Whats the significance in adding extra freebore to a chamber. I know it can be done through various means from a reamer with added freebore to throating reamers. Whats the advantage if any. Id really like to know more about this.
It's done when you're trying to get more bullet out of the case and more powder in.
Basically, the juncture point of the bullets bearing surface / boat tail is held just ahead of the juncture point of the case neck / shoulder. The free bore is cut accordingly. This also helps to avoid the donut area. On the longer calibers like 300wm, 7mmRM, 300 Ackley etc. you enter the land of the single shots when this is done.
When having new reamers ground this is one thing I look at along with chamber neck clearance. It does make a difference at times but cant really be used on larger, longer calibers if mag feeding is desired.
in order to keep your case capacity when shooting heavy bullets much longer than the ones chamber was originally designed for, you need to lengthen the freebore.
i'll give an example of something i am going to work on as soon as the bottom metal shows up.
right now we have a 7wsm chamber set up for 180vld touching the lands at 2.94" oal so it will fit in a modified aics short action magazine. this puts that long bullet way down into teh powder column. once we get the seekins bottom metal, we are going to throat it for that same bullet touching the lands at 3.1" oal. the rest of the chamber is going to stay the same but the freebore is getting longer to allow the bullet to seat out further.
I ordered a reamer from Dave Kiff at PTG.
He wanted to know the bullet I would use, so he could make the throat short enough so I could reach the lands, but long enough so I would not seat into the case neck base doughnut.
For pressure and velocity:
I have overloaded lots of cartridges and found the threshold of short brass life. The longer the bullet is seated, the more powder can be used and stay inside of long brass life. Seated longer and longer, until the bullet jams into the lands. Then the amount of powder that can be used spikes down.
So if I want to set a personal velocity record with some cartridge, I would make the throat so long the bullet cannot reach it.
Roy Weatherby made magnums with very long throats, that made for great velocity, but compromised accuracy.
When I shoot reduced loads in a straight wall cartridge, I can get quiet but powerful loads by seating down into the case. This gets the peak pressure up, even with low gas mass. The 2" long jump to the lands makes the loads very inaccurate, even at 10 yards.
Boots mentioned back one time, think of a 60° center vs a 45° center slammed in the back side of a chucked up chamfered barrel..The 45 comes out nice and quick, the 60° is a bit stickier..What he was alluding to is a long tapered leade to the throat can be sticky...Make it long enough to seat the bullet, but also make the break to the land edge not too slow..