I'm building an 18" AR right now with a SS WOA barrel.  I've decided on a Leupold Mark 4 in a Larue mount, but there are so many options with the Mark 4's I don't know where to start.

I was comparing the 3.5-10X40 LR/T to the 3-9X36 MR/T, and the first thing I noticed was the lack of focusing knob or ring on the MR/T.  The LR/T has one, which I assume to be from 50 yards to infinity.  Why does the MR/T not have one?  What yardage is its focus fixed to?  

Also, with the 18" barrel, which power should I go with?  2.5-8X36 or 3-9X36 or 3.5-10X40...  Is the TMR worth the extra money?  What is the difference between M1, M2, and M3 target knobs?  Thanks in advance for any replies, Arfcommers.

I can't answer all your questions but can help with a few.
M1 knobs are 1/4 moa adjustments
M2 are 1/2 moa adjustments
M3 are 1 moa adjustments on elevation and 1/2 mao on windage.

The M3 offers quick range adjustments but are very coarse and the ele knob is used as a bullet drop compensator knob (M2 knobs also) for specific loads. Chances that it matches your loads, velocity, temp and all that are slim. They are close but not perfect.
M1 knobs are more precise but take several turns to reach to say 1000 yds where the M3 does it in one turn.

The MRT is for mediem range and the LRT is long range so I am guessing leupold felt the Mrt didn't need a paralex adjuster.

I have several of the 3.5x10 M3s  that I like but I am not going for the tightest groups. If I get close to the center and hear the steel plates I am happy. The M3s work great for me out in the desert where I just toss the plates where ever.

The TMR will only be worth it if you plan on learning to use it to range and for hold overs. Same with mil dots.
I have taken a half assed interest in learning mil dots but find I don't use them for hold overs, just for range. I use the elevation knob for different distances instead of the mil dots which is why I like the M3s. So I'm lazy.

Quoted: I can't answer all your questions but can help with a few. M1 knobs are 1/4 moa adjustments M2 are 1/2 moa adjustments M3 are 1 moa adjustments on elevation and 1/2 mao on windage. The M3 offers quick range adjustments but are very coarse and the ele knob is used as a bullet drop compensator knob (M2 knobs also) for specific loads. Chances that it matches your loads, velocity, temp and all that are slim. They are close but not perfect. M1 knobs are more precise but take several turns to reach to say 1000 yds where the M3 does it in one turn. The MRT is for mediem range and the LRT is long range so I am guessing leupold felt the Mrt didn't need a paralex adjuster. I have several of the 3.5x10 M3s  that I like but I am not going for the tightest groups. If I get close to the center and hear the steel plates I am happy. The M3s work great for me out in the desert where I just toss the plates where ever. The TMR will only be worth it if you plan on learning to use it to range and for hold overs. Same with mil dots. I have taken a half assed interest in learning mil dots but find I don't use them for hold overs, just for range. I use the elevation knob for different distances instead of the mil dots which is why I like the M3s. So I'm lazy.

It seems to me that the LR/T is for ALL distances, not just long ones.  I'm just wondering about the lack of a parallax adjuster on the MR/T.  Even if it's not for 600+ yard distances, wouldn't it be a good idea to have one anyway?  How large is the error created by the lack of one?

Quoted: Quoted: I can't answer all your questions but can help with a few. M1 knobs are 1/4 moa adjustments M2 are 1/2 moa adjustments M3 are 1 moa adjustments on elevation and 1/2 mao on windage. The M3 offers quick range adjustments but are very coarse and the ele knob is used as a bullet drop compensator knob (M2 knobs also) for specific loads. Chances that it matches your loads, velocity, temp and all that are slim. They are close but not perfect. M1 knobs are more precise but take several turns to reach to say 1000 yds where the M3 does it in one turn. The MRT is for mediem range and the LRT is long range so I am guessing leupold felt the Mrt didn't need a paralex adjuster. I have several of the 3.5x10 M3s  that I like but I am not going for the tightest groups. If I get close to the center and hear the steel plates I am happy. The M3s work great for me out in the desert where I just toss the plates where ever. The TMR will only be worth it if you plan on learning to use it to range and for hold overs. Same with mil dots. I have taken a half assed interest in learning mil dots but find I don't use them for hold overs, just for range. I use the elevation knob for different distances instead of the mil dots which is why I like the M3s. So I'm lazy. It seems to me that the LR/T is for ALL distances, not just long ones.  I'm just wondering about the lack of a parallax adjuster on the MR/T.  Even if it's not for 600+ yard distances, wouldn't it be a good idea to have one anyway?  How large is the error created by the lack of one?

It has fixed 100 yard focus.  The further from this distane the greater the error per amount of your eye being off center.  If your eye is always dead center it wont really matter.  If you are off center you can have significant shift (couple inches) at say 300 yards. The diffenrence in a hit or miss at long range.  Just set up a gun and look at 300+ yard targets and move your head around while the gun is set up to not move on sandbags and you will see the crosshairs move around on your target.

Thanks for the replies.  Final question:  which one to get?

I will say you should get the TMR over mildots! The TMR is sooooo much better. I bought the mildot before I new alot about range finding and that's all that I would use it for. I went went the LR M3 on my ar but I'm shooting a .308. If you are going to shoot inside of 600 than I would say get the Mid Range.

Here is an article you may want to read:Optics for Practical Long Range Shooting

Realistically what are you going to use this optic for?
Do you want to shoot small groups, furry creatures, 25-300 yards, 75-500, long range plinking?  Let your use dictate the optic.  

I like the following:
25-250 or 300: 1.5-5*20 MR/T M2 SPR
70-500+: 2.5-8*36 MR/T M2 TMR
Punching paper or small furry guys at long range: 3.5-10*40 LR/T M1 (or even more 4.5-14,6.5-20)

For the size and versatility of an accurate 18” AR I would go with the 2.5-8 personally.  I think the TMR is worth it…that’s why I have one the way.  

FYI... Per the Leupold manual their riflescopes are set parallax free at 150 yards.  
Also perhaps I have missed it, but I have never seen parallax adjustment on a quality scope under 9x as the higher magnification you go the more chance there is for parallax error.   (Airgun scopes aside)