7 RUM jump vs free bore

[dinmax82]

Trying to find data on the 7RUM and the information I've found is a bit confusing.

Some sites say that the cartridge is designed to use large free bore of 0.40". I'm used to jumping bullets 0.015 or so only. They say the extreme free bore prevents over pressure and adds higher velocity. Will this still be an accurate round?

I want to use the 7RUM to push the new Berger as fast as possible with available brass and chamberings. I won't shoot it everyday obviously so barrel life isn't an issu3.

 

[rcoody]

Trying to find data on the 7RUM and the information I've found is a bit confusing.

Some sites say that the cartridge is designed to use large free bore of 0.40". I'm used to jumping bullets 0.015 or so only. They say the extreme free bore prevents over pressure and adds higher velocity. Will this still be an accurate round?

I want to use the 7RUM to push the new Berger as fast as possible with available brass and chamberings. I won't shoot it everyday obviously so barrel life isn't an issu3.

Read about Roy Weatherby and freebore. I believe his rifles are still accurate.

 

[coyotezapper]

Factory Remington's can have as much as .400" freebore but I believe they do this for safety reasons. The 7RUM is very overbore and can be over pressured very easily during handloading. With a tighter chamber pressure can jump dramatically with as little as .5 of a grain of powder. The freebore helps alleviate the sudden over pressure and makes pressure signs more gradual like other cartridges. Don't let this scare you. I have a rem700 custom shop 7RUM that had this freebore and it shot great. I have since shot out the bbl and it has a chamber designed for the 180 Berger VLD with a .060" freebore now. If you think of it like a Weatherby chamber like the post above mentioned its like a pressure vessel and most Weatherby's shoot well.

 

[dok7mm]

Dinmax, Just got through working up loads for my son's 7 RUM, so I feel for your problems adjusting to a mile of freebore, but it's there for a reason. I finally settled on the 168 Berger Classic Hunter hybrid and found it jumped very well. Tried IMR 7828 and Retumbo. Reached max with 7828 @ 81.3 grs and best groups @ 80.5 gr. Retumbo was the winner for speed and accuracy -- 3188 fps w/ SD of 4.0and 1/2 moa @ 400 yds. Load was 90.5 gr. in Nosler brass, WLRM primer. I tried several seating depths and a CBTO of 2.940" was the clear winner. Pressure jumps up on you quickly, so be sure to work up carefully to these loads in your rifle.

 

[Lefty7mmstw]

Trying to find data on the 7RUM and the information I've found is a bit confusing.

Some sites say that the cartridge is designed to use large free bore of 0.40". I'm used to jumping bullets 0.015 or so only. They say the extreme free bore prevents over pressure and adds higher velocity. Will this still be an accurate round?

I want to use the 7RUM to push the new Berger as fast as possible with available brass and chamberings. I won't shoot it everyday obviously so barrel life isn't an issu3.

Let her jump a good bit in the 7rum.

My last 7rum loved 140 sierra pro hunter bullets pushed by rl25. I was seating the bullets to mag. length... the jump was right at 1/2" and the bullet was actually clear of the case before it hit the rifling.
The rifle would shoot in the .3"s at 100 all day long and was positively awesome in the field. I brained a trotting mule deer with it at 200 yards once and pulled two 700 yard plus shots in twelve hours with it. One was a white tail at a trot with a cross wind.

My current 7rum is shooting 175 hornady or 180 sierra over a mild dose of retumbo; she is still cruising at nearly 3100 fps at start charges, jumping a couple tenths of an inch, and shooting 1/2 moa...

 

[dinmax82]

So if I get a custom barrel made I can make it so the jump is very little, like 0.015" and bve ok as long as I start my loads at the low end and work up in in .5 increments instead of 1gr?

 

[coyotezapper]

.5 grain increments are only a recommendation for this size of case. IMO, .5 is the largest you should go in any case. I find powder nodes at times within half a grain and would have missed it had I been testing in 1 grain increments. Also, don't get caught up in jump. If you make good handloads that are concentric jump will not be an issue.

 

[rcoody]

So if I get a custom barrel made I can make it so the jump is very little, like 0.015" and bve ok as long as I start my loads at the low end and work up in in .5 increments instead of 1gr?

I don't understand why you would want to change something that obviously is working. If you don't like the freebore of the RUM then pick another caliber.

http://www.longrangehunting.com/forums/f19/7mm-ultramag-freebore-accuracy-68435/

 

[Jokostel]

The freebore is there for a reason. To control pressure spikes.

 

[Jokostel]

i don't understand why you would want to change something that obviously is working. If you don't like the freebore of the rum then pick another caliber.

http://www.longrangehunting.com/forums/f19/7mm-ultramag-freebore-accuracy-68435/

#this

 

[Edd]

... the jump was right at 1/2" and the bullet was actually clear of the case before it hit the rifling...

Has anyone ever heard of this scenario keeping the case neck and shoulder from sealing and causing the shoulder to collapse?

 

[coyotezapper]

Has anyone ever heard of this scenario keeping the case neck and shoulder from sealing and causing the shoulder to collapse?

I had collapsed shoulders with my 7STW when I first loaded the "original" Barnes X bullet. I ignored their seating depth recommendation and put them too close to the lands. At the time nobody could tell me what caused it. I even showed the cases to Mic Mcphearson and he had no idea. Jumped the bullet and problem fixed. Final load that I developed with the original X jumped around .180" and shot sub 1/2" at 3400 fps and was absolutely deadly. It was literally DRT every time.

 

[Lefty7mmstw]

Has anyone ever heard of this scenario keeping the case neck and shoulder from sealing and causing the shoulder to collapse?

in a loosely built rifle with work hardened brass (possibly also rather mild pressure loads)it might be a possibility.... but I think you would be as much or more concerned with crappy accuracy if the rifle was that sloppy inside...