Deep throat: Increasing freebore by 0.5"

[Lurq]

Hello all, have always appreciated the information I could find on the Hide.
Not a reloader or builder but interested in an experiment with the following goals, use readily available components to convert a .300WinMag into an unusually long COAL round with great downrange characteristics while still using .300WM brass.

Have the components bought and reloading expert paid for, about to work with the reamer. My plan is to have a friend build me the extra long rounds, but if cost/material becomes an issue for the long term, will the extra long freebore allow me to still shoot factory-length ammo safely and without damage?
Would having the freebore of a .300WM currently at 3.467" extended by 0.540" to accommodate a very long bullet cause me problems if I then shoot factory .300WM in it? The barrel is 24" with 1:9.3 twist (prefer longer and faster but want to confirm theories first).

Damage or rapid wear due to jump? Blowback/pressure issues due to more open freebore? Major accuracy issues from stabilizing ?
I've looked up costs to rebarrel so I can have both chambers, but my preference is to leave the blue printed bolt action as is after the reamer's work is done.

As you can tell, I'm a rank amateur at experimenting (which is why I have experts helping me), but any thoughts would be appreciated.
Lurq

 

[Carlos Danger]

Subscribed.

 

[Supersubes]

So what bullet are you extending the throat for? Dont know where youre starting from but standard winmag throat is .130-ish, so a total of .660 fb after your alteration seems excessive.

Whats your current freebore diameter?

 

[Bandit320]

I load my 300 WM to about 3.7” with 225 ELDMs. This is a Criterion match chamber, which I believe is to SAAMI spec. An extra 0.5” seems excessive.

 

[918v]

Maybe he bought a bore rider bullet but his bore diameter is too tight so he mistakenly thinks he needs a longer freebore.

 

[TheOfficeT-Rex]

interested in an experiment with the following goals, use readily available components to convert a .300WinMag into an unusually long COAL round with great downrange characteristics while still using .300WM brass.

That doesn't really tell us what you want it to do, beyond just stuff more powder in a winmag case. If you want 300 Norma performance, buy a 300 Norma. If you want a reamer to run solids or a particular bullet, that's a more answerable question.

Have the components bought and reloading expert paid for, about to work with the reamer.

Seems like the cart is in front of the horse, and/or you already know what you want since you've bought stuff. Carry on.

 

[Lurq]

Thanks all for your replies.
The experiment is to load a 250gr Atip but much further out, to about 4.01" COAL. Avoiding changing chamber or shell dimensions and not wanting to go to any maximum loads, but if I make the throat length change, did I make shooting factory-length ammo a problem?
I say "about 4.01" because I want to have the bullet's outer surface and shell's inner neck surface to have full contact but at the very rearmost part of the bullet for least drag when launching. Want to have as much of the bullet out the front as is safely/effectively possible.
Standard .300WM factory rounds look traditionally stubby, where for example a .416Barrett has a higher proportion of the bullet extending beyond the neck.
No, not looking to maximize powder space for more powder; actually, curious about launching a high BC long round with medium powder charges (to extend barrel life but gain wind handling characteristics). Not really looking to emulate other more powerful magnums either, I think the 250 is more common in .338LM.
Appreciate any thoughts.

 

[Supersubes]

Thanks all for your replies.
The experiment is to load a 250gr Atip but much further out, to about 4.01" COAL. Avoiding changing chamber or shell dimensions and not wanting to go to any maximum loads, but if I make the throat length change, did I make shooting factory-length ammo a problem?
I say "about 4.01" because I want to have the bullet's outer surface and shell's inner neck surface to have full contact but at the very rearmost part of the bullet for least drag when launching. Want to have as much of the bullet out the front as is safely/effectively possible.
Standard .300WM factory rounds look traditionally stubby, where for example a .416Barrett has a higher proportion of the bullet extending beyond the neck.
No, not looking to maximize powder space for more powder; actually, curious about launching a high BC long round with medium powder charges (to extend barrel life but gain wind handling characteristics). Not really looking to emulate other more powerful magnums either, I think the 250 is more common in .338LM.
Appreciate any thoughts.

