I am at my wits end.  The Grendel I put together has suddenly stopped shooting the .5" groups it did when I put it together.

Now, it shoots 2", punctuated by .75 now and again.

It is a wilson arms 1-8 stainless fluted barrel with matched bolt, midwest gen 3 free float and vortex 4-16 hst.

It began behaving erratically, and I noticed the epoxy midwest sends with the gen 3 had broke loose, and the tube would touch the gas block and throw shots.

I rebedded the tube, cleaned the bore and went back to developing my 129 ABLR loads.  I figured that my barrel didnt like the bullet, but my pet 123 ELDMs shot terrible.

Any ideas folks?

Round count? Frequent mag dumps?

Around 350, no mag dumps, or anything that could even be considered rapid fire.

I’d change the scope first to rule that out.

I thought an optic problem would be more erratic.  I have checked my rings and base, all are tight.

Before living on this grendel, it lived on my old 20" heavy .223 varmint upper, so it hasn't really seen recoil.

I have a 2.5-10 vortex on my 18" spr in the same burris mount I can cross check with.  I will do that tomorrow.

Did you have a burr at the gas port by any chance?

I've seen gas port burrs do this to guns.

Is the barrel bedded into the upper with Blue Loc Tite?

Is your receiver face square?

If the barrel is impacting the inside of the handguard, that won't help with grouping.

I wondered about this, are there actual tests that show that blue loctite on the barrel extension enhances accuracy? I've read about it multiple times but considering the heavy vibrations and acceleration the barrel undergoes I did'nt think it would make any difference/would be significant.

But of course I could be wrong, did anyone actually test it?

The Army Marksmanship Unit has, but I don't believe those results were released.

It takes an extra couple of minutes to do, so I just do it.

ETA: OP, verify that the handguard isn't in contact with the gas tube.  I've had this happen before and it resulted in similar problems.

Yes.  Several AR15 accuracy smiths, starting with the AMU, have been doing it for many years now.

I've been doing it on all my builds once I learned Bill A. and AMU did it.

I've witnessed several builds that were thrown together or not bedded in the past, that I broke down and trued/bedded, go from 1.7 MOA to .6" or less, on both AR15s and AR10s.  Specific examples include an 18" RLGS JP barrel, and a .308 Krieger barrel in Iron Ridge Arms billet receivers.

JP told me many years ago that he thermo-fits his AR15 barrels, where the receiver tunnel is too small for the barrel extension, and needs to be heated to open up for it.

I do it as a standard practice, and enjoy immediate positive results with my builds, but I also select good or high-end barrels from known sources.

Sorry for the delay.  I was working on my range and shooting.  Changing optics did not help.

Gas port was clear when assembled.  I do always check that.

Admittedly I have never trued or bedded an upper.   This upper is an aero m4e1, which I thought would be pretty good out of the box.

The handguard no longer gives and touches or influences the gas block in any way.

I must admit when I changed lowers and shot a group today it was half what it normally is.  I do not think I shoot my rise trigger very well.

I guess I need to look into lapping the upper.