Re: Remington 5rSS .308 400 yard
I don't know if the 5R suffers from this like the other Remingtons, but quite often there is an excessive free bore. Heres why:
Long ago, previous Remingtons had proper throats. Long story short, due to lawyers and hot hand loads, that has changed. When most modern centerfire cartridges are loaded correctly to length, the projectile is at or just off the lands. Well, hot hand loads are what caused the problem. Hot hand loads that are "jammed" are even more of a problem. Basically, a "jammed" bullet is seated in the lands. When the rifle is fired, there is excessive pressure, as there is already resistance on the projectile from contacting the lands and it does not as easily begin its forward movement as it would have in a free bore. This excessive pressure can turn a "hot load" into an "explosive load." To get around this issue, Remington started chambering their rifles with excessive head space, so that no matter what a hand loader did, he couldn't get his "hot hand load" close to or into the lands.
Basically, they ended up with a free bore that is so long that when you load the 308 to maximum magazine feeding length, it still isn't even close to the lands. Its simply a lawyers fix to keep hot hand loads from over-pressurizing, blowing up rifles, and consequently, having lawsuits filed against Remington.
I don't care who you are, its generally safe to say, that a bullet seated at the lands or jumped .015, it will be the most accurate. The Houston warehouse projects proved this. Accuracy always deminished after more than a .015 jump. Sometimes a small caliber VLD will like a little more of a jump, say .030, but not often.
So you are left with a problem. Accurate rounds need to be seated at the lands, or jumped or jammed by about .015. But there is no way to get that on a Remington because of the excessive freebore.
I've seen some 22-250 Remingtons, that no matter how far out the bullet was seated, the case neck simply wasn't long enough to get the bullet out there! I had one that I simply could not load for because the OAL was so long, in order to get close to the lands, I'd need a case with a much longer neck. And there are a bunch of 308 Remingtons that suffer from the free bore being so long that even when hand loaded to maximum magazine length, projectiles are still excessively jumped.
And this leads us to why gunsmiths set brand new barrels back as part of an "Accurizing Package." You would be amazed at how well a stock Remington barrel can shoot when its set back and re-chambered.
Now I'm not saying that this is your issue. But I think you need to figure out how much free bore you have. Then you need to figure out your OAL with the projectile sitting .015-.010 off the lands and see if you can even load a round that will fit in your magazine. At that point you have three options:
1. Use your 5R as a single shot with some proper loaded rounds, simply because the OAL is too long to fit in the magazine.
2. Use your 5R as intended, suffer with the excessive free bore, and get it as accurate as you can with what you have.
3. Take it to a smith. Have the action blueprinted, have the barrel set back, re-chambered, and crowned. I suggest you have GAP do it, if they are willing, and their turn around fits your timeline. If you do this, I bet you would easily print 3/8 inch groups with some 175gr FGMM or 175 Black Hills Match with your stock 5R barrel.
The excessive freebore engineered into Remingtons is problematic. It saves them from a lawsuit, but takes from from the shooter and the accuracy potential of one of their rifles.
Hope that helps in your quest for accuracy. As always, when handloading, start at least 10% low and work up.