What is the theoretical precision of a riflescope.

[jlow]

So here is the question I have come up with from previous discussions.

If you look at how a riflescope is constructed the erector tube which contains the reticle is moved by the windage and elevation turrets. These turrets are not directly attached to the erector tube but is kept pushed against them by a spring – so the whole mechanism is free float. Since it is free float, it can be affected by the recoil. So the question is what is its ability to maintain its focus on target in response to this recoil.

Movement of the erector tube in the x, y direction (windage, elevation) means the center of the reticle will also move which translates to a potential different point of aim. When a point of aim changes, where the bullet will go will also change. Since a target at 100 yards is a long way away, even a slight shift of the erector tube can open up a group which is independent of the load, rifle, and shooter.

Any thoughts?

 

[jonbearman]

Most quality scopes don't have these problems. I have had a few cheapies that would change by a foot at 100 yds,hence they found the trash barrel. If you stay with recognized name brand scopes like leupold,burris,nikon,sightron(s2 and above), and the german scopes the probability is good they will track very well.I am in no way saying that the top brands don't ever have problems but for the most part they perform very well. In recoil things probably do move but they should return to zero instantly.IMHO

 

[LHSmith]

You might find your answer by reviewing SR Benchrest Tournament records and comparing "fixed" scopes (those with the internals fixed for no movement and are adjusted for windage and elevation via external adustments -similar to Bausch and Lombs BalVar scopes) against production model scopes. Google "Jackie Schmidt fixed internal scopes", or "Bob Brackney External scope mounts".
Bottom line is that if a production high quality scope is functioning properly, there is NO difference in the amount of shift in POA vs POI between an internally adjusted scope and one that has the internals fixed and uses external adjustable mounts.

 

[jlow]

Thanks guys! This is very useful and it makes sense.

So here is a related question: I recently talked to a scope tech from a highly regarded scope company and he mentioned that shooting a gun in a rest that is heavily clamped down (i.e. a rest that prevents gun movement) is significantly detrimental to the scope because the recoil is transferred to the scope and can potentially damage it. The question I have is this true and in the same fashion can shooting a scoped gun in that fashion cause a POA shift?

 

[jonbearman]

The scope tech is correct as far as poa I don't think it would change on the first shot but who knows.If you don't like recoil get a muzzle brake put on so you can shoulder the gun comfortably.

 

[jlow]

It's not a matter of the recoil. I only shoot 223 and 308 and neither has significant recoil at least to me. I only use a rest to help sight the gun for load development.. Thinking of switching over to just using my bipod and rear bag if it makes a difference.

 

[LHSmith]

If your talking about the LedSled type of rest, yes they have been known to cause damage to both rifle and scopes. I believe those rest are totally useless. First, the POI will be way off with a LedSled vs. shooting from typical tripod front rest/bag and rear bag setup. The POI will even be different from shooting off hand vs. a BR rest, or even a Bull's bag shooting bag. The bullet is still in the barrel as early recoil begins, so depending on the rest/ rifle interface the muzzle will rise, stock will torque differently,and barrel harmonics will be affected. Second, if your trying to ensure peak accuracy for load development, that's fine IF you use the contraption exactly in the same manner for all your shooting i.e. strapped in with the same force, same weight, same bench surface. In BR just a subtle change like moving the front bag (or rear bag) an inch fore or aft will likely affect accuracy.

 

[jlow]

I should clarify that my rifles are all free float barrel and only the handguard touch the rest not the barrel for the exact reason you mentioned i.e. it will affect harmonics.

I also do not use the sleds with lead for the reasons mentioned earlier which is I am not trying to reduce recoil.

 

[LHSmith]

There is a significant POI change when one fires a full blown BR rifle on BR quality rests and benches between free recoil and firing with the buttstock just touching ones shoulder.....everything else the same, only the finger touching the trigger. Anyone who shoots free recoil can tell instantly, that they screwed up with bench manners.

 

[jlow]

I apologize for not saying this up front but as you can imagine, there is a lot more detail to firing a rifle.

I agree with what you say but even though I use a rest, I don’t shoot free recoil but pull the butt stock tight up against my should as my precision rifle instructor has taught me to do.

