AR308 Point of Impact change question

[reynolds25]

I have a Smith and Wesson M&P 10 that I have converted to 260 Remington. It has a JP hand guard and I run it with a heavy stainless steel suppressor. The barrel is a medium profile barrel.



For a while I thought I was losing my mind. The rifle would shoot round after round through the same hole then the POI would change on my next group by an inch. Then I would take a break and the POI would go back. At first I thought it was heat or the scope (SWFA HD) had a wandering zero. Today when I went out I remembered that before all my mods and when it was still a 308 I was shooting it at the range once, pounding steel at 300 yards. I handed it to another guy and he was high on the 300 yard target consistently. Then he handed it back to me and it was back on target. At that point I realized it was the amount of pressure we were applying on the hand guard. I told him to apply less pressure and PING he was now hitting those steel targets.





That sequence popped in my head today. I tested my theory and sure enough if I put a small amount of pressure on the rifle the rounds would group nicely but high. If I took all the pressure off the hand guard they would group a full 2 inches lower at 100 yards. We are not talking huge amounts of pressure here BTW.



I guess my question is - Do you all think that my upper receiver is weak and that is the cause for the shift? Do I need a higher quality upper and if so who makes an SR25 compatible upper that is very strong. I would be willing to do a new hand guard as well as I am not in love with the JP but it did have this problem with the stock hand guard a while back as well.

 

[qorban88]

I have used the JP handguards before. They are nice, but they are intentionally offset from the barrel center and have a small diameter inside. Check to make sure your gas block, or anything else, isn't near the handguard, which will affect your barrel whip as mentioned. You should have .050" clearance between anything and the barrel. Check it with the suppressor on. A heavy suppressor will actually pull down the barrel with its weight.

If that's clear, I would check the torque on the barrel nut. If I remember, part of it is held with loctite.

I don't think it's your upper's strength. Worst case.. the threaded portion is off and not perfectly in line with the handguard. Take some measurements with a caliper and look down the barrel to see if it's centered. Good luck.

 

[Sticky]

I have a couple 308's with JP handguards and another thing to check is that the bbl nut is not bearing on the front of the upper receiver. There should be a little daylight between the upper and the rear most barrel nut iirc when you install them.

They are offset as mentioned and yes, the first bbl nut should be loctited in place before the barrel and barrel nut are installed and the rail installed. The first bbl nut needs that little bit of clearance to the upper. You could be getting some interference on the underside of the gas block with the offset tube (if it is like mine) when bearing down too hard on it as mentioned above..

 

[reynolds25]

Thanks guys, I realized from your posts that I may have not installed the hand guard properly, I am going to check it all out tonight and see if it looks good on both of your suggestions.

 

[DaemonAssassin]

Is your handguard free floated?