I received this question via a private message and thought it was interesting enough to answer it in some detail.  I have been thinking a lot about appropriate optics for gas guns recently, so this came it at the right time.

Here is what I ahve been messing with last few days:

Above: 18" WOA barrel 5.56 AR-15 with Burris XTR PS 5.5-30x56 on it.

Below: 22" Satern barrel 224Valkyrie AR-15 with Delta Stryker 3.5-21x44 on it.

For obvious reasons, I am not posting who asked the question, but what I'll do is send him a link to this article, so that he gets an answer and we might have a good discussion.
Here is what I received:

Had a question for you
I have a Knights Armament SR25 6.5CM with a 14.5" barrel - I plan on shooting 100 - 600 yards at targets at 100 yd intervals
I am favoring the TT 315P with an Aimpont Acro mounted on top -( see you liked the TT315M) - would that be enough magnification?
I saw the SB SB 5-20 which offer more top end - I like the turrets on the Tanget and tooless Zero - what are your thoughts between the 2?
If I stayed 400 and under - how do you see the TT315P with the Acro Red dot against the SB 1-8 Short Dot (saw your review on that)

There isn't really enough information in the question for a simple answer, so let's consider a couple of options.

What is the gun for? 

For shooting medium range (out to 600) with occasional close range use?  If yes, go with a conventional scope and offset or piggybacked RDS.

For shooting at close distance with occasional med/long range use? If yes, so with a nice LPVO like the above mentioned S&B Short Dot 1-8x24 https://alnk.to/3J8vlFv.  It is quite capable at long range, but where it will really knock your socks off is up close and personal.

The question of what is enough magnification is pretty personal.  For me, 3-15x is plenty, but I shoot a lot off of the tripod and off of barricades which likely influences my decision.  For that, my 3-15x50 TT315M Tangent works great and 6mrad of elevation is plenty.  https://alnk.to/gVNkUXB  175gr #08 out of a shoter tube will drop in the 5 to 5.5mrad range at 600 yards.

For a dedicated long range semi-auto, other than the looks, there isn't much downside to a larger scope, which is why I set up Burris' XTR PS on the 5.56 AR pictured above.  It is for my son to learn to shoot at distance and to us in next year's BOTC match.  https://alnk.to/gp33YuR

If I were setting up an accurate semi-auto and wanted a high mag scope for it, I would likely lean toward March 4.5-28x52. https://alnk.to/dLjHgjh  It is compact, relatively light and very good optically for any use other than the dead of the night (for which we should be using clip-ons anyway).

S&B Short Dot is a very nice scope, but March is a newer design and, other than the lowest light, has an edge in terms of image fidelity and stray light control.

TT315P Tangent is a veyr ncie scope and toolles turrets are appealing.  However, there is a weight penalty to that.  For me, that was not worth it.  I ahve 5-25x56 and 7-35x Tangents and those toolless turrets are spectacular.  However, in a 3-15x50, I am quit ecomfortable with simpler 6mrad per turn knobs on the TT315M.

When I tell people that I am using Hornady's factory ammo for matches, I get occasionally surprised looks.  Occasionally.  Most of the time, I get fairly condescending looks from handloaders who would not let a factory round pollute the chambers of their precision rifles if their lives depended on it.

To be fair, if I had the time, I might be reloading as well.  I might be able to do better than modern factory ammo, but the truth is that I am not a quarter MOA shooter, so a lot of that would be lost on me.  The UKD matches I fidn most interesting can be done by a competent shooter with a 1MOA gun.  I am working on that first part.  I reload when I have to, but if I can get factory ammo that's not outrageously expensive and that shoots well, I will go with that every time.

I looked around and realized that I have about 600 rounds of Hornady 147gr 6.5CM factory ammo from two lots.  400 rounds from one that I will use during the match later this month.  The remainder I will likely use up practicing prior to that.

I woke up early and snuck out to the range while my family was still asleep earlier today.

I set up in the 100 yard tunnel we have.  I took my four shots to zero with the lot I plan to use for the match.  Then I fired a six shot ~0.7MOA group.  That's accurate enough for my purposes.

Here are the chronograph results:

It is slow, but accurate and consistent.

Then I shot a five round group of Hornady 147gr ammo from the other lot.

Both of those lots are not too shabby for factory ammo.  POI was within 0.2mrad between the two lots, all laterally.  

I had some other ammo on hand, so I fired two five shot groups with Berger 144gr hybrid target ammo and Sako TRG 136gr.

Here is what I got for five rounds of Berger:

That's not a ton of statistics, but it is a data point.  The group was ~1.1MOA

Here the data for the Sako TRG 136gr ammo:

This ammo showed some mild pressure signs, but not too significant.  

Again, this is not a ton of data, but Hornady is looking pretty decent here.

I'll repeat the zero retention and consistency tests a couple more times between now and June 18th when we head to Wyoming.

I might pick up a couple more boxes of Hornady 147gr from a different lot to get some more data.

More to come.

My first PRS match is officially behind me.  It was, I think an officially sanctioned regional PRS match at the shooting range called Del Norte about an hour from me (just east of Rio Rancho).
As I discussed previously, rather than using the rifle I originally prepared for it, I ended up having to change gears at the last moment and slapping a 5-25x56 Tangent on my daughter's 6ARC MiniFIx.  A lightweight 16" Proof barreled 6ARC is not an ideal set up for PRS competition, but, in the grand scheme of things, the rifle did hot hold me back at all.  I had a couple of weights that fit the Q-cert handguard, so I slapped them on there as well, which got the rifle to right around 12lbs.  The balance point was almost where I want it, but not quite.  In all other ways, this was a very handy rifle to shoot.  I am actually thinking of getting a couple more weights that I can stack on it (Sawtooth weights are stackable), so that I can shoot some future local matches with it.  It shoots Hornady's Black 105gr ammo very well (it also shoots 108gr ELD-M and 103gr ELD-X well, but I had a larger supply of Hornady Black ammo on hand).

It chrono'ed pretty consistently for factory ammo.  Here are the measurements for a 20 shot group.

If I were to take out the fastest shot, SD drops down to 8fps.  

In terms of wind performance, it is a 5mph rifle, so slightly better than the 308 ammo I was going to use originally, but not as good as the 6.5CM that was my intended match rifle for this year (we make plans, but then life happens).

I do have to admit that I really enjoyed shooting the match with the MiniFix.  So much so, that I am seriously thinking of building another one for myself, since this rifle is technically my daughter's.

The closest shot we had was a bit over a couple of hundred yards.  The furthest was around 1150.  That was a bit challenging for the 16" barreled 6ARC, especially since the terrain there is tricky and wind is doing strange things.  Morning was relatively quiet wind-wise, but then the wind becamse gusty with changing directions.

It was a one day match with about 30 shooters (six squads) and ten stages.  The 17 year old kid who won the match was in my squad with his dad.  Apparently he is on the US team, so it should not be surrpising that he won.  He was very good.

Overall level of the shooters in the match was quite good.  I really was not sure what to expect.  Somewhat paradoxically, the only matches that I have shot in so far are the much larger two day NRL Hunter matches.  In those, I am just out of the bottom third in terms of skill level.  In this match, it was pretty much in the same spot, I think.  

The terrain was a little bit like Cameo, I think (I have been there, but not shot there), with some angle changes and sufficient vegetation to make getting on target occasionally troublesome.