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.

I will. it may take a while to track it down. Really not sure what to try next. I think i will put the questionable lower on my other upper and see if it throws the POI off as well. Maybe i have an out of spec lower although it seemed to lock up good.

Well. I changed the buffer itself and it did the trick, i had the same POI as my carbine lower. They were both rifle length buffers but one was heavier than the other. Hard to believe that it made that big of a difference. Wish I had more info. I know the heavier one was an ARStoner the other I think was an aero.

Just catching the thread now but yes you got problems with your buffer/spring and the problem is likely there. Have you ever considered a captured buffer? Your going to be able to get rid of the whole buffer retainer idea,you can run them in rifle/carbine length tubes with the same lock time/consistency and can also be easily swapped from gun to gun when you switch uppers. Once you try one you wont go back to the spoon over the spring twang.

SOUNDS LIKE YOU ARE TRYING TO RUN A CARBINE LENGTH (sorry) gas system with a rifle butt stock, maybe an issue, plus you need to get the buffer retainer fixed so it stays put!!

with a properly sized and located carbine gas system, thats just a phrase used to describe that gas port location. it has nothing to do with pairing a stock with it.

please consider that a standard a2 (or similar) style fixed stock "rifle" buffer = 5.2 oz. a "carbine" H3 buffer is 5.4oz. the length difference -5.900" vs 3.250" - is to accommodate the buffer tube length difference.

they even sell nylon spacers you can put in behind a carbine buffer to use them in a rifle length fixed stock buffer tube.

in theory - the buffer weight should only effect lock time and overall recoil impulse. the bolt in an ar-15 rifle will not even start to move until well after (relatively speaking since we're dealing with MS here) the projectile has left the barrel.

which makes it even more wild in this case that swapping out the buffer seems to have corrected the issue.

were they otherwise dimensionally the same? should both be 5.900" long.

have you weighed them out?

those would be to very interesting data points to have.


this really has me scratching my head because, in theory at least, changing your buffer should have absolutely zero effect on POI/accuracy in general since a closed bolt is a closed bolt, and the AR system is designed to not have the bolt start moving until the bullet has left the barrel.

does beg to wonder if for whatever reason you were getting some wierd bolt tilt, or excess bolt speed causing the projectile to hit on something during feeding and either setback a bit, or tilt just a little so that it was coming into the rifling at a bit of a cant because of the deformation on chambering.