Rifle Scopes : parralax on 1-4x scopes

Thank you both. I appreciate all of the input. Even with the maximum error you have calculated I am still inside the 1 MOA X ring at 600 yards on the Service Rifle targets.

Ilya - that is part of the equation on opening the rules. 

USMC requested they be able to shoot the rifles they train with (A4 or M4 with ACOG) in Service Rifle class in CMP and NRA competition. Certainly does also encourage veterans with experience using these systems to participate.

The secondary impact is it allows shooters who have a scoped AR (3-gunners and such) to compete with the rifles they already have. Many have said they never tried Known Distance shooting with service rifle because they don't own an A2 or A4 with carry handle sights and front sight base. This takes that obstacle out of the way for them to try the sport.

Tertiary impact... Older shooters who quit shooting service rifle when they could no longer see the post clearly enough to shoot their rating can use the optic to get back in the game. Many of these shooters are running match rifles with round sights now and still shooting, just not service rifles.

The shield and  the sword part of the addition of optics as a service rifle is now it becomes and arms race with scopes and mounting systems adding to the cost of building a competitive rifle.  My attraction to service rifle was I knew that the $1300 rifle and ammunition I was using was pretty much identical to the ones being used by the national champion shooter from the US Army AMU team on the line next to me. Any difference in the scores was purely the shooter. The Indian, not the arrow or the bow. That and shooting 20-300 and 600 yards across the course of fire with iron sights on an AR is just plain cool.

White Oak Armament,  Compass Lake Engineering, Keystone Accuracy and several others all make these uppers for about $800 and anyone of them will shoot 1/2 MOA with the proven loads that everyone knows. No load development required. A2 lower and a Geissele NM trigger for another $500 and you are in business.

90% of the rifles on the line at a Service rifle match are an A2 with pinned 1/4x1/4 MOA sights, Float tube under the handguards and over a 1:7 or 1:8 Wilson, Kreiger, Pac Nor, Shilen or other quality maker barrel with a Wylde chamber with a 2 stage Geissele  or Rock River trigger  set at 4.5 # shooting a 75 or 77 grain OTM bullet from a red green or black box at 2750 fps with 23-24.5 gr of Varget, RE-15, H4895 and IMR 8208XBR as the most popular powders.

I do not expect  a 4x scope to make a huge difference in scoring as no scope holds the rifle still for you but I do not see how using a scope would not increase X count of a shooter capable of shooting a good score with iron sights.