Upper reciever true and square?

I have been looking around in my search for parts in a new build and keep hearing about reciever straightness. While i havent had any function issues with the plain ones i have, i do have a couple questions.

For builders who check this what exactly are you checking and how do you do it? What would be considered a ok tolerance?

I really want to buy a Vltor Mur or maybe a Larue Stealth but dont have the extra cash at the moment so that will have to wait a little while. In the mean time i would like to check out what i have just to quiet the perfectionist inside. I do realise that the MUR and the Stealth are made better and more rigid than a "normal" upper receiver but really getting down to it can you really have a functional or accuracy difference in an ar15 platform?

With all that said ill still buy one eventually because of the machinework, quality and how good they both look. So what do you think about this?

Grant (or maybe it was Wes) once wrote up a terrific guide to checking upper receiver dimensional trueness. That was back around the time that Mark came out with his Stealth upper and the boards when crazy with debate.

For what it's worth I never found a CMT that was true on its face -- out of a sample numbering something under 12. Every Sabre upper I've purchased has been dimensionally right-on-the-money -- out of about 5.

I realize this doesn't answer your question about how to go about the actual checking but I think I'll let the guys with the big credientials speak to that point.

Best of luck. :)

What difference does this make?

Quote:

---

Originally Posted by Ellery Holt

and the boards when crazy with debate.

---

I'm glad I missed that one. There's so much irrational worrying in the gun realm. :rolleyes:

After my recent experiences I would strongly recommend getting your vendor to supply a "matched" set IF you do go the "precision" (MUR or Stealth) route. The permutations are too long to list, but I will say from trying 4 Stag lowers, an LMT lower, and 3 DPMS lowers with a CMT upper and 3 different Vltor MUR uppers, that I only got 8 VERY GOOD fits. Most were too tight, particularly on the Stag, and the LMT lower had the loosest overall fit of them all. The DPMS/MUR combination was the most ideal......tight but one is able to push out the pin with a finger.

If you must, try Les Baer matched set, though again, I can't figure out why...

I was mainly looking for a description of how the vendors who do look for the reciever to be true and square actually check the recievers. I already have the lower im going to build on and may use my existing upper so its mostly for informational purposes.

I have thought hard about buying a Voltor MUR but dont know if ill see a real benefit from it over the standard reciever. I have never had an issue with the normal recievers but since the upper im building will be capable of better accuracy than normal i thought id give it every chance i could. Within reason that is. I do like how the MUR looks and know it will be made correctly and more stout but a 2x the price id like to know how much of an issue it really is (stiffness and uppers being out of spec).

From what ive gathered the difference it makes is that the top rail could be canted and the face the barrel contact points could be a little off. To how much this will effect the end user who knows. How good they are probably has allot to do with that.

Quote:

---

Originally Posted by blkexp98

I have been looking around in my search for parts in a new build and keep hearing about reciever straightness. While i havent had any function issues with the plain ones i have, i do have a couple questions.

For builders who check this what exactly are you checking and how do you do it? What would be considered a ok tolerance?

I really want to buy a Vltor Mur or maybe a Larue Stealth but dont have the extra cash at the moment so that will have to wait a little while. In the mean time i would like to check out what i have just to quiet the perfectionist inside. I do realise that the MUR and the Stealth are made better and more rigid than a "normal" upper receiver but really getting down to it can you really have a functional or accuracy difference in an ar15 platform?

With all that said ill still buy one eventually because of the machinework, quality and how good they both look. So what do you think about this?

---

Couple things. Do not buy a billet upper for its rigidity as forgings are stronger. Buy a billet upper for its trueness.

IMHO, the only time a billet upper makes sense is if you are going to put glass on it. If you are shooting irons or using a red dot optic, it does not matter.


C4