M4 issue

[bigwakes]

I have an M&P15 and just built an AR with a Vltor Mur upper. With my Eotech sited on the M&P15 it's dead on. When I mount the same optic on the Vltor it's 4 inches low at 25 yards. I put the same optic on a buddies Colt and it shoots where the Vltor does. I would like to figure out why, but also come up with a solution so I can move optics between guns without resighting them in. They are both flat top ARs and I am putting the optic in the same position on each weapon. Can anybody shed some light on this?

Edited October 24, 2010 by bigwakes

[Reservoir Dog]

......but also come up with a solution so I can move optics between guns without resighting them in.

That's probably not going to happen. Different ARs will shoot differently. Subtle differences in each AR (different rail systems, barrels, etc...) will effect the point of impact. Even removing and remounting an optic to the same rifle can effect the POI.

[Dolomite_supafly]

Every rifle is different no matter how close they are. You could have two guns with sequential serial numbers and more than likely they will require a different zero for each. I have even found that identical yet different barrels on the same gun will produce a different zero.

Free float tubes vs standard handguards can cause major differences in zero as well as change overall accuracy, even on the same gun. If the M&P has standard handguards and the Vltor doesn't then this may be the issue. To check, remove the handguards on teh M&P and try shooting some groups again. Just make sure not to rest the gun on the barrel as this can influence the rifle's point of impact.

I am a huge fan of free floating the barrel on any gun when possible. Bare bones, brand name free float tubes can be had for <$35 + shipping. I have free float tubes on all my AR's and rails will be added to the tube as I have a need for them.

Hoped my rambling helped

Dolomite

[bigwakes]

I was thinking it would be Easier than that. The wierd thing is the Eotech can go on my GSG5 and has the same point of impact as the M&P. I guess I just need to put an Acog on my Christmas list.

[Dolomite_supafly]

I would check to see if the handguards are causing the shift. It is as easy as removing the handguards and shooting a few groups, you might even find it shoots better. Removing standard handguards are easy as pie and takes less than a minute. You don't have to loosen anything, just pull down on the delta ring and slide the handguards out.

If the handguards are what is causing the issues then a railed free float tube is way cheaper than any ACOG you can buy. Almost all decent railed free float tubes are under $300 and there are a fair amount of decent ones at less than $150. Any way you shake it having a barrel free floated tends to make for a more accurate rifle.

Dolomite

[bigwakes]

I have Magpul MOE on the M&P and a Daniel Defense Omega 7.0 on the Vltor. I'll try to remove both handguards and see what that does.

[DaveTN]

Can anybody shed some light on this?

Barrels are not perfectly parallel to the upper. As different barrels are screwed in they will “runout†different amounts because of the machining. A few thousandths of an inch at the barrel can translate to a few inches or a few feet (depending on the distance) at the target.