Thinking of buying a Loopie scope for a planned SEBR, and am torn between the 4.5X14-50 M1 and the 3.5x10-40 M3 w/BDC cam.

What will the M3 with the BDC cam do?

Or should I just get the variable IOR they are selling with the SEBR??????

Works like this:  The M1 has the familiar .25MOA per click adjustments that we all know and love from many other scopes.  It's great for target shooting, because you can dial your zero in to within 1/4MOA, which is far finer than most people (or rifles) can shoot anyway.  It is slow, and requires that you either have your comeups memorized or written down, and it has the fatal flaw of not having enough adjustment to move from 100 to 1000 yards in only one turn of the elevation dial.

Enter the M3.  A fellow at Leupold (whose name escapes me at the moment) analyzed the above set of problems and came up with the following solutions:

1) the elevation turret would adjust in 1 MOA increments, enabling you to adjust between 100-1000 yards in one rotation of the dial

2) the turret would be matched to a specific round at a specific muzzle velocity, so that "comeups" as we learned them would be unnecessary.  On acquiring a target at 400 yards, you simply rotate the dial to '400', hold dead center (yes, I see you in the back waving your hand talking about wind.  Shuddup for a minute, I'll get there.) and shoot.  If you haven't fucked up your range estimation you will be rewarded with a center hit.

What does all this mean to you?  Depends.  If you really think of your rifle as a people-puncher you're better off with the M3.  Its adjustments are less precise, but much faster.  If you go to a sniper comp, you're going to have problems with the M1 - eventually you'll move from 400 to 800 only to find out you're one full rotation out from where you need to be, and miss miserably.  

YMMV.
QS

Bullet Drop Compensator.

The M3 adjusts your POI to the target range with the turn of one knob.  It is numbered in hundreds of yards (or is it meters?).  You sight in at 100 and set the knob to "1".

You then have a target at 300, turn the knob to "3" and you should hit the target, assuming your bullet matches the one one that the BDC is based on.

Also, when sighting in the M3, the elevation clicks are 1 MOA and the windage 1/2 MOA.  IF you are anal about hitting the head of the match rather than the book, you might want to go with the M1.

With the M1 series, you have to use "comeups" of some number of 1/4 MOA clicks to adjust for range.  The advantage is you can determine the exact number required for your load and rifle, if you have a range of sufficient size.  Te drawback is that you can lose track of how many turns you have made and be way off (like a whole revolution) at long range.

The M1 is more precise, the M3 more practical.

I was taught by my father how to use the mil dot reticle itself to adjust for holdover, hence only having to adjust the M1 turret one rotation out to 1000 yards.

If you are quick at determining your target range using the mil dot system, using the mil dots for holdover is just as easy and more accurate and can be catered to your specfic load.  

A [i]Bullet Drop Compensator[/i] is a fancy name for a range dial, nothing more.  Maybe the intent is to charge extra for it because of the exotic name?  For a sight to work on several different rifles/ammo there's some engineering involved, but it's still just a range dial when all is said and done.

You can set the [i]BDC[/i] on your M16A2 rifle to 300, 400, etc, and several ranges in between the big numbers.

-- Chuck