SPR Project

Craftsman 441

Craftsman 441

CGN Regular
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Location
Wainwright
Over the last few months I have been building building myself an SPR and it's nearly done. The recievers are new Stag Arms recievers from Arms East Inc. (thanks again Walter!) and fit very snug to each other.

So as I got to thinking about accurizing the AR, I started wondering about just how square the end of upper reciever is to the bore for the barrel extension. They true and square everything on bolt guns, right? I did some research and found that some shops do this very procedure for their builds. Armed with an idea, I bought a foot of dia. 1.25" stainless bar, designed a mandrel and stayed late in the machine shop for a couple of nights. Here's what came of it:

Mandrel done and mounted between centers.
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I zeroed my dial indicator at the highest point and turned the chuck by hand until I found the lowest point. As you can see this reciever's front face is only 0.007" out of square with it's bore.
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Since I was set up anyway, I wasn't going to let 0.007" go unchecked. I mean, that's not bad at all, but I was set to cut anyway.
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When I was finished, the cut was nice and flat and square. After facing off the 0.007", the indicator didn't even move. The mandrel idea worked. Since I can't afford to build another SPR (everthing is a top grade part and quality optics are expensive), I sent the mandrel to Walter at AEI.
I know he is a very busy guy so it may take some time before he starts offering the service.
 
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So the next step is the barrel. It wasn't a big problem for me since I found the perfect piece when I was travelling through NB on vacation this summer. I bought a 26" stainless Lothar Walther AR barrel blank from Blue Mountain Sports in Brown's Flatt in NB. It was already threaded and chambered in .223, so all I had to do was figure out how I was going to profile it.

Again, I researched my piece, so I bought books, surfed the interwebs and measured the barrels I had access to. What I came up with is this drawing.
SPRBarrel.jpg


The blank was 26" long and 1.050" in diameter with a shoulder for a 7/8" gas block and a .825" muzzle end. There was a bit of cutting ahead of me, so the only logical thing to do while I worked on machining the barrel, was to order parts.
 
If you dont mind me asking, how much did you pay for a stripped up receiver? I have a Stag Arms lower and my Norc upper does not mate well with it. So I want something a little better. Keep up the great work/ keep us informed!
 
Awesome! Nice project, good info, And thank you for the mandrel.

We're just about caught up with backorders, and am planning my SPR build as well, I will definitely use it for the first time there. Impressed to see the receiver was only 7thous out (Stag Arms = Quality), but 0.000" is better than 0.007". :)

I also just spun up one 18" barrel with almost the exact same dimensions, but .80 rather than .84 under the handguards, looking forward to finishing it and seeing how she shoots.

sl66ICEcuba, Stag Arms uppers (complete with forward assist and ejection port cover) are available through the AEI webstore.
 
dangertree said:
Awesome! Nice project, good info, And thank you for the mandrel.

We're just about caught up with backorders, and am planning my SPR build as well, I will definitely use it for the first time there. Impressed to see the receiver was only 7thous out (Stag Arms = Quality), but 0.000" is better than 0.007". :)

I also just spun up one 18" barrel with almost the exact same dimensions, but .80 rather than .84 under the handguards, looking forward to finishing it and seeing how she shoots.

sl66ICEcuba, Stag Arms uppers (complete with forward assist and ejection port cover) are available through the AEI webstore.
Click to expand...

Are you guys making barrels now? I remember a mention of it awhile back, but never heard anything further.


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Not to rain on your parade or anything:redface:, but if you do some reading into AR precision builds (Accuracy Speaks comes to mind) especially with respect to the AMU work, trueing and blueprinting the barrel and receiver doesn't do diddly (according to a whole lot of folks that have tried and confirmed it).

Anyone who wants some insight on acurizing ARs, I'd highly recommend reading 'The Competitive AR15 - the mouse that roared' by Glen Zediker. Lots of excellent info on what does and doesn't matter.
 
The bit to get right is mating the barrel and the barrel extension, this is what the bolt locks into and hence determines the alinment of the bolt with the bore.
 
My blank was already threaded and I didn't make the barrel extension, it came from AEI, so indexing was pretty well out of my hands. What I could control, though, was the concentricity of the barrel and the barrel extension.

I mounted the barrel in a four jawed chuck so that the chamber end was sticking out, then dialed it into concentric. I cut the large diameter, re-squared the shoulder after the threads and threaded on the barrel extension with the tool from P&L. I checked with headspace gauges and my bolt. Everything was good to go. My next move was to dial the extension and gauge it's squareness. It was off by 0.002", but I faced each side anyway. The muzzle end was next, and I dialed it in using the lands of the rifling. The whole thing was then turned between centers and there is a slight taper (0.005" on the dia.) from the gas block back to the 45 deg shoulder. My lathe is not new, and it's the shop workhorse, so it was the best I could get after a few free cuts.

The barrel is held on by the steel barrel nut provided with the Daniel Defense Lite Rail 12.0. It is an awesome idea for a rail mount, and the only downside I can think of is that you need to buy a seperate barrel nut wrench. Then again, the $30 for the wrench is drops in the bucket compared to the rest of the build.

I never took any pics of the barrel in progress, but here is everything as it stands today. I'm leaving the barrel natural for now. I took the time to polish it on the lathe, then brushed it with a handful of 000 steel wool. I think it is a nice contrast to the rest of the gun.
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All I need now is the GG&G FLT Accucam scope mount from Wolverine. That's it.
That and M3 knobs for my Mark 4 specific to my load.
The only reason the wait for the mount isn't killing me is that I can zero my irons and work up a load to a velocity I like.
I will likely add a collapsible stock later, but it's a lower priority than a scope mount, of course. I put the SPR upper on my carbine lower with the LMT stock, and all I can say is SCHWING!
I already have one, and I don't want all my guns to look the exact same, so I'll probably go with a VLTOR EMOD if one comes up, or maybe a UBR if I come across one.
 
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