Gunsmithing Building an accurate AR...

[Jason280]

All right, I'm in the process of building another AR, which will basically be a 20" SPRish clone. Lower is a Rock River with 2-stage trigger, while the upper will be a standard flat top with free float railed YHM forend and flip up front sight/gas block unit. The barrel I am using is a stainless 20" Lothar Walther 1:8 twist heavy barrel, with the Wylde chamber.

My plan is to build a sub MOA gun using 75 and 77gr bullets, and everything is ready except for the upper. So, here's my question. What's the best way to go about picking an upper for the barrel? Would it be worthwhile to go ahead and buy a receiver face lapping tool to square the face, or should I simply pick an upper with the tightest fit to the barrel? Without the tool, is there an accurate way to verify the face is square prior to buying an upper?

 

[Johnsusername]

Re: Building an accurate AR...

Yes.
Remove the muzzle brake, sight, hand guards.

Slide the barrel into the upper muzzle first.

Put some dykum on the face of the upper and then rotate the barrel extension against the face.

If you remove most of the dykum it's square.



John

 

[Jason280]

Re: Building an accurate AR...

Damn, that's a pretty good idea.

 

[TheRifleman]

Re: Building an accurate AR...

Square it on a lathe for best results. The anodized buildup (plating thickness) varies over the surface, and is often uneven. Take care to not remove too much.

 

[Controlled Chaos Arms]

Re: Building an accurate AR...

as far as picking an upper, there are many that'll work and be reliable with a blend of high quality. just be sure to check it over real good before doing your building. make sure things fit... the carrier needs to work well in the action bore, the charging handle needs to fit, the upper needs to fit into the lower with the .250 pins installed, etc. nothing like building a weapon and then finding out something is amiss... i used to slap lowers together like they were going out of style... then a guy shows up for his weapon and a magazine won't even fit through the lower since the well is under spec... not good.

also, in a heavy barrel scenario and with some potential for extra and/or heavy vibration in cases where the projos are large or in alternate chamberings, i highly recommend using red loctite on the barrel extension...

some guys will piss and moan about this, but these uppers were never designed to hold a ton of weight... combine that with some vibration and the barrel nuts do come a bit loose and/or the extensions are a bit sloppy to begin with providing a poor scenario... the blue loctite isn't enough. weapon heat will run blue out usually. red can be removed with a propane torch when gently applied, so no worries about removal.

i still use antiseize on the barrel nut, but never on the extension. same on the little screws for the yhm quad rail. those are notorious for either coming loose or getting stuck bigger than shit.... so add anti seize compound and torque properly... if your YHM front flip up is too snug like most, then drop a couple drips of kroil on it and with time it'll be fine. those are sticky for some reason, but they usually come out of it...

hope this helps.