I recently completed a suppressed 308 AR10 with a Primary Arms 5X. The barrel is a Ballistic Advantage.

I don't expect it to be shooting sub MOA groups.

I put 20 rounds of Malaysian 147gr through it to get a zero, and I have to say I was pleasantly surprised with the groups. Where the Malaysian surplus has been said to be a 3-4MOA round, I had some groups that we're around 2-3". For cheap planking ammo I'll take it. Got a decent zero, and fired off the last 5 or so rounds which were all patterned around my POA within a 3" circle (at 100 yds).

No rapid fire, slower fire, with cool down breaks in between strings. Barrel/suppressor still got hot. Could see mirage through the scope (first time, pretty cool).

I let it cool down a few mins, and then loaded up 4 rounds of 168gr FGMM so I could see how it shoots and what the difference in zero is between the 2 rounds.

Well of the 4 rounds (I know we do zeroing in 3's and 5's but I had 4 left in the card and didn't want to leave one alone, and didn't want to take 1 round out of another sleeve), 2 were dead on in my POA, less than a half inch of each other.
The other 2 rounds were approximately 2-3" lower, but also about an inch from each other.

What caused this?

At first I thought I had it figured out. I chopped down my forend so that it would be a perfect match up to the suppressor, and they do in fact make contact, and I was shooting from a bipod. But wouldn't this cause the impact to go higher? Either way, I should probably trim the rail back a ball hair so they it's not making contact.

Is the shift caused by the heating of the barrel from the first shots? It was already warm, sonits not like going from cold bore to hot bore.

I should also explain that I am not sure what order the hits were in.

Thanks!

Sample size is too small as you stated. Also, the contact between barrel and rail should be addressed as you are no longer "free floated". You cant infer where the shots will go because of that contact either.

Trim the rail, and run more than 5 round groups.

My first shot with a suppressor there is frequently a cold bore POI shift. So the first shot always opens up a 5 shot group. But those 4 shots are usually tighter than unsuppressed so group size doesn’t change much. When it does I look at the mount tightness, or try to figure out where I might have gotten sloppy. Usually it’s me and my next group settles in a bit.

It's being trimmed back a bit and as soon as I get it back I'll be cleaning it and getting out with more FGMM to shoot some more groups

Yep, contact between the handguard and the can not even worth your time.

If I had to guess... it would be one of three things.

1. Random coincidence.
2. Change in shoulder pressure on back of stock or cheek pressure on top of stock.
3. Moved your off hand position, head position, or bag position after first two shots.

For sure seasoning is a thing. First round is the most important. Next 4 is much less significant but still has an effect. Biggest difference is in .22 lr ammo.