Freebore

[azgutpile]

From what I understand, it is pretty common to order a chamber reamer with a shorter than SAAMII spec freebore; however, I would like to get a better understanding of some of the risks. I have two builds that I'm currently working on that i would really like to feed from the magazine.

From reading Long Range Long Time's review of the McMillan EOL Outdoorsman, I found out that McMillan along with EOL have made a modification to their 300 RUM chamber (300 EOL Long Range) that allows it to feed from the magazine and still touch the lands. In addition, it can still fire factory 300 RUM cases safly. The only way that I can figure they modified the case was by shorten the freebore a bit. That being said, could I order a custom reamer with a shorter frebore and get the same results?
Please let me know if I'm on the right path, or if I'm missing something.
Here is a link to the review.
http://www.longrangehunting.com/forums/f19/mcmillan-eol-outdoosman-family-outing-91004/


At the same time, I understand that Remington and Weatherby both use excess freebore on their larger magnums to gain velocity without excess pressure. If I start loading rounds .020 off the lands like my other rifles, will I be able to reach expected velocities without excess pressure? I have read that by using a shorter throat, or seating the bullet close to the lands, the pressure can be greater.

Mike E's comment in the attached link gives me some concern. http://www.longrangehunting.com/forums/f19/match-chamber-not-53414/

I had a similar experience to his with my other custom 300RUM where it was a match chamber with I believe a standard freebore. I was seating the bullet at .020" off the lands and getting book velocities with almost 10 grains less powder. The accuracy was excellent, so I left well enough alone and forgot about it until now.


The other rifle that I'm building is going to be a 7mm SAUM on a Stiller Short action. With this rifle, I would like to reduce the frebore from .3077", which is the standard SAMII in the 7mm SAUM down to, or close to .1884", which is the SAMMI for the 7mm WSM. My goal is to have the cartridge under 3.1" OAL which would would allow me to feed from the magazine. What should I expect from this and in a pinch, would I be able to use factory ammo?

Thanks,

 

[elkslayerjc]

I have a 300 RUM as well and what I found to solve the problem of being able to load for the magazine and be .015 off the lands was to install the Wyatts extended box magazine (CFE-9) which allows you to extend your bullets to 3.999 O.A.L if need be. Minor smithing is needed but is really easy to install. Just a thought and you can go online and check out the different options or you can call them and discuss your situation. Or you can go about it the way that you are but I really beleave that bullet seating depth and how much bullet is below the shoulder of the case makes a big difference. The deeper the seat the more of the explosion is around the bullet and not directly on the bottom of the bullet.

 

[azgutpile]

I have the Wyatt box on my other 300 RUM and it works fine, but I would like to use a detachable box magazine, which won't get me to 3.99".

 

[rscott5028]

You can simply build your dumby round and send it to a good smith or the reamer mfg.

Or, obtain a reamer print from PTG or other; stick it in a CAD program; overlay the cartridge and desired bullet verifying COAL for your magazine; and modify the reamer print.

But, there's no free lunch...
shorter throat = seat deeper = less case capacity = lower velocity potential

I'm sure there's some optimal freebore+lead angle for a given rifle/magazine/bullet/chamber/brass/powder combination. Guys like Shawn Carlock and Kirby Allen have played with this and may even be working on patents for their designs e.g. 338 Edge +P.

But, it seems to me there's more to loose than to gain by experimenting for the average guy unless you have the instruments, tooling, and/or funding to experiment.

Custom throating can perhaps be done by hand. And, you can go deeper. But, once you remove metal, you can't put it back.

other options to consider...
wyatts mag box
long action vs short for your 7 rsaum
different cartridge
different bullet

Did McMillan/EOL ever say if their modificaton was to the mag box or freebore, and/or lead angle?

-- richard

 

[notajeep]

I have been wondering about the same issue for over a year. My M-700 VSF shoots pretty well. As in 2.5" groups at 500YDS with out weighing brass or trickling powder. I'd like to improve the rifle accuracy as well. I have played around with bullet seating depth and with the Hornady A-max 168Gr with the bullet just sitting inside the brass it barely touches. So I jumped to the SMK 180Gr to see how the fit was. Long story short the free bore in my M-700 factory original dimensions must have been made to accept the longest of 30cal bullets. Well this stinks, I prefer the 168's or something a bit smaller. I'm interested in a shorter free bore as to reduce bullet jump. With the benefit of being able to use the magazine and hopefully improved accuracy. I guess I'll be looking into having this done this winter.