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7mm Ultramag, Freebore & Accuracy

 
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  #1  
Old 02-19-2011, 11:45 AM
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Join Date: Oct 2004
Location: Northamptonshire England
Posts: 676
7mm Ultramag, Freebore & Accuracy

One of my shooting buddies and i were discussing the future build of his 7mm Ultramag LR Deer rifle, when the topic of the cartridges generous freebore and posible loss of accuracy when using bullets around the 140-150grn range came up.

The rifle will be built on a custom action so in order to use lighter bullets the idea of a shorter freebore chamber was aired.
A little research revealed that a chamber cut with a freebore of half that of the cartridges original .400" dimension can result dangerously high pressure spikes, obviously something we want to avoid.

The problem over here in the UK is that we must by law use game style bullets with reliable expansion on deer, and bullets in the lighter weights are most readily available.
The whole idea behind the project and the cartridge choice is to deliver hard hitting, accurate flat shooting projectile reliable onto the vital areas at long range with quick immobilizing kills.
What we dont want to loose is either velocity from using heavy bullets, or accuracy from the more suitable and favoured lighter bullets, which clearly would suit a shorter freebore.

It would be hugley appreciated if any of you guys out there who are shooting light bullets satisfactorily through your standard 7mm Ultramag and those who may have gone down the route of a shorter freebore, could share their experiences.
Many thanks in anticipation.

Ian.
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Old 04-29-2012, 04:05 PM
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Join Date: Apr 2012
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Re: 7mm Ultramag, Freebore & Accuracy

Hello Ian,

I realize that I am replying to a post that is more than a year old.
Maybe this is no longer of interest but, .......

I have shot bullets in the 140mm weight exclusively in my 7mm Ultramag.

With a 139 gr Hornady SST loaded at the recommended depth of 3.580", the bullet doesn't engage the throat until it travels a whopping 0.442"

I have never seen a rifle with anywhere near this amount of freebore..
I'm sure this is necessary to get the bullet up to speed on a cartridge that is massively overbore.
Otherwise the pressures would be tremendous.

The good news is that my Ultramag is extremely accurate with these light bullets.
It stands in defiance of logic but the freebore doesn't seem to matter.
My rifle routinely shoots around 1" groups with Remington factory ammo.
It performs equally well with handloaded Hornadys.

Even with the heavier bullets, the freebore is only reduced about .050"
I wouldn't advise seating them out further even though the longer bullet might permit it.

This caliber demands and abundance of caution.

Tls
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  #3  
Old 04-29-2012, 04:43 PM
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Join Date: Apr 2012
Location: England
Posts: 22
Re: 7mm Ultramag, Freebore & Accuracy

Ian,

I'm shooting a 7mm Rum - with 180gr Berger VLD's

My smith built mine with a generous free bore which I questioned at the time as I was used to loading VLD's as close to the lands as possible. To be fair this rifle shoots very well (clover leaf at 100 yard - around an inch or two at 600 yards - it might do even better with a better pilot!

I was hoping for higher velocity (I get just about 3000 fps) but the high BC makes up for it over using a .300 win mag.

Spec:-

Bat action, McMillan A5, border cut 32" barrel, jewell trigger.
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  #4  
Old 04-30-2012, 01:53 PM
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Posts: 181
Re: 7mm Ultramag, Freebore & Accuracy

I think your friend would benefit from the heavier 7mm bullets with the RUM. The Berger 180 Hunting VLD is one killing machine with plenty of power and laser flat trajectory and now that they will be bringing out the 195 grain pill and there are others up to 200 grain pills! The VLD's I have found like to jump so I would really talk the project over with my smith to get the right twist and freebore for the build.
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Old 04-30-2012, 04:23 PM
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Join Date: Apr 2012
Location: England
Posts: 22
Re: 7mm Ultramag, Freebore & Accuracy

I would totally agree with F.E. but saying that my rum does run well. My gunsmith said the critical thing was the presentation of the bullet squarely to the lands - if I'm honest I didn't get how seating of the lands was better in this case it just works?!

That's good news for me regarding the 195gr's - can't wait to get some of them.
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  #6  
Old 05-01-2012, 05:59 AM
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Posts: 2,114
Re: 7mm Ultramag, Freebore & Accuracy

Quote:
Originally Posted by Nitroexpress View Post
I'm shooting a 7mm Rum - with 180gr Berger VLD's. . . . To be fair this rifle shoots very well (clover leaf at 100 yard - around an inch or two at 600 yards - it might do even better with a better pilot!
Is an inch or two at 600 yards the worst it shoots? That's amazing and virtually unheard of for that round!!!!
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  #7  
Old 05-01-2012, 09:53 AM
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Join Date: Jul 2004
Location: Texas
Posts: 5,450
Re: 7mm Ultramag, Freebore & Accuracy

Freebore has long been a controversial subject.

There are two definite schools of though, One is to get accuracy you must seat against the lands.
(Benchresters often do this for round to round consistency) but they are not using cartriges that
will hold 100+ grains of powder.

The other faction is the "Freebore dont scare me" group. these are the shooters that shoot the
over bore cartriges in order to achieve Warp Speed from there bullets without the problems of
excessive pressure.

One poster said that his gunsmith said it was a mater of presenting the bullet to the lands
squarely.This is very true.

Freebore is a distance that the bore (Same diameter as the bullet) of the barrel has no rifling.
This allows the bullet to start moving before it engages the rifling lowering the initial pressure
required to engrave the bullet. This allows the use of more powder,producing more gas,to
propel the bullet faster in a longer barrel where it can be used.

Contradictory to some peoples belief, Roy Weatherby Did know what he was doing and proved
it by loading the fastest cartriges in the world for a time. As to the amount of required Freebore
the cartrige designer determines that based on what he is looking for (The reason for coming
up with a new cartrige to fill a gap left by others). Most of the Weatherbys have a generous
amount of Freebore 0.361 to 0.756. In spite of this they can be very accurate if chambered
and loaded well still using the standard Freebore (SAMMI).

The big 7mm RUM is extreemly overboard and needs plenty of freebore and longer barrels to
reach it's potential. Top velocity and trajectory can be reached with bullets in the 140 to 160 grain
class with the 168 match bullets not far behind. It is my opinion that is is a great choice if
someone want's to use the 175 to 200 grain 7mm bullets for high wind areas and shooting.
But if flat shooting is what you want the 140s 150s and 160 are the way to go.

All of my big Magnums have SAMMI Freebore and will shoot under 1/4 moa with some shooting
less than 1/10 MOA Some are belted and some are not (Another controversy for another day).
and they are all loaded to magizne length for quick follow up shots if needed.

The trick to accuracy with lots of Freebore is simple. Concentric chambers that fit the brass perfict
with little or no head space (Less than .0005 to .0001) and concentric ammo with little or no runout.
The rest of it is good load componants for the bullet weight.

Some will not agree with this philosophy and that's not a problem but it works for me and I
believe in it because it works.

So chamber the big 7mm RUM with a SAMMI chamber and Freebore, Load good ammo and enjoy
one of the flattest shooting cartriges in the world and if you use premium bullets that perform well
on game it will be a joy to own and shoot,

J E CUSTOM
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