Factory ammo? No, it will be dog slow and shoot like shit. There will be massive blowby because the bullet will be clanking down the excessive freebore for a half an inch. Your idea of seating the bullet such that the neck is grabbing just a sliver of the bullets bearing surface makes no sense whatsoever. Youre trying to solve a non-existent problem, while creating new problems. Pushing the bullet out of the powder space, restoring capacity, is perhaps the only valid reason to lengthen freebore in your scenario.

 

[Ledzep]

Once upon a time I chambered a barrel for .30 Luger (7.65x21) with an especially long free bore. It mimicked .300 blackout subsonic performance and my thought was that a shorter un-intruded powder column would produce better ES/SD performance...

Moral of the story is that super-long freebore with a shit load of bearing surface sticking out of the case means that your bullet seating concentricity has to be PERFECT or else the bullet will interfere with the throat and cause some issues closing the bolt. Either that or you clearance the diameter to allow for misalignment in seating the bullet and risk principle axis tilt and the resulting bullet deformation and precision issues.

Somewhere around 0.75-1 calibers is about all you typically want/need for free bore length, IMO.

ETA: No harm as far as safety concerns in running an uber long free bore with factory ammo. Pressure and velocity will be lower than standard. Accuracy may or may not suffer.

 

[Taylorbok]

He want's to seat the bullet above the neck shoulder junction, lots of people do that. Build a dummy round and then measure BTO and compare it to where your lands currently are to get an idea how much you are actually going to need to extend it. you can jump bullets .150" but .500" is going to be excessive.

As was said the main reasons for seating out far is to increase powder capacity and avoid donuts. I think you might need to look at your ballistic calculator a bit more. When you start sacrificng velocity for BC in a heavier projectile there is a point that it's to far, especially under 1500 yards.

I'm not going to run the numbers for you but I bet running the 230 a-tips at WM speeds will net you more gain than running those 250's much slower.

Not to mention if it's a factory rifle I doubt the barrel has the twist to stabilize either of those bullets

 

[Supersubes]

Thanks all for your replies.
The experiment is to load a 250gr Atip but much further out, to about 4.01" COAL. Avoiding changing chamber or shell dimensions and not wanting to go to any maximum loads, but if I make the throat length change, did I make shooting factory-length ammo a problem?
I say "about 4.01" because I want to have the bullet's outer surface and shell's inner neck surface to have full contact but at the very rearmost part of the bullet for least drag when launching. Want to have as much of the bullet out the front as is safely/effectively possible.
Standard .300WM factory rounds look traditionally stubby, where for example a .416Barrett has a higher proportion of the bullet extending beyond the neck.
No, not looking to maximize powder space for more powder; actually, curious about launching a high BC long round with medium powder charges (to extend barrel life but gain wind handling characteristics). Not really looking to emulate other more powerful magnums either, I think the 250 is more common in .338LM.
Appreciate any thoughts.

Thats not what he’s written. The 300 wm has a .260” ish neck length. He’s adding .540” to the existing freebore, so about .690 freebore total. So neck shoulder junction to the end of the freebore is going to be in the .950” range. The 250 atip has a .700” bearing surface approx. So he’s either going to jump .250”, or be .250” above neck shoulder junction. Both his posts indicate the latter.

I wholeheartedly agree that seating a 230 a little deep in the existing chamber is likely better than his plan.

 

[Taylorbok]

Thats not what he’s written. The 300 wm has a .260” ish neck length. He’s adding .540” to the existing freebore, so about .690 freebore total. So neck shoulder junction to the end of the freebore is going to be in the .950” range. The 250 atip has a .700” bearing surface approx. So he’s either going to jump .250”, or be .250” above neck shoulder junction. Both his posts indicate the latter.

I think it's what he is trying to say but is having a hard time articulating it or having any actual measurements since he is very new to all this
(I assume he doesn't have the tools).

