More chamber questions

[Switchbarrel]

I thought I saw a post regarding chamber dimensions and the various bullet weights, but I cannot find it. So...

I'm looking at a pre-threaded barrel for one of my rifles. The shop says the dimensions of their reamer are:
freebore of .243 X .100 with a .271 neck.

Most of what I've been able to find says you want .113 freebore to shoot the heavier bullets and load them out of the donut area. I want to shoot the 105-107 gr. bullets, will this reamer be suitable? How far into the case will I be seating a 105 a-max, say boattail vs neck/shoulder junction? I assume I'll be re-reaming the necks to get rid of the donuts when loading. Thanks for any help.


-Rick

 

[Forum Boss]

Rick,

You don't need that much freebore. I have a 0.73 Freebore and that gets the 105 Scenar, Berger 105 and SMK out of the doughnut. That Freebore is short enough that I can shoot the 95gr Bergers as well, and even go into the lands with the 80s.

I recommend an 80-90 Freebore as a good do-it-all choice, as long as you understand you may have to jump the Berger 95s which have a REALLY short bearing surface.

 

[Switchbarrel]

Great. Thank you for the help.


-Rick

 

[shcal]

I have read the Freebore discussion quite a bit on this forum because I am in the early stages of putting together specs for a new 6mm BR barrel/chamber for my existing rifle. I shoot primarily varmints at long distances,200 yds to 500 yds) in a predominantly windy environment so I have pretty much decided on an 8" twist barrel. I will be shooting the heavier varmint bullets. I have not seen mention the Fowler 80 gr. flat base bullet and recommended Freebore dimensions for it. Does anyone have any information on the Freebore for this bullet?

Here's a compendium of the Freebore information I have gleaned so far from the forum.

Thanks in advance, shcal.

Consensus Freebore Measurements
Assumes bullet base seated .050” above the neck/shoulder junction.

FreeBore Dimension Bullet

.113” Freebore 107 gr. SMK – Optimum

.071” Freebore 87 gr. V-max – Optimum

.045” Freebore 95 gr. Berger – Optimum

.054” Freebore 90 gr Lapua Scenar – Optimum

.080” to .090” Freebore Good do-it-all for 107 gr. SMK, 105 gr. A-Max & 105 gr. Lapua

.073” Freebore 105 gr. Scenar, Berger 105gr. and 107 gr. SMK out of the doughnut. Freebore is short enough that you can shoot the 95gr Bergers as well, and go into the lands with the 80 gr. Bergers.

 

[Caduceus]

shcal,

I saw that last bit about .073" as well and think it may have been posted by the Moderator. I don't quite understand he can have a 107 SMK out of the doughnut while Dave Kiff in his interview says that you need .113" to get out of the doughnut using the same bullet?

How far above the Neck-Shoulder junction are we talking about here? Kiff is talking about 50 thou so with everything else being equal, this would put the base only 10 thou out with the shorter .073" freebore. Maybe that is enough but I think I'd prefer more. I've seen doughnuts in a tight neck 223 and they looked like they come more than just 10 thou up from the NSJ.

Another posibility is that we may be talking about different leade angles. Mostly people use 1.5 deg but changes to this angle can certainly change freebore requirements.

Anyone,Moderator?) care to elaborate?

 

[vegeta]

Yes, I would also like to know how leade angle afects freebore requirements. I have a JGS reamer with a 3 degree leade angle and 0.112 freebore and I still can load 70gr Nosler ballistic tips to touch the lands. Maybe I am doing something wrong.

Bobby

 

[Forum Boss]

The .073 Freebore was cut with a Kiff reamer that he created after we sent him a bunch of dummy cases. This will put bottom of the bearing surface on the Scenar 105 about .050 away from neck-shoulder junction and the SMK 107 a little closer,the Sierra has slightly longer bearing surface). The .113" throat is recommended for those who shoot the SMK 107 and Berger 105s pretty much exclusively because it gives the case more boiler room and you won't see such dramatic pressure changes moving in and out of the lands.

As noted, slight differences in lead angle will make a difference, as will where you define the "doughnut" area. I'm talking about the actual neck-shoulder bend when I say the 105 Scenars are .050" away. But put in .030" on your caliper and you can see it's not much. We spec'd the .073 as a do-it-all length to shoot the 90gr and 105gr loaded Lapua ammo. As a bonus, I have just barely enough neck to hold a Berger 95 VLD, but that's pretty marginal. Anywhere from .090 to .120 will work optimally with the 107s, and we know people who are shooting them with success in a .060 Freebore!

The 6BR case neck is about .310", about 8mm. The difference between a .073" freebore and a .113" is just .040", about 1mm. So you can see there is some room to play. The longer throat offers more space for powder, while the shorter throat lets you seat the shorter bullets into the lands. You make the call.

Attached is the print on the .073 Freebore and a Print for the SMK 107 optimized reamer. Bottom line, the .073 is on the short end of the usuable scale while the .113 is on the long end. Both will work with the 105s and 107s, but the .073 will be more subject to pressure spikes because you've got the heel of the bullet sitting right on top of the powder column.

 

[shcal]

Moderator, Thanks for the explanation and the drawings. As you can see, I'm in a conundrum trying to figure out a good freebore dimension for shooting a variety of heavier Varmint type bullets. As I mentioned before, do you have any experience with the Fowler 80 gr. Flat Base bullet? I understand this is a pretty decent varmint bullet in addition to the Hornady's and Berger MEF's.

Thanks again, shcal.

 

[Forum Boss]

Shcal,

I've got a box of the 80gr Fowlers, but haven't shot them yet. I did send Dave some 80 Fowler Dummy rounds and they will work fine with a .073 Freebore, with enough shank in the neck.

The 87gr Vmax has a pretty long bearing surface for its weight, but again it will work fine with a freebore in the .070-.090 range. What I'd do is load a dummy with the shortest bullet you're planning to shoot in quantity, with at least .200" shank in the neck and see where you end up. Keep in mind that some of the boat-tail varmint bullets in the 80-90 grain range actually have a shorter bearing surface than the equal weight Flat-based bullets.

You're going with an 8 twist so that will let you pretty much shoot anything. I think the real limitation will be if you decide you NEED to use the Berger 95 VLD. With its ultra-short bearing surface, I'd go with a .060 Freebore, maybe even less. If you don't need to shoot that bullet, a .080 Freebore would be a good choice.

 

[shcal]

Thanks Moderator. As the project progresses I will let you know how things work. Now for a better informed discussion with my smith.

Thanks again, shcal.

BTW tried to shoot today with my PPC at 200 yds+ and the wind is just howling. Shooting 65 gr Hornady V-Max, and getting blown all over the place. Needless to say, the critters had a good day.