Low vel long range?

Ok, so I have pursued higher vel (for a 10 twist .308) at 500yd and been moderately successful. My solution was a heavy bullet (Accurate 31-210E) at 2300fps. Was fairly accurate, but, the recoil became more than I really wanted. So, I've gone back to the XCB (167gn) at slower speeds and limited my shooting to 300yd. Found a nice sweet spot at 1750fps. It works with powder coating and conventional lube. I've been using the lubesizer just because of the convenience.

I was at the 1000yd range one day shooting the 6BR and had targets also set at 500 and 200yd. It was a very calm day so I was having way too much fun :)

Got the .308 out with my cast loads and had several good groups in a row (<1.25MOA). I looked at the 500yd target and figure what the heck. Calculated the hold (a LOT) and touched off a shot. It landed close to the gong! Proceeded to shoot some more and got a couple hits (5" gong). So, put some on paper and had a 6" group. I am now kinda interested in this.

Today went out and put targets at 600yd. A little wind, ~10mph quartering headwind. Shot an 11" group.

So, why does this combination work? Anything I would come up with would be a SWAG, but, the bullets are shaped like the long range BPCR stuff, so maybe there is a correlation. I do know the wind blows the things around a bit and they take a while to get to the target :)

As circumstances permit I will continue to shoot these 'lazy' bullets at longer ranges just for fun. FWIW, at 600yd my scope ran out of adjustment and had to hold over on the reticle. Maybe need a 20MOA rail for this one :)

Several things I have noticed over the years.
If you get the chance chronograph both loads at 10' and 200-300 yds. the super sonic load will loose more velocity over the distance than the sub sonic.
A more violent transition from super sonic to sub sonic also.
Your lighter shorter bullet may be staying stable better than the heavier longer as the velocity drops below.

The trick to shooting long range isnt so much velocity its knowing the come ups and reading wind. Ive watched and have shoot 500-600 yds with a BPCR in 45-70 with good results, but the drop or come up required is amazing.

The shape of the 210gn bullet is not good for transonic, it is a FP cone shaped nose. During transonic the shock wave would make an abrupt transition from body to nose which probably upsets the bullet flight.

And, yes, supersonic drag is quite different than subsonic.

Also consider bullets with the COG (Center Of Gravity/mass) close to the COP (Center Of Pressure) will transition thru the sonic zone smoother.