Hear (read) me out.

Is there a go to optic that you all use for the express purpose of testing out a rifle for groups?  Something that you don’t permanently attach, something you really trust?   An optic that you wouldn’t necessarily use on the gun, but something you use to test the gun?  No moving around, just bench testing.

Example I want to evaluate uppers and decide which one I’ll keep.  Different calibers, just to evaluate groups at 100 yards.  To test the system (barrel).

I really would like to know.

I like to borrow a Nightforce NXS 8-32X56 Mil-R.

You can SEE it with that!

I don’t like taking optics off a gun.  I don’t like to zero them, much less re-zero them.  I suppose it’s because I suck at it.   I really do.

A holosun would work well for that. Something for a point of aim and that's it. I can shoot a tight group at 100m with a 1x optic.

In all reality, it's going to take more than making a few groups with an upper for me to decide whether I like an upper.
Most of the time, I'm not checking if how accurate or precise an upper is because I buy from reputable manufacturers and trust them. Most of the time, I'm checking weight, length, and rail.

What target?

Yes click here

I can’t shoot a tight group with a red dot.  It’s a personal failing, one of many.

I, personally, don’t trust any manufacturer.  They all fail, have issues, even if seldom.  That’s on me to test my gear.

I think I’m going to get a 4x or 6x fixes scope on a quick detach mount to use with my lead sled.  If I put the cross hairs on the mark, at least I’ll know that part is consistent.   I know I can’t do that at 100 yards with a red dot.   I mean I can’t, you may be able to.

Rear aperture and target 4+ MOA wider than the dot.

Agreed about the aperture.   I’m only going down this path to determine with set up is more accurate in an artificial scenario involving a lead sled and bench type shooting.

Yes, it’s a determination on which one to downsize.  It’s a factor.

Edit.  One factor to compare relative to the uppers.

If it's something that's only going to be used to test out uppers, I'd get something cheap and reliable to create groups. Primary Arms and what not...

I use an SWFA Super Sniper on an ADM cantilever just for this purpose. A red dot isn't going to be able to show true accuracy when I do load development or testing for consistency. I get very strange head shaking looks from the operators at the range while those obviously smarter than them ask what kind of load I'm working with. It's a dead give away when I'm using a yuge scope on an SBR or a simple carbine or mid length.

@GSL

That’s exactly the scope I was looking at!  In 10x or 12x, they are only $300 off Amazon.  Thanks for the validation of concept.

I have a good Burris Black Diamond target scope - 6-24x50 with 1/8 MOA tall target turrets and fine 1/8 MOA reticle.  I have it QD mounted in a Larue mount and move it around from rifle to rifle, for load development and accuracy testing.  It was a $1000 scope at the time and I think I paid about $800.   You can pick up scopes like that in good used condition pretty inexpensively, now.

Another option would be a relatively high powered fixed magnification scope, like an 18x or 24x.  It does not need illumination.

Any bench type scope would benefit from having a very thin reticle for shot placement, and benefit from having either an adjustable objective or side focus, if it is a variable.

With high magnification, you need the best glass.  Do without other frills and focus on the glass and the reticle.  Even turrets don't matter much for shooting small groups at 100 yards, as long as the scope will hold zero once sighted in for that load at that distance.

I have a Bushnell HDMR 3.5-21x that I use for that sort of thing. It's a solid scope but nothing particularly special. It's just easy to swap around on different guns and re-zero with three shots and has more than enough magnification to make groups as tight as I am physicially capable of. Good reticle for poking holes at farther ranges and figuring out your dope too, just a simple basic mil tree.

@ms556

I don’t have a scope for this purpose yet.  As stated previously I am looking at SWFA scopes at 10x fixed.

I’m open to other brands.   What would you recommend specifically?   I’d rather keep it around $500.   New or used is fine.   At this site, a lot of AR (duh) scopes are for sale in the EE. Where would be a better site to look at for used, >10x fixed power scopes?

For your budget you can get the SS and the ADM or LaRue mounts and still be able to do your task and purpose quite easily.

Will do.   I currently have a spare mount.

Or you can go straight to the source:
https://swfa.com/swfa-ss-12x42-tactical-30mm-riflescope-105770.html

I don't buy used scopes that I cannot evaluate personally.  However, SWFA does have a section of their website where they sell open boxes and scopes with minor blemishes.  They will accurately describe what they offer and prices are good.

For about $500 you can get a new Leupold Mark AR 6-18x40.  It is "old school" in that it has only a 1" main tube, no illumination, an adjustable objective for parallax rather than side focus and a now less fashionable but quite usable mil dot reticle to go with its mrad turrets.  The glass is good, the turrets repeatable,  It is a good basic scope for long range shooting on a budget in addition to shooting groups at closer distances and load development.

Here is a good review.  Although this is a couple years old, you can get a decent evaluation from an objective source.

http://www.snipercentral.com/leupold-mk-ar-6-18x40mm-full-review/

Another good single purpose choice would be the fixed power Weaver target scopes, like the Weaver T-24.  I think they are priced at about $425 or so right now with most vendors.  A lot of entry level or mid level target shooters on a tight budget use the T-24 or T-36.  I'd not go above 24x as higher magnification makes eye positioning very rigid and unforgiving when looking through the scope.  Even the slightest movement to the side and you are lost.   24x is as high as I can use and stay behind the scope very long without fatigue.

If you want to test the absolute maximum accuracy of a rifle at 100 yards, you need more than 10x.  100 yard benchrest rifles are shooting with 36x and 45x scopes.  Such scopes are useless for any other purpose, but even at 100 yards, the top competitors are using magnification that high.  Just don't cheap out on the glass, because as magnification goes up, especially above 12x or so, the requirements of the glass quality get higher to maintain a usable image.  What is as important as magnification is resolution:  the ability to resolve, or see fine detail, like the need to see and aim at the same location inside 1/4 MOA boxes on the target.

Another budget fixed power scope to try is the Bushnell Elite 3200 fix 10x.  Scope can be had with mil dots and moa adjustments or 1/10 mil adjustments

I have a older Trijicon TR23 4 - 16 x 50 (I think they have re-numbered it the TR29 ) which would do rather well for that purpose, I think. Great glass.