I have a 6mm ARC upper I bought from Stag and shot it all last year with a carbine lower I built years ago for my .223. Long story short it shot great but I wanted to build another lower dedicated for the Arc. I used a a stag stripped lower and used a rifle length stock and buffer system. I went to the range and the POI of impact shifted over 3' and I ran out if adjustment on my scope, I put on my other lower and it shot where it always had. I did notice the buffer retainer pin doesn't catch the buffer all that well and when i opened to the lower the buffer came out and deadheaded against the hammer of the trigger group. I used the Magpul MOE stock and a basic buffer and i believe a tubbs rifle buffer spring. Any thoughts as to why is has such an impact change between the lowers?

What type of buttstock was on the lower that shot well? What type of trigger on the lower that shot well? Did you say the POI shift was 3 feet or 3 inches?

TC

Yes 3 feet i ran out of scope adj with the rifle length buffer system and stock. The other lower that shot well is a Magpul ACS3 carbine. Both triggers are drop ins from velocity and rise armament.

And optics were never removed from the upper?

No, just swapped the lowers and re- zeroed the carbine lower from last year. Scope tracked back up just fine and grouped.

Changing just the lower should have no effect on the POI.

Exactly what I was thinking.

I thought the same. I am really confused with the situation.

Not sure what’s going on but here were some thoughts I had.

Cheek weld, head position when shooting. Are you closer to/further away from the scope, head at a different angle?
Is there a lot of slop between the fit of the upper and lower?

I still don’t understand how it’s 3’ difference. Doesn’t make sense.

Maybe that buffer being forward against the BCG was putting some abnormal pressure forward and causing something to be off. Did it group at all on the new lower?

That's a new one on me, I could see an inch or two maybe.

There's got to be something wonky for sure, now it's just isolating it. The fact the buffer won't stay retained is probably a clue I guess, but I can't tell you how. I wouldn't be surprised at a change between carbine and rifle buffers, but not that much.

Until you fix the lower to function correctly, check how the fit is between the lower and upper in the curve. The radius must match.

In one statement you say you used rifle length stock and buffer tube. In another you say you used a Magpul MOE stock. Unless I am mistaken the MOE stock is for the carbine length tube and does not work on a rifle length buffer tube. Did you put a rifle length spring in a carbine length tube? Wouldn’t affect accuracy but would possibly push the buffer out. As for the buffer coming out, is the tube screwed in so far that it does not let the pin go up and down causing it to hold the pin down? The tube should only screw in far enough to keep the retaining pin from popping out completely. You should be able to push it down and it spring back up.

There is a moe rifle stock.

Was not aware of that. I have only seen the carbine

This is interesting.
Please post the solution when you find it.