Chamber design: Freebore vs a long leade

[wind gypsy]

When considering a discussion about SAAMI chamber vs custom, a common difference is that many SAAMI cartridges don't have freebore but rather a leade with larger diameter and constant taper until it is below groove diameter. Ex: 284 Win saami chamber has no freebore but rather a long and shallow angle leade that starts @ 0.290" diameter. Most custom reamers will have freebore diameter of 0.2845" before the start of the leade.

Based on the fact that modern cartridge designs intended for precision shooting applications use freebore and pretty much all of the custom reamer specs for competition shooters do as well, I assume there is a benefit to doing so. I can imagine a tight freebore might help the bullet stay centered in the bore as it enters the lands but was just curious if that is correct and is that the only benefit?

Basically, what are the benefits/compromises of using freebore like in most modern "precision focused" chamber designs vs something like the SAAMI 284 throat design?

 

[4O6shootist]

When considering a discussion about SAAMI chamber vs custom, a common difference is that many SAAMI cartridges don't have freebore but rather a leade with larger diameter and constant taper until it is below groove diameter. Ex: 284 Win saami chamber has no freebore but rather a long and shallow angle leade that starts @ 0.290" diameter. Most custom reamers will have freebore diameter of 0.2845" before the start of the leade.

Based on the fact that modern cartridge designs intended for precision shooting applications use freebore and pretty much all of the custom reamer specs for competition shooters do as well, I assume there is a benefit to doing so. I can imagine a tight freebore might help the bullet stay centered in the bore as it enters the lands but was just curious if that is correct and is that the only benefit?

Basically, what are the benefits/compromises of using freebore like in most modern "precision focused" chamber designs vs something like the SAAMI 284 throat design?

A 1° lead will degrade much faster than a modern 3°, the 3° matches ogive angle of majority of newer match projectiles.

 

[wind gypsy]

Isn’t 1.5 degree more common?

 

[4O6shootist]

Isn’t 1.5 degree more common?

1.5 per side, 3 total yes. I forget what the shallow angle lead was on the old 284. I know the modern day rums I believe we're 1° side, 2° total, and with their overbore nature moved pretty fast.

 

[Macht]

I think it's Lilja that has an interesting write-up on their site about matching throat angle to the projectile you plan to shoot.

Unsure if there's any merit to it, but if you went that route you could optimize the throat angle that way and then add or subtract freebore length to set the jump/OAL you're after.

 

[Edds]

There are a lot of cartridges that have a cone shaped entry into the barrel vs having a parallel freebore. I have asked about this a few times on different websites. I’ve never gotten any useful information about it.

 

[4O6shootist]

There are a lot of cartridges that have a cone shaped entry into the barrel vs having a parallel freebore. I have asked about this a few times on different websites. I’ve never gotten any useful information about it.

Every rifle has a cone shaped lead into the lands, it's mandatory for low psi engraving. A guy could square off, force jam, prob not a good idea tho. Every saami chamber has a leade angle. Most often there's full diameter cylinder between chamber neck taper and the lead(.0005-.0015 larger than bullet) that allows heavies to ve seated out of powder space.

The whole design of the 284 @wind gypsy is asking about, was because it was originally built in a short action mag fed fed lever gun with light bullets. Light 7mm bullets had minimal bearing surface, so the near no freebore Chamber with gradual lead angle kept engraving forces and initial chamber pressure down. If you look at the typical F class 284win or 284 shehane chambers, they are very different than the saami 284 win, because that purpose is vastly different than upon the cartridges inception.

The case geometry of the 284 win is a major influence upon today's modern cartridges. Low taper, steep shoulder, hold psi very well.

 

[Edds]

Every rifle has a cone shaped lead into the lands, it's mandatory for low psi engraving. A guy could square off, force jam, prob not a good idea tho. Every saami chamber has a leade angle. Most often there's full diameter cylinder between chamber neck taper and the lead(.0005-.0015 larger than bullet) that allows heavies to ve seated out of powder space.

The whole design of the 284 @wind gypsy is asking about, was because it was originally built in a short action mag fed fed lever gun with light bullets. Light 7mm bullets had minimal bearing surface, so the near no freebore Chamber with gradual lead angle kept engraving forces and initial chamber pressure down. If you look at the typical F class 284win or 284 shehane chambers, they are very different than the saami 284 win, because that purpose is vastly different than upon the cartridges inception.

The case geometry of the 284 win is a major influence upon today's modern cartridges. Low taper, steep shoulder, hold psi very well.

If the 284 chamber is considered as it relates to the OP's question, the SAAMI chamber has a .199" freebore.

 

[4O6shootist]

If the 284 chamber is considered as it relates to the OP's question, the SAAMI chamber has a .199" freebore.

You sure? Saami shows a zero freebore 0.5° lead long throat.

 

[Edds]

Yes, I'm sure. From .290" diameter to .2845" diameter at a .7925 degree angle is .1988".

 

[4O6shootist]

Yes, I'm sure. From .290" diameter to .2845" diameter at a .7925 degree angle is .1988".

I see what you're saying, odd tapered freebore. Most freebore is constant diameter to a tapered lead.

 

[spife7980]

The new chambers have freebore, the old ones just start wide and taper down, see 243, 270 etc in the saami drawings.

 

[Edds]

The new chambers have freebore, the old ones just start wide and taper down, see 243, 270 etc in the saami drawings.

FREE BORE​

A portion of the chamber, usually cylindrical, forward of the case mouth of a diameter larger than the projectile in which rifling is not present.

 

[4O6shootist]

The new chambers have freebore, the old ones just start wide and taper down, see 243, 270 etc in the saami drawings.

I'm in agreement here. Freebore, and an oversized throat with long shallow lead angle, are not the same thing. Typical 7mm freebore is 0.285 diameter, supporting and aligning the bullet to the lead. The old 284 starts at 290, then tapers down to 277 bore diameter. Maybe the geometry says you'll have 0.199 distance before the taper reaches 284 or 285 but this isn't freebore. It's just Linda Lovelace or Heather Brooke style throat!