 

[Doom]

Sounds like turning it into a Weatherby. The 300 Weatherby Mag has about 0.4" freeborn. I also question if the case has the capacity to sufficiently make use of a 250 Grain bullet. Someone with Quickload could probably help here.

Thats not what he’s written. The 300 wm has a .260” ish neck length. He’s adding .540” to the existing freebore, so about .690 freebore total. So neck shoulder junction to the end of the freebore is going to be in the .950” range. The 250 atip has a .700” bearing surface approx. So he’s either going to jump .250”, or be .250” above neck shoulder junction. Both his posts indicate the latter.

I wholeheartedly agree that seating a 230 a little deep in the existing chamber is likely better than his plan.

I don't think the WM has .15" free bore per SAAMI.

 

[parshal]

I once ran a .740" bore rider throater in a 65SS just see what it would do. I couldn't reach the lands with a bullet still in the case. It dropped velocity by ~100 fps. I never did re-work a load to compensate or check for accuracy.

The answer is you can run factory loads but, as someone else said, it'll probably shoot horribly since you can't tweak the load. You'll definitely lose substantial velocity.

 

[Supersubes]

Sounds like turning it into a Weatherby. The 300 Weatherby Mag has about 0.4" freeborn. I also question if the case has the capacity to sufficiently make use of a 250 Grain bullet. Someone with Quickload could probably help here.

I don't think the WM has .15" free bore per SAAMI.

I think you're right on the WM freebore, its a little less. The OP’s numbers are so large though, a little lost in the noise wont matter.

 

[Doom]

I think you're right on the WM freebore, its a little less. The OP’s numbers are so large though, a little lost in the noise wont matter.

SAAMI looks to not have any free bore. I don't shoot the WM so I don't know what is actually being done with different manufacturers/reamers. Might be like Remington 308 chambers, IIRC my 168 SMK is about .150" from the lands when seated at 2.800" COAL.

 

[918v]

There’s SAAMI and there’s reality. I’ve never seen a 300wm without a freebore.

 

[spife7980]

For those not in the know:
Lots of old saami chambers dont have "freebore" -243, 270 etc- they have a long coned transition that starts wider and pinches down.
So its not a clearanced cylinder with 90 degree corners but they cut the corner at an angle and split the difference.
It starts wide at .315 at the end of the neck and tapers down to the .300/.308 bore and groove.

Here is a more modern design with a cylinder of freebore before the shallower leade angle.
It immediately cuts down to .309 and then is free for .1241 before it starts the leade angle at the .300/.308 bore groove.

 

[Doom]

For those not in the know:
Lots of old saami chambers dont have "freebore" -243, 270 etc- they have a long coned transition that starts wider and pinches down.
So its not a clearanced cylinder with 90 degree corners but they cut the corner at an angle and split the difference.
It starts wide at .315 at the end of the neck and tapers down to the .300/.308 bore and groove.
View attachment 7976422

Here is a more modern design with a cylinder of freebore before the shallower leade angle.
It immediately cuts down to .309 and then is free for .1241 before it starts the leade angle at the .300/.308 bore groove.
View attachment 7976429

Thanks for posting. I didn't have time to get that put together. Another point about SAAMI is that they allow for variations for various reasons as long as the variations do not increase the pressure. Adding free bore or additional free bore to a cartridge decreases pressure so it is still consistent with SAAMI as this is minimum Chamber dimensions.

 

[Taylorbok]

Thanks for posting. I didn't have time to get that put together. Another point about SAAMI is that they allow for variations for various reasons as long as the variations do not increase the pressure. Adding free bore or additional free bore to a cartridge decreases pressure so it is still consistent with SAAMI as this is minimum Chamber dimensions.

"SAAMI does not “approve” or “certify” firearms or ammunition products. SAAMI creates the voluntary standards for performance, dimensions, and other characteristics. In order to determine if a particular product was made in accordance with those voluntary standards, contact the manufacturer."
-saami.org

Saami doesn't control anything they just provide some standard measurements whether the mfg wants to follow them is totally up to them