated with the tail intruding into the powder capacity of the case. It can get more complicated when ogive and leade taper come into consideration, but this is the general idea.
Forum Boss
Administrator
Reed gave a good description. The other practical factors to consider are:
1) Most people hate dealing with the brass ridge or "doughnut" that can build up at the neck-shoulder junction. This interferes with bullet seating and is a pain in the behind to eliminate. By setting the freebore so the bottom of the bearing surface,max diam. bullet shank) is .050"+ above the doughnut zone, you can pretty much ignore it.
2) As a general rule, long bullets require more freebore, but with long skinny bullets, that's not always the case. The 90gr boat-tail bullets are fairly long, but have an average bearing surface so they don't need a lot of freebore. The Berger 95 VLD has a super short bearing surface so it needs very little freebore--less than the 60gr flatbase bullets for example.
3) Bullet seating depth can have a huge impact on pressures and velocities. For example, just today I was doing some chron testing. With the same 105gr bullets and exact same load, I chron'd a velocity of 2927 fps with the bullets .015" into the lands. Then I tried them moved back to .012" away from the lands, a difference of .027" or roughly 1/32nd of an inch. The velocity spiked to 3031 and I got stiff bolt lift. How can that little bit make such a difference? Well, jamming into the lands raises start pressures, but it also increases cases capacity. By moving the bullet back I reduced case capacity and also got the bullet to release at a different micro-second in the burn cycle.
What will I do in this situation? I'm going to reduce the charge and reduce the jump a little to see if I can keep some of that extra speed without spiking the pressure.
1) Most people hate dealing with the brass ridge or "doughnut" that can build up at the neck-shoulder junction. This interferes with bullet seating and is a pain in the behind to eliminate. By setting the freebore so the bottom of the bearing surface,max diam. bullet shank) is .050"+ above the doughnut zone, you can pretty much ignore it.
2) As a general rule, long bullets require more freebore, but with long skinny bullets, that's not always the case. The 90gr boat-tail bullets are fairly long, but have an average bearing surface so they don't need a lot of freebore. The Berger 95 VLD has a super short bearing surface so it needs very little freebore--less than the 60gr flatbase bullets for example.
3) Bullet seating depth can have a huge impact on pressures and velocities. For example, just today I was doing some chron testing. With the same 105gr bullets and exact same load, I chron'd a velocity of 2927 fps with the bullets .015" into the lands. Then I tried them moved back to .012" away from the lands, a difference of .027" or roughly 1/32nd of an inch. The velocity spiked to 3031 and I got stiff bolt lift. How can that little bit make such a difference? Well, jamming into the lands raises start pressures, but it also increases cases capacity. By moving the bullet back I reduced case capacity and also got the bullet to release at a different micro-second in the burn cycle.
What will I do in this situation? I'm going to reduce the charge and reduce the jump a little to see if I can keep some of that extra speed without spiking the pressure.
""3) Bullet seating depth can have a huge impact on pressures and velocities. For example, just today I was doing some chron testing. With the same 105gr bullets and exact same load, I chron'd a velocity of 2927 fps with the bullets .015" into the lands. Then I tried them moved back to .012" away from the lands, a difference of .027" or roughly 1/32nd of an inch. The velocity spiked to 3031 and I got stiff bolt lift. How can that little bit make such a difference? Well, jamming into the lands raises start pressures, but it also increases cases capacity. By moving the bullet back I reduced case capacity and also got the bullet to release at a different micro-second in the burn cycle.""
Moderator I find this extremely interesting. I would never have thought 27 thousands would have this kind of effect. I understand exactly what you are saying but it still doesn't seem right. Please let me know of your later findings.
Changeling
Moderator I find this extremely interesting. I would never have thought 27 thousands would have this kind of effect. I understand exactly what you are saying but it still doesn't seem right. Please let me know of your later findings.
Changeling
Forum Boss
Administrator
Changeling said:
Well it surprised the heck out of me too. Could be a chronograph malfunction, but I had four readings in a row, between 3020-3033.Oddly, cases looked fine,no ejector mark, primers still round), but pressure was high enough to create a stiff bolt lift. Maybe the chron readings were a fluke. Possibly what happened is that the reduction in OAL created a compressed charge. We'll test again and see what happens.
FYI Darrel Jones said he was getting 3015 fps from a std case in a 30" barrel. That seemed way high to me... but now I'm not so sure. He was running about .003 into the lands, but with a short freebore so his SMK 107s were sitting right on top of the powder column as well. What I can say is the 6BR case seems to deliver very good accuracy right at the top of the pressure zone, much like the 6PPC.
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