Hi Doug, I have a bit of experience with the freebore of the 7mm RUM and experiments.
In my first 7mmRUM barrel, I was also suprised by the freebore. Worse still, I was using an M70 action which has a shorter magazine that the M700. Bullet jump for my loads was .400"
The rifle shot well but I never quite got down to the .3-.4" range I was hoping for. The rifle was a favorite with hunting clients of ours but during the course of one trip, I neglected to tape the muzzle, a small amount of moisture got in with the damp weather and in one shot, completely ruined the barrel. I only noticed when groups opened up dramatically and while checking the muzzle over, noticed the slightest swelling on the outer barrel, just the slightest bulge. So off the rifle went again.
Round 2- we had Dave Kiff regrind the reamer to halve the throat length. there would still be some freebore to smooth out pressure but atleast the bullet would engage the rifling while still having some of its shank in the cartridge case. I hoped this would get me down to .3" etc.
The rifle was now a monster. With the 160 grain flat base Speer Hotcor (very easy bullet to do preliminary accuracy/ load development work) I started at 89 grains of Retumbo for 3100fps (26" barrel). the fouling shot gave 3100fps, the next load gave 3300fps and was difficult to extract (was thinking I had stuffes up a load), the next gave 3100fps, the next gave 3300fps, destroyed the case and I needed a mallet to open the action.
I emailed Kirby Allen and he got straight to it- directional ignition from too much powder space. Kirby suggested I switch to a slower powder but we do not have this locally. Before doing anything else though, I upped my loads. Velocity spreads and pressure settled with 92 grains Retumbo at 3200fps but case life was short- talk about a small window.
Round 3- The rifle went back to the Smith and he (very patiently) re-cut the throat for the original .400" jump. The rifle has shot OK ever since but is very limiting. To get down in accuracy, I eventually discovered that the freebore has a huge detrimental effect on the bullet stability factor. Theoretically, a 1-9 twist should stablize 162-168 grain bullets but in the RUM, this does not work. During the huge bullet jump, the slightest deviation in bullet to bore concentricity is magified as twist rate is increased. To shoot 162-168 grain bullets, the best twist rate for the RUM would be 1:10. With the 1:9 twist of my bore, I got down to .5" and less using either the 175gr SMK or 180gr VLD. 91gr of Retumbo drives both at 3184fps but load to load deviation is still 20fps for those concerned with the finer details. Asvelocity is increased, regardless of pressure, the groups with 162-168 grain bullets opens up to 3" plus. The 168gr Berger VLD suffers the worst.
Ok, so I have a long range rig that won't shoot under .75" unless I use either the 175gr SMK or 180gr Berger. Serious throat erosion has been seen in another customers rifle in less than 150 shots so I can basically expect that now I have done all my test loads and a bit of hunting with the new standard throated barrel (about 100 rounds so far), I will only be able to maintain this level of accuracy for the next 100 rounds or so- am only guessing.
A worse move was when I had a brake fitted to aid accuracy. During tests, the brake proved to reduce groups by 50% so all kudos to the smith. The problem is that the rifle kicks up dirt so bad that I have to close my eyes for a few seconds as I let the trigger off- its a very fine line between a procedure versus the development of a poor habit. The noise is horrendous, even with earplugs. A few weeks ago, I made a terible mistake, was setting up for a long shot over a favorite spot when an animal walked out in front of me at point blank. I had no time to put earplugs in, fired the shot, lost my hearing for two days and was uncomfortable for several days thereafter.
Anyway, have pretty much had enough of this. I should mention, I have both a 9.3RUM and a .375 RUM (Dave Kiff reamers) and the freebore is the same. Test rifles in the other RUM's are the same. The difference with my two big RUM's is that both are ***** cats to load for and I would summerize them as being "well balanced". My M700 .375 RUM shoots one hole groups at velocities ranging from 3250fps with 225gr bullets through to 2950fps with 300 grain bullets. Rifle is 12lb, barrel length is 26, recoil is shear hell. The 9.3 does exactly the same with like weight bullets, best powder is H4350 but both can easily be downloaded with H1000 to drive, for example, the 293/300 grain bullets at 2500fps. Totally different to my 7mm.
My next move will be to re-barrel the 7mm to a smaller cartridge. I am having an internal debate about this which lead me to visit here after not visiting for a year or so. I think my ideal case capacity would be something to the tune of the 7mm-300 Win mag but I can't find any feedback about it. My major worry is the short neck. I don't think I will go the route of the Dakota as I want to retain the normal bolt face. Am not to sure about consistant brass supply for another candidate, the 7mm-375 Ruger (no neck turn). It can be hard to find a constistant supply for less common cartridges such as the .375 here. The other option, which friends have encouraged, is to go back to my old favorite- the 7mm Rem Mag. Before anyone comments, please let me expalain my situation best:
We have high winds, average is 10mph and gusting. The 7mm RUM probably the only cartridge I have ever used that showed an almost immunity to this our to 450 yards. I constantly stuff up shots beyond 400 yards with the .308 and match bullets due to wind gusts in the valleys (ridge to ridge shooting). I can go back to the 7mm RM but will have to lift my game somewhat if I want to learn to shoot well at long ranges.
The terrain is steep ( I hunt different routes when not guiding clients). I can't go any higher than 11lb all up.
The 7mm 162-168 grain bullets give the fastest killing on what I am hunting out to the greatest distances. I have been shooting game weighing between 90 and 130lb and leaving the big animals for clients. Hogs are the main target species and even at 90lb, take a lot of knocking down at extended ranges.
The rifle will be used at both point blank and long ranges of up to 800 yards- no further. When I loan the rifle to clients, it will be used out to 400 yards and will be loaded with a contolled expanding bullet for mature hogs with a 1" plus cartilage shield.
I don't want another 300 Win mag as I want to go down a step in recoil this time around. Yes, I admit it is a very good cartridge.
Will probably shoot 300 to 600 rounds per year once I step down from the 7RUM. Am currently toting both a .308 and the RUM on the quad bike- do most hunting each week with the .308 and a few "luxury" shots with the RUM to minimize wear and cost.
Sorry to steal your post at the end Doug. Hope the initial info is useful.
Nathan.
