Suppressors First shot high?

[SupressYourself]

I wasn't finding previous posts that seemed to fit, so I'll ask:

My brother is shooting a Mack Brothers Varminter 3.0 in 30cal on a JP AR-10 in 260 Rem.
He recently replaced the JP barrel with a Dracos Straight Jacket barrel. -- It's a little odd in that it uses an adjustable gas key instead of an adjustable gas block.

Anyway, what he's seeing is that in every group, the first shot is 1-1.5 inches high. The next x number of shots group together as long as he fires them 20 seconds (or less) apart.
If the gun is allowed to sit for a minute or two, the first shot will be high again.

We shot it without the can, and it didn't seem to happen, or if it did, it wasn't nearly as distinct.
The odd part is it didn't seem to happen with the previous JP barrel.

The one test we did not do (and should have) was his can on my gun and vice versa. I've never seen this shooting my 2 different cans across 6 different rifles.

My thought is that it may be something like: the first shot changes the atmospheric conditions inside the can, so the rest of the shots are lower until the gun is allowed to rest and the gases dissipate from the can. Any ideas?

 

[SupressYourself]

Upon further testing, this seems like it might not be caused by the suppressor. The can only seems to change the behavior a bit.
With the suppressor off, it does a similar thing, only the first shot is left and low instead of high. It seems like it's related to the "Dwell Chamber" / adjustable gas key configuration used with the Dracos Straight Jacket barrel. He called Dracos. They have no fricken clue -- which was clear when the guy started talking about the "mating" between the upper and lower receivers... wtf? -- He would have sent it back already if not for how excellent it groups (minus the first shot).
Let me know if there are any ideas. I'll see if I can find a Dracos thread to post this to...
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[black500]

Sounds like a cold bore shift may be contributing. My primary rifle I use has a .4 Mil shift up on the first shot and then groups like a champ. Happens suppressed or unsuppressed. Its the same vertical shift on multiple cans. Horizontal shift varies by .1 Mil from can to can.

 

[SupressYourself]

Thanks for the reply, but I don't think it's a "cold bore" thing, at least not in the classic sense. He can shoot 20 rounds and then let it sit there for a minute or two, and then it will send the first shot high again. I don't think a suppressed 260 would cool down that quickly, although I'll admit that construction of the Dracos barrel is a wildcard here.

 

[JW329]

I remember reading/watching somewhere that the rifles gas system will chamber a round differently than how the charging handle does, different force/speed and that will amount for a slight shift but 1-1.5 is yuge!