I have this thread also running in milsurp.
So far ive pretty much decided to want my ross in 7mm Rem Mag. The bolt requires no mods for the conversion and should function right with the mag.
That would be a good reason to go with a 7mm Rem Mag.
I would think that 7mm RemMag would *not* feed from the magazine very well, if at all; there's an enormous difference in case taper between it and the .280 Ross (or .303 Brit - not sure which round your rifle was built for).
However, most target rifles are built without a magazine. It's simpler that way. You put a round in the loading port, close your bolt, shoot, extract the empty, put another round in, shoot it, etc.
Are you looking to build a target rifle that would (or could) be used in competitions, and be competitive? Or do you just want a nice accurate fun long range plinker?
A few more reasons in favour of 7mm Rem Mag:
- it's a very high performance cartridge. With the right bullets (i.e. hpbt match bullets like the 168 Sierra or the Berger 180) it will perform very well at 1000m and beyond.
A few reasons against the 7mm Rem Mag:
- it's a very powerful cartridge. Even if you build your rifle to be 15# or 18# in its shootable configuration, there will be a LOT of recoil. Not that it will hurt you, it won't; rather, it will be enough recoil that it will be *very* difficult for you to shoot the rifle extremely well. A 15# .308Win is challenging enough; it takes some people (cough cough yours truly cough) *YEARS* to get over bad habits like flinching. And regardless of recoil levels it will also have a LOT of muzzle blast, which makes it more difficult (mentally speaking) to truly excel in your shooting of it.
- being extremely high performance, you'll see a decline in your accuracy after 1000 rounds or so. This might not necessarily matter, but it should be understood.
- if you ever do want to try it in formal or informal competitions (and I highly suggest you come out with us and give it a go!), you'll find very few fellow competitors with similarly-chambered rifles (just about everybody else shoots 223 or 308). Of course we'll welcome you and have you shoot with us, but you might find it more rewarding and meaningful to be shooting a .308
But what im unsure at the moment would be the proper length and twist rate for the barrel for my needs.
Would anyone know of a reference with the trajectory info for bullet weight and barrel length
For target rifle shooting, trajectory isn't terribly important. What we're after is heavy, sleek bullets that retain their velocity well and get blown around by the wind as little as possible.
When building up a target rifle, you generally choose which bullet or classes of bullets you'd like to shoot (for example, I might want to shoot 155 grain match bullets in my .308; you might choose 180-class target bullets for your 7mm Rem Mag). From that you determine the twist rate needed (either from tables from the bullet manufacturers or barrel makers, or various formulas ("Greenhill") or calculators (JBM Ballistics has a "Miller stability formula").
You choose a barrel length and profile suitable for your needs. Oftentimes we choose as long a barrel as possible, typically 30", just so we can get every reasonable bit of velocity possible (cons are that you need a longer gun case, longer barrels put more bending load on your receiver, and longer barrels are floppier). You choose a barrel thickness related to your weight target. A non-223/308 F-Class target rifle has a weight limit of 10kg/22#, so people tend to choose particularly heavy barrel profiles and heavy stocks, in order to get to this weight budget in a way that gives you an accurate and easily-shootable rifle. A 223/308 F-Class rifle is limited to 8.25kg, so you need to be more careful in your barrel choice - depending on whether you choose a light or a heavy bipod, or a light or a heavy scope, you might need to get a medium-weight barrel instead of a super heavy weight barrel profile.
