Problem AR10 build

I've had fun with a James Madison Tactical 80% lower and Palmetto State Armory upper in .223, and I wanted to build one in .308. I machined the lower but am having problems with the assembly. I'll let the pix tell the story.

When I try to put the upper and lower receivers together, the charging handle contacts the top of the lower.



Because of that, the takedown pin holes don't line up.



The mag also doesn't fully seat.



If I remove the charging handle, you can see the difference in height better.



With the charging handle removed, the takedown pin holes line up and the pin goes in smoothly...to a point. It seems like the pin head is the wrong shape or size. If it was slightly smaller in diameter, it'd fit in the recess. OR, if it had a chamfer to match the lower, it'd fit. But as-is, it doesn't fully seat.



But even if it did fully seat, the pin still looks too short.



And the pivot pin is too short, too.



Also, the mag release and its recess in the lower are tight. So tight, that friction alone holds it in place here. On the other end, in the mag well, it's an easy slip fit. The mag release measures .246" all the way across. The recess in the lower measures .248 on the loose end and .244 on the tight end.


The only thing that comes to mind are that I got a lower from JMT that's out of spec in a couple of spots and the wrong upper parts kit from PSA.
The takedown and pivot pins look too short--that's a parts kit problem. Contact between the charging handle and the back end of the lower--looks like the lower is too tall, which stops the pin from lining up--pin and mag don't line up. Remove the charging handle and the pins and mag line up. But the mag release being tight...again, that's a JMT flaw.

Oh, and JMT says their lower is DPMS compliant. Because I had zero problems with my AR15 build using components from the same manufacturers, I didn't expect DPMS/AR compatibility issues.

Thoughts?
Thx

Wow, lots of issues...I think you're on point with it being out of spec. Is that an AR15 parts kit (mistakenly) and not a 308 parks kit (smaller pins)?

Ditch the plastic lower for the .308. It likely won't hold up. Lots more mass slapping around compared to a 5.56.

FWIW, the fitting is pretty common for polymer lowers imnall the areas youbate having problems. Time to break out a file and some layout fluid to see where the high spots are.

3paul10, I'm thinking the pins, at least, are maybe AR15.

clandestine, I've never had fit problems with any other 80% poly lower I've built, and the polymer on my Rem700 .308 seems to be holding up just fine. But I did send inquiries off to both manufacturers. I'll post their responses when I get them.

I put together a polymer lower and many of the issues you are experiencing are common from what I've seen. You will have to do a little fitting for most of the issues. However, like Chad said, I would not do the polymer for the 308. But if you're willing to work through the problems, it will go bang. Just don't know for how long.

Good luck with this build. Welcome to AR rifle assembly 101.

You would think everyone would be standardized? I built a Juggernaut tactical 80% lower ant put it on a Aero M5E upper. I ended filling the upper maybe .0015 inches of rear radius to make everything line up and I ended up filling the internal radius of rear take down a bit to make it all fit. Welcome to building. Only way out is buying matched sets I suppose from same manufacturer


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This is my 4th poly 80 build. I'd never had anything like this happen. Sounds like maybe that wasn't the norm.

Does anybody know if the AR10 and AR15 pivot and takedown pins are different length? If so, that's just a packing error from PSA.

DP12 said:

This is my 4th poly 80 build. I'd never had anything like this happen. Sounds like maybe that wasn't the norm.

Does anybody know if the AR10 and AR15 pivot and takedown pins are different length? If so, that's just a packing error from PSA.

Click to expand...

https://www.wingtactical.com/blog/what-parts-are-interchangeable-between-ar15-ar10/

I read that already. It says "This is due to the differences in the takedown pin, bolt catch and other lower receiver pins." But it doesn't say what the differences are.

Also, the bolt catch that came with the PSA kit seems to fit and function just fine.

Polymer receivers are reinforced making them wider in key areas. This requires longer takedown and pivot pins.

I did a Polymer AR15 and after many problems, cut it in half and got a replacement lower. Not sure I will bother to mill it but will definitely do another in aluminum. Polymer worked well for my Glock builds, but not so well for anything else.

DP12 said:

clandestine, I've never had fit problems with any other 80% poly lower I've built, and the polymer on my Rem700 .308 seems to be holding up just fine. But I did send inquiries off to both manufacturers. I'll post their responses when I get them.

Click to expand...

Polymer isn't the problem. It's the use of polymer for the wrong designs.

Polymer is not a good thing for larger caliber AR lower, even if it has a steel skeleton.

AR15, not bad, but for AR10 receiver it's a downgrade.

I live and breathe AR's. I'm just trying to save you some pain.

That's the strange part. The pins on a poly AR15 I built seated perfectly and were flush on the opposite side. I've put 500 or so rounds of Wolf classic through it without a hitch. Same manufacturers supplied me on the AR10 that I'm having the problems with.