At what distance do the advantages of the boattail start to be significant?  

Two examples of interest to me:
1) Hornady 55 ball, vs Hornady 55 SP.

2)  In particular, I have a shipment of 180 gr Spitzer (flatbase) bullets coming in, that I will load for the Garand.  At times, I intend to reach out to 600 yards.  I am wondering at what distance (if any), the 168 gr boat tail matchkings I also have, will have a ballistic (accuracy and wind) advantage over the flatbase 180 gr.

250 yards and further.

My understanding is that the boat tail bullet's advantage comes at distance because being more aerodynamic, they lose speed slower that flat based bullets. Plus, they seem to go through the trans-sonic zone more smoothly.  All the discussion I've heard has been that for distances up to 300yds, flat based bullets are better and over 300yds boat tailed are, however, there is an overlap depending upon velocity such that flat based may work well out to 500yds in some instances.  But I'll differ to those who have more specific knowledge.

Just look at the ballistic coefficient difference and decide for yourself.

There is a distance where wind will push one farther than the other to a point you will decide is too much for your needs.

The slower, lower BC will be worse at all ranges.

If you get 2675 out of the 168 you may get about 2590 out of the 180 at the same pressure(roughly) so take those velocities with your BCs and put them in a ballistic calculator.

The 168SMK will ALWAYS have a better ballistic coefficient than the 180 PH.  

The BC for the 180PH is (listed) .407, the 169SMK is .462 and the 168TMK is .535.  

You will really start to notice the difference over 200yds.  But at closer distances, why would you use such a heavy bullet?  

I'm assuming you got these cheap and now are trying to "make them work."  But if you want to shoot out to 600yds, use the SMK.    

The Sierra HP isn't a bad bullet but at distance there are much much better bullets out there.  Ones that are much better suited for the M1 too.

For closer work, the 180's are unnecessary.  Why not shoot a 150-135 or 125?  Save your self the distraction of recoil?  

Just my humble opinion........................      

Good reply - and you're quite right.  To answer your rhetorical question:  $0.16/bullet for Sierra 180's - is why.  That's a Heck of a price for a heck of bullet.  So heck, I picked up a couple thousand.  If I change my mind, I figure I can dump them on the local club bulletin board for more than I paid.

Well this is an outrage - you people have not done all my work for me.  

Sigh - fine.

OK, Transonic warbling can begin at around 409 m/s (worst case)
A Sierra 180 gr Pro hunter SPT launched at 2475 FPS (Hornady way-over-Conservative max load is 2550 fps, but I'm backing off that even), at Sea Level (Camp Perry will be better) is moving at 1402 FPS (according to shooter, which is 426 m/s) at 600 yards.  So it's not transitioned to transonic quite yet (though margin isn't particularly high, but it should work).  My calculator gives me a danger flag at 860 yards for definite transition to subsonic.

600 yards:
A Sierra 180 gr Pro Hunter at that speed will have a wind drift in 10 MPH wind of 6.9 MOA.  And an elevation drop of 17.8 MOA.
A 168 gr Hornady BT sent at 2737 FPS (a warm load) will have a wind drift in 10 MPH wind, of 5.3 MOA, and an Elevation drop of 15.6 MOA.
For reference, M2 ball has a winddrift of 6.4 MOA at 600 yards.

That's a big enough difference that yea, missing wind calls is going to start blowing you off into the next ring with the 180's at 600.

So the 180 gr isn't nearly as good of a wind-bucker as a 168 gr bullet, past.  oh, about 200 yards really (as you guys say).  BUT! if you happen to zap a hog at 200 yards, that 180gr is going to be impressive I bet.