So it still comes down to my original question. Which is if I am doing load development on such a rest in such a manner, does it affect first my scope in terms of potential damage and would it give me inconsistent POA because of the recoil effect on the scope?

 

[LHSmith]

The inconsistent POA comes from the way the rifle is fired....not due to any scope issues. Any deviation in the way a rifle is fired will result in "some" deviation in the POI. Most Hunter-class score shooters, hold the rifle in much the same manner as you were taught......and if they do not hold the rifle in the EXACT manner with the EXACT amount pressure for each shot (whether it be a light hold, death grip, or anything in-between, they will likely miss the 10 ring.
It is unclear what type of restricted rest you use and how you use it, but even machine rests have the means to absorb recoil energy.

 

[jlow]

I don’t really look at my rest as 100% restrictive – first, it is not of a quality that allows it to do that and second, I do not put lead on it to reduce recoil.

I agree that the shooters method, skill, and ability to reproducibly do the same thing can of course affect precision. One thing that finally convinced me to look outside of myself (and BTW that was after 3 years of trying, so I am not one to shrug blame easily) is I was getting sub-MOA groups relatively easily shooting a high quality 22lr using the same rest and same scope. Since there is almost no recoil, this started my present line of thinking.

 

[LHSmith]

In order for load development to be meaningful, that is: to obtain repeatable results, one must have:
* a rifle that is capable of the degree of accuracy you seek
* a proven rest set-up on a solid platform
* a scope that holds POA of sufficient power for resolution
* some means to monitor wind at intervals along the bullets path
* experience in condition reading skills so one knows when and when not to shoot, and gives one the ability to reason why a shot went out of group (i.e. a sudden switch or let-up, or mirage ). Sudden wind changes will not be apparent without flags, and an otherwise good load may be rejected.

 

[RHux]

Go to precisionrifleblog.com to read their latest scope tests .

 

[jlow]

I did not start but ended up with pretty much everything including sub-MOA rifles, proven rest (as per the 22lr results), a high quality scope.

Wind was not a major factor since I was looking mostly at verticals. Quality ammunition (FPGMM, and handloads that had single digit SDEV).

As mentioned earlier, this is not something that I blamed and came on easily. In fact, if you have read the other thread that I had on scope problems, just like mikecr, I pretty much blamed anything and everything except the scope for 3 years...

 

[LHSmith]

Wind was not a major factor since I was looking mostly at verticals. Quality ammunition (FPGMM, and handloads that had single digit SDEV).

Sorry, I thought we were talking shortrange, 100 yards was referenced in post #1. I understand the need for LR BR to minimize vertical, but at the avenues I shoot here in the Northeast, any wind or lack of it will be a major factor in developing a load and fine tuning it well enough for matches. You have to know how a load performs in various wind conditions, may shoot well in a L>R and poorly in a R>L.... so you can pick a condition which offers you the best advantage. Also, the winning competitors in BR shooting 6 ppc's or even 30 BR's will have a large spread in SD. In my tenure in the SR game, I do not see much correlation between low SD and accuracy, results on target trumps any chrono numbers.

 

[jlow]

As it turns out, I don't shoot BR and generally see good correlation between SD and accuracy. But we are really digressing since my accuracy proof is with factory ammo. Let's stay on topic please.

 

[LHSmith]

Apparently our accuracy goals are far apart.
As far as topics, I tried my best to answer your specific questions. From your past posts, you seem to be very analytical and meticulous in trying to understand what it takes to produce accurate reloads. But to ignore the impact that conditions play on bullet POI is kind of odd. IMHO, you are not taking full advantage of all your exhaustive reloading preparation. Review the wind deflection tables again and you will see how large a role it can play.

 

[jlow]

Not at all. I am completely with you in terms of the importance of wind on accuracy. I only want to stay on topic as it relates to the scopes. You are absolutely right that I am analytical in nature - this comes from 30 years of research. What I learned is although many factors affect you ultimate goal and all of them should be taken seriously, it is important to at least at first take each individual items individually and optimize them without trying to do everything at once. This is the reason I was to stay on topic.

 

[LHSmith]

Well, maybe you're problem was not as much scope related as much as wind related. Just throwing it out there.