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Posted: 3/24/2015 5:35:12 PM EDT
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I recently aqquired an MGI upper and decided to build an AR for use with multiple barrels. I figured I'd use my pistol lower with the (gasp) Sig Brace so I could switch from .300 ACC to 556mm back and forth. I pulled the barrels out of my pistol uppers and put the MGI upper on my pistol lower. I then installed the pistol barrel in the upper & the weapon would not cycle. The bolt carrier bound up and wouldn't function. When I put the upper on another lower, no problem with either barrel. This is what leads me to think the lower is a little out of spec.
The lower in question is a Delton & my others are Bushmaster, Rock River, & PSA. It works fine with the dedicated flattop 7.5 flamethower/upper but I wanted to run 10.5's in both calibers(.300/556). Looks like I'll be building one of my stripped Bushmaster lowers into another pistol. What do you think? Hessian-1 
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Quoted:
Are you sure it's not something with the buffer tube? buffer tube(Sig brace on KAK tube) worked with standard flattop uppers & MGI upper worked with other lowers. It appears that the particular pistol lower (Delton) is not a match for the MGI upper & the buffer tube may be slightly misaligned causing the bind. I thought that it could have been the MGI upper but it works fine with my other lowers. Luckily I have a stripped Bushmaster that seems to fit well with the MGI upper I can build into a pistol lower. Keep the conjectures coming. I may be overlooking something. Hessian-1
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