Growing pains.

[knowledge_is_power]

Ok, so as I've said in most everything that I've posted, I am new to this. So here's my issue/curiosity. I shoot a .308 Rem 700 with a 20" 1:10 bbl and I've been getting into reloading for it. I've noticed that when I load my rounds with less powder I hold tighter groups vs the higher load outs. I haven't tried every mix and match out there yet, just 168 gr and 175 gr Sierra's and for powders I've been using Reloader 15 and Varget. I'm basically wondering if the reason my groups spread out with the heavier loads is because of mechanics, or me? And if it is the first, what can I do to fix it? Thank in advanced!

[Unclenick]

CAUTION: The following post suggest possibly working up to loading data that may be beyond currently published maximums for this cartridge. USE THIS INFORMATION AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.

Recoil and firing set up several vibration and deflection modes in the barrel. In a sweet spot load the time the bullet takes to clear the muzzle turns out to be synchronous with the muzzle being in a relatively dead spot in these influences. So when you load for accuracy you are loading to time your bullet exit for one of these dead spots. There are typically several.

You can tune the loads for a deadspot several ways. The obvious one is adjusting the powder charge. Another is adjusting the bullet seating depth. Another is changing powders. For any given muzzle velocity, faster powders provide shorter barrel times because they provide a larger portion of their pressure and acceleration earlier in the bullet position in the barrel. That makes it go down the rest of the barrel faster, getting out sooner. The reverse is also true, obviously. So simply matching velocity to that of a known good load doesn't work to tune loads.

IME, the Remington 700 typically has a long freebore, which tends to reduce pressure, so it can often tolerate a little bit more powder than the books call for. Also, in .308 the brand of brass makes a difference in case capacity, with Winchester having the most capacity and needing the largest charge to reach a given pressure or velocity. Military cases typically have the least capacity, and commercial brands other than Winchester fall inbetween. The difference between Winchester and IMI military brass, for example, can be 2 grains of powder.

Anyway, the bottom line is there are factors that can throw book data off. You need to learn to work loads up while watching for pressure signs, then stop when you hit one for your gun.

For one systematic approach to identifying sweet spot loads, read Dan Newberry's pages.