40 mm vs 50 mm Advice

[freedom-lover]

Just completing an AR308 build. I am interested in usage for tactical and also 600 m target shooting. Budget is $500 - $1000, plus another $100 or so for the mount. My initial thought was something along the lines of 4-14x40 (like for example a Leupold Mark 4 with a mil dot reticle, although that model doesn't have external adjustable turrets, which I think I would like).

[jtv3062]

If you're planning to shoot when the sun goes down, then the 50mm.

[GeoffLinder]

Yep, the objective size is all about light gathering. The downside is a larger objective diameter and the higher scope mount position needed because of this. In most cases 40mm will be plenty.

[ExtremeX]

When it comes to optics, there is always a trade off...

Personally I like larger objective scopes... unless you have a specific application where you want to reduce size and weight (like on a hunting rifle), I generally always opt for the larger objective. Most of my larger optics all sport a 50mm objective.

For optic mounts specific to the AR platform, like the Burris PERP, Larue LT104, or similar mounts generally put your height over bore around 2.7" which will easily accommodate the larger objective size. I also use the same mount for my optics with smaller objectives like a 1-4x scope because it places the optic pretty much where it needs to be.

In your price range, I would look hard at the Vortex Viper PST line. SWFA also came out with a 3-15x42 which would also fit the bill if you opt for a smaller objective size.

[SloChicken]

I just am finishing a build (6.8 AR build)
I am using a 2.5-10x with a 56mm objective lens. I have it in a QD Burris PEPR mount and it sets up perfectly. Same height as my red dots on my other ARs.
that are set up at lower 1/3 co-witness with their BUIS
as a side note,
the Scope is not really co-witness capable (although 2 eyes open is okay at 2.5x, I went ahead and got the Dueck Defense offset Iron sights They are great and cheek weld is the same for both the scope and the sights.
Light pickup is excellent at night with the scope I got. This build is specifically for low-light, early morning, or late evening hunts. Even night hunts, although that is not my norm.

The scope pictured is a Trijicon 2.5-10x 56mm with an illuminated green dot over a mil-dot reticle. No batteries required and the green dot really helps over low light areas or at night.
I paid about $800 for the scope and another $100 for the mount. The Iron sights were $200 in case you are interested in them.





Hope this helps! And good luck!

[freedom-lover]

Thanks for the advice. Is a 20 MOA canted scope mount helpful?

[HK Dave]

I think Killer Bee didn't read the part about $500-1000. Far as I know, the only quality scope available with a 35mm tube and 50mm objective is a Vortex Razor coming in at $2K. :P (BTW I don't mean this with any disrespect to Killer Bee, but the internet might make it seem that way. I say this jokingly like a buddy would having beers around a table)

OP, if you're shooting out to 600m, the size of adjustability is unimportant. Just about any decent quality tactical scope will get you to 600m. It's only when you're reaching out to 1000m+ that something like a 35mm tube "might" be needed. I do just fine with a 30mm. :P

If 600 is as far as you'll ever shoot, the 20moa is not required on most scopes... if you however will ever be reaching out past that, the 20moa cant MIGHT be useful depending on your scope.

I too prefer larger objectives whenever possible.

[Whiterabbit]

I prefer smaller objectives myself.

If you are choosing between good glass at 40mm or acceptable glass at 50, I'll take glass over size.

If that's not an issue, knock yourself out.

[OpticsPlanet]

I think Killer Bee didn't read the part about $500-1000. Far as I know, the only quality scope available with a 35mm tube and 50mm objective is a Vortex Razor coming in at $2K. :P (BTW I don't mean this with any disrespect to Killer Bee, but the internet might make it seem that way. I say this jokingly like a buddy would having beers around a table)

OP, if you're shooting out to 600m, the size of adjustability is unimportant. Just about any decent quality tactical scope will get you to 600m. It's only when you're reaching out to 1000m+ that something like a 35mm tube "might" be needed. I do just fine with a 30mm. :P

If 600 is as far as you'll ever shoot, the 20moa is not required on most scopes... if you however will ever be reaching out past that, the 20moa cant MIGHT be useful depending on your scope.

I too prefer larger objectives whenever possible.

+1 for HKDave. Even with most 1'' scopes with medium magnification range a 20 MOA base usually isnt needed. Some higher magnification scopes with limited internal adjustmnet might need it. If you stick to a 30mm tube that will give you plenty of adjustment in most cases. You mentioned a Mark 4...maybealso take a look at the PST. Vortex has a 4-16X50 PST with a 30mm tube with 75 moa internal adjustment. Plenty out to 600+. Great scope for the money.

Trevor B.

[ExtremeX]

If you are choosing between good glass at 40mm or acceptable glass at 50, I'll take glass over size.

I can agree with that (Quality glass > objective size)... objective size means nothing if the glass is crap...

Chances are a 40mm scope with quality glass with better lens coatings will also be very efficient in light transmission.

All things being equal, not so much.

[HK Dave]

Oh yah, forgot about IOR, great scope... but it certainly isn't with $500-1000.

I think a Vortex Viper would suit the OP well.

[freedom-lover]

What are the thoughts on first focal plane reticle vs second focal plane?

[HK Dave]

Depends on the style of shooting you do. If you do a lot of holdovers instead of dialing in adjustments, it's well worth it to get FFP.

If you're the type that always shoots at max magnification or always dials in adjustments, then SFP will suit you fine.

Generally speaking, it is more difficult and costly to build an FFP scope over an SFP. So in money terms, you would probably get more bang for the buck getting an SFP with all other things being equal.

With scopes that max out at 10x, I personally don't see a reason for FFP. The reason that is... generally speaking, when shooting in a range where you're using less than 10x, you're not doing large holds. Also when you are shooting a range where you would do holds, then i'd be using the scope maxed to 10x anyhow.

Also the reticle at say... 3x, the hold dots are so tiny, you'd have to be a super eagle man hero of the universe to use them properly.

With scopes over 10x, I will never buy an SFP scope again. All my scopes that magnify beyond 10x are FFP. If I was shooting benchrest or something where all my shots are at 1000 yards... i'd consider an SFP again, but I don't do that type of shooting.

[ejhc11]

Go 40mm. Your eyes can't take advantage of larger light gathering capability over 40mm, just an overkill.
Read this.

http://www.chuckhawks.com/straight_scope.htm

[zio707]

Do the math and figure what works best for you for a light gathering lens.

For example a scope set at 8 power with a 56mm Obj will have an exit pupil of 7mm... Or 56mm/8 = 7

42mm/7 = 6 (42mm at 7 mag)

Just divide as follows.... Obj. mm / power = exit pupil.

I used a 42mm and 56mm to show the differences, if you plug in a 50mm you'll see no great advantage over a 42mm.

Now age comes into play as well with what your eyes can really do with this. Youngsters can take advantage of 6mm to 7mm exit pupil while us older folks it starts to diminish down to 5mm. Not an expert, just learning like the rest of us.